Development of a Biomechanical Model and Validation of Assessment Tools for Personal Load Carriage Systems
BY
Wm. Alan H. Rigby
A thesis submitted to the School of Physicai and Health Education in confonnity with the requirernents
for the degree of Master of Science
Queen's University at Kingston Kingston, Ontario, Canada
September, 1999
copyright 8 Wm. Alan H. Rigby, 1999
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Abstract
This study was part of a larger military project to improve personal load carriage
systems for soldiers. The goal of this study was to develop and validate a personal ioad
carriage system biomechanical model. The mode1 would serve as the basis for a personal
load camage system design tool, which would provide a better understanding the pack-
person interface and in tum help development of new systems for soldiers. A sub-
problem of this study was to develop and validate an improved pack testing system for
evaluation of the biomechanical modei and for fbture scientific and field studies.
A load distribution mannequin and force platform were part of the comprehensive
testing system designed to provide the necessary measurements to validate past and
current personal load carriage system biomechanical models. In addition, hkro new
devices, a strap tension probe and an instmmented test pack, capable of measuring pack
strap tensions and lumbar contact forces respectively, were created. These new
measurement tools were validated within a 5% average error and suggestions were made
for funher irnprovements.
The current biomechanical model was the third iteration in a series of persona1
load carriage system models. The current model employed two different techniques to
predict pack-person interface variables. The first technique used the principles of static
equilibrium of the pack-person interface to determine unknown variables and preûict
contact forces between the pack and the human form. Tension in the upper shoulder,
lower shoulder, and load lifter straps supported much of the pack mass. Through fiction
and anatomical geometry the waist belt and lumbar region provided vertical lie. Al1
unknown elements of the equilibrium needed to be solved. The second technique used
logical relationships between different elements of the pack-person geometry, interna1
forces and moments, and contact forces to predict unknown variables and pack-person
contact forces. Again, tension in the upper shoulder, lower shoulder, and load lifter
straps supponed much of the pack mass. The relationship between the shouider straps
was modeled using a modified pulley equation. The waist belt and lumbar pad lie forces
were predicted bas4 on fiictional contributions and vertical components of reaction
forces.
The current model could not be used as a robust xientific tool. Equilibrium
predictions of pack-person interfiice variables were quite poor compared to measured
values (average p-values less than 1 . 0 * 1 ~ ~ ) . The large coefficients of fiction at the
shoulder, lumbar region, and waist caused the predicted ranges of the regional models to
be so large that their ability to contain the associated validation measurements were
suspect, despite the predicted ranges encompassing an average of 7 1.3% of the rneasured
values. On the other hand, the geometric components of the model were valid as the
model predictions and measured values were statistically correiated (average p-values of
0.99). The sensitivity analysis proved that the equilibrium expression that predicted the
outputs were highly sensitive to input variables, implicating the load lifter strap model as
a potential cause of the "ill-conditioned" system. Regional model and geometric outputs
were less sensitive to changes in input variables.
In general the model, modeling process, and sensitivity analysis provided insight
into and qualitative information about the pack-person interface. In addition, two new
measurement tools were validated and can be added to the personal load carriage system
battery of test. Future directions outlined in the thesis showed how the current rnodel and
validation techniques could be used for the next iteration of the model, other scientific
studies, and field tests.
Acknowledgements
I would like to acknowledge the Defense and Civil Institute of Environmental
Medicine for their financial and materials contributions to this thesis. 1 found
this work very stimulating and enjoyable, and 1 appreciate DCiEM's and Major Linda's
Bossi's help in making this research possible.
1 would also like to take this opportunity to thank a number of individuals,
without whom this thesis would no€ have been possible: Dr. Joan Stevenson, for ail your
help, guidance, and encouragement over the past 4 years; Dr. Ron Pelot and Dr. Tim
Bryant for your invaluable contributions and advice; Gerry Saunders of the Clinical
Mechanics Group for your precision and tireless efforts in construction of the strap
tension probe; The gang in the lab: Jon, Wayne, Pat, Derek, and Sue, for making
yourselves so available for al1 the day to day help.
1 would also like to recognize my many friends and family, who's support has
rneant so much. A special thank you to my mother, Sandy Rigby, for everything you
have done, your credits are too long to list.
Finally, 1 would like to acknowledge Jenn Ellis. Your motivation, support, love,
and understanding throughout this thesis and in life continue to be my inspiration. Thank
you, Jenn. 1 love you.
................................................................................................................. Abstract
................................................................................................. Acknowledgments
....................................................................................................... List of Figures
......................................................................................................... List of Tables
......................................................................................... Chapter 1 : Introduction
General Project Focus ................................................................................
Review of Literature ..................................................................................
..................................................................... Biomechanical Model
................................ Objective Evaluation of a Pack's Effectiveness
General Review of Pack-Person Interface Literature .......................
Chapter 2: Development and Validation of a Biomechanical Assessrnent Tool ........
Introduction ...............................................................................................
Test Mannequin .........................................................................................
Force Platforrn ...........................................................................................
Test Pack ...................................................................................................
................................................................................... Strap Tension Probe
.................................................................. Strap Tension Probe Validation
Accu racy ........................................................................................
Reliability ....................................................................................... . . Precision .........................................................................................
.............................................................................. In Yivo Analysis
...................................................... Strap Tension Probe Validation Resuhs
Calibration ...................................................................................... ................................................................. Accuracy and Reliability
In Vivo Analysis ..............................................................................
................................................ Strap Tension Probe Validation Discussion
................................................ Strap Tension Probe Validation Conclusion
1
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Chapter 3 : Biomechanid mode1 of a Personal Load Caniage System ....................
Introduction ...............................................................................................
The Mode1 .................................................................................................
....................................................................... General Pack Model
......................................................................................... Notation
The Shoulder and Related Pack Elements ...................................................
.................................................................................. The Shoulder
................................................................... Geometric Calculatiorts
....................................................................... The Shoulder Model
The Waist and Related Pack Elements ........................................................
The Waist .......................................................................................
................................................................................ The Waist Belt
The Lumbar Region and Related Pack Elements .........................................
The Lumbar Region ........................................................................
The Lumbar Pad .............................................................................
The Rote of Friction ...................................................................................
Chapter 4: Biomecha~cal Mode1 Validation Methods ............................................
............................................................................................... Introduction
.................................................................................. Validation Procedure
The Model ......................................................................................
............................................................... The Measurement System . . .
Sensitivity Analysis .....................................................................................
..................................................................................... Statisticd Analysis
................................................................................................. Chapter 5: Results
............................................................................... Coefficients of Friction
...................................................................................... Model Predictions
....................................... ......................... Geometric Predictions .... Shoulder Mode1 ..............................................................................
Lumbar Pad Mode1 .........................................................................
Waist Belt Model ............................................................................
List of Figures
Figure 1-1 : Phase 1 shoulder based pack-person interface mode1 ............................ ..................................................................... Figure 1-2: Phase II waist belt model
................................................................... Figure 1-3 : Phase II lumbar pad modd
....................................... Figure 1-4: Phase II personal load carriage system model
.......................................................... Figure 2- 1 : The load distribution mannequin
............................................................................... Figure 2-2: Modified test pack
........................................................................ Figure 2-3: Modified surgical pliers
Figure 2-4: Modified pliers open and closed Mews .................................................
......................................................... Figure 2-5: Schematic open and closed views
............................. Figure 2-6: Geometncally simplified strap tension probe system
Figure 2-7: Change in fùnctional strap length .........................................................
Figure 2-8: Stifiess of pack system .......................................................................
Figure 2-9: Strap tension probe validation setup .....................................................
Figure 3-1 : Elements of a standard commercial pack and the biomechanical mode1 .
...................... Figure 3-2: Biomechanical model of a personal load carnage system
Figure 3-3: Determination of strap and wrap angles ...............................................
................................................................... Figure 3-4: Net shoulder contact force
......................... Figure 3-5 : Anatomical simplification of a transverse waist section
................................ Figure 3-6: Wedge shape of hips in Frontal and sagittal planes
............................ Figure 3-7: Complete anatomical simplification of the hip region
Figure 3-8: Liff capability of the waist belt .............................................................
... Figure 3-9: Transverse view of a typical quarter section of a waist belt in tension
.............. Figure 3 - 1 0: Anatomical simplification of the lumbar region, sagittal Mew
.................................................... Figure 3- 1 1 : Lumbar region - lumbar pad model . . ............................................................................. Figure 3 - 1 2: The role of fhction
vii
List of Tables
th Table 2- 1 : 50 percentile male human fonn mannequin .......................................... 18
Table 2-2: Calibration coefficients and error measures ............................................ 31
Table 2-3: Accuracy and reliability results with respect to tension .......................... 32
Table 2-4: Accuracy and reliability results with respect to stifiess ......................... 32
Table 2-5: Precision analysis results ....................................................................... 33
Table 2-6: In vivo analysis results ........................................................................... 33
Table 4- 1 : Contribution of fnetion over the shoulder, waist belt, and lumbar pad .... 61
Table 4-2: Test setup configurations ...................................................................... 63
..................................................... Table 5- 1 : Coefficient of Ection determination 66
Table 5-2: Shoulder geometry model predictions compared to actual measured values 67
Table 5-3: Upper shoulder strap tension (Tt) mode1 validation results .................... 69
Table 5-4: Shoulder contact force ( ~ 3 mode1 validation results ............................. 70
Table 5-5: Shoulder contact force (sNX) mode1 validation results ............................ 71
Table 5-6: Shoulder contact force (!Snz) mode1 validation results ............................ 72
Table 5-7: Shoulder fnction force (FR) mode1 validation results ............................ 73
Table 5-8: Lumbar contact force (Fx) mode1 validation results ............................... 75
Table 5-9: Lumbar pad lifi force ( F ~ ~ ) model validation results .............................. 76
Table 5-10: Waist belt lift force (FZ) mode1 validation results ............................... 78
Table 5- 1 1 : Waist belt - lumbar pad complex lifi force (Fz) model validation results 79
Table 5- 12: Summaiy of sensitivity analysis for geometric outputs ......................... 80
Table 5- 13 : Summary of sensitivity anaiysis for regional mode1 outputs .................. 81
Table 5- 14: Summary of sensitivity analysis for equilibrium expression outputs ...... 82
Mii
Chapter 1
Introduction
General Project Focus
Many different devices exist to improve the load carriage capability of humans.
History has shown the backpack to be the most comrnon choice for both civilians and
military personnel. Throughout its military use, the pack's basic design undenvent very
little change. However, the ment recreational boom has led to many vked designs that
have magnifieci the diflerence between state-o'the-art civilian packs and current military
systems. Only by understanding the effect of these changes can design advances be made.
Furthermore, understanding what factors make pack designs effective pnor to
construction of prototype designs would be even more valuable for cornfort and fit of
systems. Until now advances in pack design have corne from costly, time-consuming, and
oAen subjective evaluations of prototype systems. A more effective method of evaluating
current and future pack designs is necessary.
The impetus for this project came as part of a larger Canadian Forces endeavor to
better outfit rnilitary personnel under the auspices of Defense and Civil Institute of
Environmental Medicine. Queen's Ergonomics Research Group has contributed to this
endeavor over the last five years. Their research has encompassed biomechanical,
physiological, and subjective analyses of persona1 load carriage systems (as well as load
carriage webbing and vests) and the pack-person interface. The main thrusts of the
Queen's Ergonomics Research Group contracts were to create a comprehensive battery of
personal load carriage system evaiuations. Part of that work included a personal load
canlage system biomechanical model. This model was to be the basis of a design tool for
evaluating and improving cunent and proposed pack designs.
If an objective method of evaluating pack designs dunng the initial stages of the
design process could be developed, then poor designs could be discarded, retaining
potentially effective ones, thus saving time and money. Not only could packs be evaluated
eariy in the design process, but dso insight could be gained into how design variations
affect a pack's usefùlness. To develop such a method, three essential elements must be
known. One, the variables of a pack-person interface that determine a pack's effediveness
must be understood. Two, objective methods of measuring the elements deemed
important must be created. And three, a method of predicting the objective measures,
based solely on pack design critetia and user-controlled inputs must be generated.
Work done by Stevenson et al. (1995, 1996, 1997, and 1998) succeeded in
completing the first two requirements of such a personal load carriage system evaluation
tool. Through subjective assessment of packs, experienced user interviews, user focus
groups, and in-field measurements, Stevenson et al. (1995, 1996, and 1997) developed a
comprehensive list of variables that predicted pack performance. Furthemore, Stevenson
et al. (!995, 1996, 1997, and 1998) created and validated a battery of objective measures
of pack-person interface variables. Finally, Bryant et al. (1997) was able to relate these
objective measures to the subjective evaluations thereby providing a method for
evaluating pack designs. If, therefore, a method of predicting the objective outcomes,
based solely on design criteria and hypothetical pack components could be achieved,
personal load cmiage systems could be evaluated objectively prior to the construction of
a single prototype.
The general purpose of this work was to develop and validate such a predictive
tool. Specifically, it was the author's intent to develop and validate a biomechanical
mode1 of the pack-person interface. Such a model would theoretically predict the pack-
person interface variables, such as forces and moments, that previous researchers, found
to be important objective measures (Stevenson et al., 1997).
Essentially, biomechanical models are variable relations and equilibrium
expressions that are used to represent a physical system. They can be used to predict the
values of variables that can not otherwise be measured thus providing insight into the
system by iliustrating the relationships between pack elements or predicting the values of
variables. While it is this latter hnction of the model, or the former two goals, dl
objectives are of interest to the author. Being able to describe variables that could not
otherwise be measured provides remchers with the ability to better evaluate current
designs and improve the ability to collect data in al1 situations. Insight gained fiom a
biomechanical model can lead to a better general understanding of the pack-person
system. The relationship between elements of the pack-person system, how specific
elements determine pack eflectiveness, the sensitivity of the system to input variables, and
the major processes that drive the pack-person system can al1 be better understood.
Finally, the modeling process itseif can generate unique perspectives and novel approaches
to new and improved personal load carriage systems.
Review of Literature
Biomechanical Modtl
This work represents a more comprehensive model than the two previous
generations of the pack-person interface biomechanical model. Stevenson et al. (1995)
initially developed a shoulder-based model for suspension of a simple pack, which was
designated Phase 1. MacNeil(1996) funhered this work by validating t his shoulder-based
model. A Phase II biomechanical mode1 was developed by Rigby (1997) as a continuation
of this work.
The Phase 1 model was based on a simple bag-and-straps representation of a pack.
Figure 1 - 1 illustrates the shoulder-based model and Equations 1 - 1 through 1 -3 detail the
equilibrium expressions. The upper and lower shoulder straps hold the load carriage
system vertically and against the body while their couples apply force to the shoulder. In
fact, MacNeil(1996) was able to show that the upper shoulder strap exerted more force
than the lower, and the difference between these two strap segments, which are connected
over the surface of the shoulder, was equal to the force of fiction on the surface of the
shoulder. As a result he was able to relate the tension in the upper and lower shoulder
strap via the pulley qwtion (Equation 14). MacNeü also showed that a third force
existeci at the lower back-lumbar pad contact point. This force wuntered the net
horizontal force created by the shoulder straps, thus satisfjing the moment equilibrium
about the centre of the pack.
Figure 1 - 1 : S houlder based pack-person intertace model, adapted fiom MacNeil(1997)
and Stevenson et al. (1995).
The following equations were taken Grom MacNeil's (1996) work and, for
continuity were adapted to use notation found in the current model. Please refer to the
notation descriptions given in Appendix 1.
Equation 1 - 1 : -Tl*(cosei) - T2-cos(û2) + Fx - W*sin(P) = O
Equation 1-2: Tl*(sin&) + T2*sin(e2) - Wgcos(fl) = O
Equation 1-3 : TI-(cos~I)*(v~ - d3+ddi) - Ti+inei)-(vX) + Fx*(vrd3) -
TI-cos(~~)-(vz-~,-~~) - T2-sin(8+(vs) = O
Equation 1-4: Ti = ~ 2 . p
During Phase II of the model, Rigby (1997) used MacNeil's (1996) shoulder-based
model and added a hip Mt-lumbar pad complex model to create a more comprehensive
version. Rigby (1997) modeled the waist as a partial half cone where the siope of the cone
represented the anatomical dope of the hips. The hoop stress equation was used to
determine the net compressive force generated by the tension in the waist belt. The
vertical reaction force component of the net compressive force generated vertical lift and
fiction was proposed to resist the tendency of the pack to slide down the hips. Similady,
the lumbar region was modeled as a flat faced surface with a sagittal plane angle
equivalent to the angle created in the lower back by natural lordosis. The horizontal force
at the lumbar pad was proposed to be converted to vertical lie by the geometry of the
lower back and friction resisted the tendency of the pack to slide dom the back (Rigby,
1 997).
Figures 1-2 and 1-3 illustrate the waist belt and lumbar pad models respectively
and Equations 1-5 through 1- 10 detail the mathematical relationships. The waist belt-
lumbar pad complex was added to MacNeil's (1996) equilibrium expressions. The
resulting Phase II pack mode1 is displayed in Figure 1-4 and the equilibrium equations are
detailed in Equations 1 - 1 1 though 1 - 13.
Low Back
Abdomen
Waist Belt
Hip
Figure 1-2: Phase II waist belt model, adapted âom Rigby (1997).
6
Figure 1-3: Phase ïI lumbar pad mode1 adapted from Rigby (1997).
Figure 1-4: Phase II personal load carriage system, adapted fiom Rigby (1997).
The following equations were taken fiom Rigby's (1997) work and, for continuity
were adapted to use notation found in the current model. Please refer to the notation
descriptions given in Appendix 1.
Equation 1-5: Fz = FBz + F~~
Equation 1-7:
Equation 1-8:
Equation 1-9:
Equation 1 - 10:
Equation 1 - 1 1 :
Equation 1 - 1 2:
Equation 1 - 1 3: O = Fx*(vz-d,) - T2-(cos8&(vz-drd3) - Fz(vx) - T2*(sinûz)*(vx) - Tl-(sine ~)-(vx) + Tl.(cosû &(d l-d3+vz)
Rigby (1 997) predicted and measured the forces and moments associated with the
pack-person interface of six civilian-style packs. Ali measurements were within a self-
imposed 10% error limit with two exceptions. The lumbar contact force error was 28%
and one waist belt - lumbar pad Lüt force error was 433.7%. The lumbar contact force
error was, however, within the accutacy range of the measurement device used (TekscanTM
pressure sensing system) and was considered acceptable and the high error in the lift force
was considered an anomaly. The entire Phase II model validation, however, was limited
by the measurement system and the relatively low statistical power resulting fiom a small
number of packs tested. Specifically, no tools existed to accurately measure the tension in
the upper shoulder strap or determine the vertical lia contribution generated by the waist
belt independent of the lumbar pad and vice-versa.
While the work done by Rigby (1997) provided unique insight into the waist belt-
lurnbar pad complex, the validation technique lefi the Phase II model with certain
limitations. MacNeilts (1996) validation of the shoulder strap tensions was accepted, the
vertical lia of the waist belt-lumbar pad complex was validated as a unit, and the
understanding of some of the intemal mechanisms of the system was lefi unclear.
Objective Evaluation of' a Pack's Effcetiveness
As part of the larger military load carriage project, Bryant et al. (1997) developed
an evaluation method for pack performance. Basically, soldien experienced in using
persond load camage systems subjectively evaluated a series of packs and objective
measures of those same packs were made. A Pearson correlation matrix was created
using the two data sets and the objective measures that accurately predicted the user's
evaluations of the packs were determined (Bryant et al., 1997). The subjective and
objective tests will be described in more detail below.
Subjective (human factors) measures were gathered as part of the larger DCIEM
project (Stevenson et al., 1995, 1996, 1997, and 1998). Experienced pack users from the
Canadian Amed Forces conducted numerous physicai tasks while wearing a series of
packs. Activities included: long distances marching, obstacle course running, donning the
pack, doffing the pack, agility circuits, mobility exercises, and lethality exercises (Doan,
1998). Upon completion of the exercises, each user completed comprehensive
questionnaires, rankings, and ratings regarding the pack and its effect on these activities.
Stevenson et al. (1995, 1996, 1997, and 1998) developed a number of objective
measurement tools for packs and the pack-person interface. The Load Carriage Simulator
rnakes dynamic measures of forces and moments at the L3 spinal level, contact pressures,
and relative motions of the pack during simulateci human motions. The Cornpliance Tester
measures the relative stEness of a pack system and the Load Distribution Mannequin
measures the relative load transmitted fiom the pack to the person over different locations
on the body (Stevenson et al., 1996, 1997, and 1998).
At an alpha level of 0.05, r > 0.67 represented signifiant correlations when the
subjective and objective measures were compared using a Pearson correlation matrix of 76
variables (Bryant et al., 1997). Significant correlations were observed between specific
human factors measures and Il displacementlforce load carriage simulator measures and
10 pressure/sti&ess variables. Bryant et al. (1997) noted that: pack displacement was
strongly correlated with posteiior hip discomfon; force and moment averages and
amplitudes were correlated with mobility and cornfort; and, a high correlation existed
between vertical force amplitude and overall pack ratings in the human trials. At lower
correlation levels, Bryant et al. (1997) reported that pressure measures were correlated
with discomfort scores and pack stifiess measures were correlated with mobility and
agility scores.
Objective measures that were shown to correlate significantly with human factors
were placed in a benchmark pool and the IO' and 90' deciles were determined using
means, standard deviations, and the t-distribution statistic (Bryant et al., 1997). In this
way, the objective measures of future packs could be evaluated against this benchmark
pool. In other words, below the 1 0 ~ decile a pack would be considered poor, above the
90' it would be considered excepiional, and scores between were deemed average (Bryant
et al., 1997).
Bryant et al. (1997) also developed a first generation threshold limit value
detemination for objective measures of pack-person interface pressures. A linear
regression (8 = 0.31, a = 0.05) of pressure measures correlated highly with subjective
measures of pressure discomfort and the confidence interval was determined for pressure
scores at which 9% of users reported discomfort. Thetefore, 9W of users perceived
discomfort at 20 kPa and it was suggested that average skin contact pressures should not
rise above this threshold lirnit value (Bryant et al., 1997). These threshold lirnit vdues
were also in the same range as those reported for blood occlusion and bedsores (Holloway
a al., 1976), other factors which are associated with tolerance to skin contact pressure.
Since objective measures of pack-person interface variables could now be used to
evaluate pack designs, the role of the biomechanical model becomes more important. If a
model existed that could predict these objective measures, packs could be evaluated based
only on inputs to the model. Prototype designs could be evaluated without the cost and
time of constmcting an actual prototype. This would certainly be of significant advantage
to pack designers and evaluators.
Gtneral Rtview of Pack-Person Interface Literature
The majority of previous literature falls into one of three categories; physiological
studies, subjective appraisal studies, and biomechanical studies (Pelot et al., 1995). The
following bief discussion has been classified according to these genres.
Numerous physiological studies have been conducted to determine the maximum
load that can be camed by individuals. Sagiv et al. (1994) had 26 male subjects walk for 4
hours at 4.5 km/hr. Based on heart rate scores, blood pressure, and perceived exertion,
Sagiv et al. (1994) determined that individuals were capable of carrying as much as 66%
of their body weight. Similarly, Holewijn (1989) indicated that as much as 37 kg for
males and 22 kg for females could be carrieci with limited impact on metabolic systems.
Epstein et al. (1988) supported Holewijn (1989) and Sagiv et al. (1994) by concluding,
based on V02 maximal tests (the maximum amount of oxygen that is consumed in
milliliters per minute per kilogram of body weight), that 40 kg would ultimately lead to
fatigue. Conversely, based on heari rate, temperature, VOI maximum and perceived
exertion, Shoenfeld et al. (1977) suggested that 25kg was the maximum sustainable load
for a 5 to 6 kmhr march. And Yu and Lu (1990) concluded that, for Chinese soldiers, an
even lower maximum value of 20 kg was indicated by energy requirement and heart rate.
These extremes seem to be best represented by Goslin and Rorke (1986) and Patton et al.
(1991) who showed that exertion increased linearly with load and speed of marching.
Maximal load carriage has also ken evaluated by subjective analysis. During a
retrospective study of self-reported perception of combat loads, Hunter and Turl (1964)
suggested 18 kg was a maximum. The Canadian Department of National Defense (1982)
determined, fiom a self-reported fatigue study, that soldiers wukl carry up to one third of
their body weight. In terms of performance ratings on an obstacle course, Nelson and
Martin (1982) indicated that performance related inversely to load rnass. Despite these
low payload values (compared to physiological limitations), specific trades in the Canadian
Armed Forces denote required loads of 66 to 70 kg. Similady, the Amencan Anned
Forces required soldiers to cany 54 to 66 kg (Iverson 1987).
In tenns of load placement, varying opinions exist in packing kit into a load
carriage system. During a biomechanical study Martin and Nelson (1982) measured
postural sway on a force plate and determined that loads placed ia the middle of the back
were easier to balance than high or low loads. Hinrichs et al. (1992) conducted a
mechanical study of pack inertia and found that loads closer to the back were more
cornfortable for the user.
Physioiogical analysis of load placement revealed slightly different outcomes than
the biomechanical studies. By measuring balance and performance on an obstacle course.
Holewijn and Lotens (1992) found that an evenly distributed load produced better results
than a concentrated load of the sarne mass. Legg et al. (1992) collected heart rate. and
oxygen uptake data and determined that carrying load in a pack was superior to
supporting load directly on the shoulders, though biomechanical analysis contradicted
these results. Bobet and Nonnan (1984) discovered that heart rate was not dependent on
a load placement for extended marches. However, Neumann and Cook (1984) found that
high load placements increased the EMG level of the gluteus medius muscle in order to
counteract the higher adduction force during gait. Yet, in a more comprehensive EMG
study, Bobet and Nonnan (1992) showed that motor patterns varied between subject
making intra-subject anal ysis difficult .
Subjective perception of load location suggests that loads placed high in the pack
are more advantageous for long straight hikes, and loads placed low in the pack are best
for shorter hikes that repuire more maneuverability (Jenkins, 1992). Conciusions made by
Jenkins (1992) pointed out that the high loads reduced the amount of famuard trunk lean
ta get the centre of mass over the base of support, reducing energy costs for long
marches. However, the lower centre of gravity aided in maneuverability by decreasing the
moment about the long and transverse axes of the body.
Numerous studies were also dedicated to rnethods of load carriage; shoulders,
waist, head, hands, etc. In an early physiological study, Bedale (1924) found that load
c d e d in a pack was significantly less energetically costly than the same load d e d by
the hands. Datta and Ramanathan (1970) further suggested that equally balanced loads on
the front and back of the torso were superior in tenns of oxygen consumption. However,
these front-back carriers were detrimental to cote body temperature. The majority of
literature on this topic was fiom a biomechanical perspective.
Martin and Nelson (1982) used force plate measures to conciude that intemal
fhne packs allow for greater postural stability relative to extemal frames. Somewhat
surprisingly, Kinoshita and Bates (1983) found no significant difference in force plate
measures between single and double carrier style packs. Similady, Martin et al. (1982)
reported no difference in gait patterns between vanous types of load caniage systems.
HoleWijn ((1990) showed that while physiological measures produced no significant
difference between shoulder-based systems and packs with a waist belt, the waist belt style
packs demonstrated significantly lower shoulder pressures. Presumably the waist belt
supported some of the load, thus reducing the net shoulder force. Kram (1991) reviewed
a unique load carriage system. Cornpliant "spnngy" poles that traversed the shoulder
supporting the loads at the ends of poles produced smaller peak shoulder forces and
ground reaction forces than standard packs. The author suggested that the poles created a
resonance situation in which the load was moving in unison with the user thereby
minirnizing inertial forces during vertical directional changes (Kram, 199 1).
The majority of subjective analyses of pack type are centered on variables
secondary to the pack-person interface such as style of pocket closures, colour, and
volume. However, Martin et al. (1982) found that users prefer a long pack frame relative
to a short fiame, despite equivdent centre of gravity positions. Yet, Kirk and Schneider
(1982) found that self repoited perceiveci exertion scores showed no significant difference
between different pack styles. Many studies did identiS, pack preferences based on
subjective measures even if no obvious objective reason existed for these preferences and
conclusions suggest soldiers can identify preferences based on codon.
The goal of this study was to develop and validate a personal load carriage system
biomechanical model. The model would serve as the basis for a personal load carriage
system design tool, which would provide a better understanding the pack-person interface
and in turn help development of new systems for soldiers. A sub-problem of this study
was to develop and vaiidate an improved pack testing system for evaluation of the
biomechanical model and for fiiture scientific and field studies.
Chapter 2
Development and Validation of a Biomechanical Model
Assessrnent Tool
Introduction The difficulty in validating a biomechanical model lies in making accurate
measures of the predicted values. As was discussed in Chapter 1, Rigby (1997) was not
able to h l l y validate his modei due to the inability to measure the necessary pack-person
interface variables. By definition a biomechanical mode1 is designed to predict values
that are otherwise not practically measurable. h, therefore, requires the generation of a
unique testing setup that will provide the opportunity to measure these forces. Under
these specific conditions, accurate measures can be made and used to validate the
biomechanical model. Then the model can be used in general, where these measures can
not be made.
The unique testing jig developed for the purpose of vaiidating a personal load
camage system biomechanical model combines four measurement tools. The first, a 50'
percentile male human form mannequin instrumented to measure the body reaction
forces. The second, a force platform to measure ground reaction force. The third, a strap
tension probe that was used to measure the tension in the four straps of the pack system.
The foutth, a modified personal load carriage system. The pack was designed to both
rneasure the lumbar pad contact forces and be adjustable so that the geometry of the pack
wuld be changed to create unique test setups. Each of these measurement devices are
outlined in detail below. The techniques used to measure the described forces are
explained and the rationale for determining indirectly measured forces are given.
Test Mannequin The mannequin was anthropometrically representative of a 5oth percentile male
based on height, and circumference measures (Table 2- 1). The head, arms and legs were
removed to make it easy to don and doff packs. The mannequin was also covered with 5
mm thick Bocklite, a substance used in prosthetics, to represent human skin (MacNeilBt
Rigby, 19%). Fixed to the base of the mannequin was a rotating vice that could be
adjusted to provide the forward lean demonstrated by experienced pack users under
heavy loads (Stevenson et al., 1995).
To measure the body reaction forces, the mannequin was cut transversely at the
T 1 0 spinal level and a six-degree-of-freedom load ceIl was inserted. The axis of the load
ce11 was onented at the antenor-posterior and medial-lateral mid-line of the mannequin
and at the level of the split. In this arrangement, the load ce11 measured the net body
reaction force acting on the torso above the leve! of the load cell. The mannequin was
not representative by weight due to the hollowing of a 15cm diameter by 20cm high
cylinder to house the load cell.
The load ceIl was an AMTIm (Boston, Mass.) MCSTM series multi-cornponent
transducer capable of measuring six channels: Fx, Fv, Fz, MI, MY, and Mz. The device
was outfitted with four-arm bridges to minimize thermal effects and the cell was designed
to minimize cross talk between the channels.
The test mannequin, known as the Load Distribution Mannequin and shown in
Figure 2- 1, was evaluated and validated by Stevenson et al. (1995). It has a capacity far
beyond the required levels for load carriage evaluations. The non-linearity and hysteresis
of the ce11 are 0.2% full-scale output (AMTI, 1991). The sensitivity is 0.13 mV/(V*lb*tt)
FG 0.50mV/(V*lbtft) Fx and Fy, and 0.20 mV/(V*in*lbtft) Mx, My, and Mz (AMTI,
1991). The ViewdacTM data acquisition system was used to apply the voltage to the load
cell, and measure the output voltage, apply the calibration equation, and display the
outputs. The data were read fiom the screen and recorded by hand.
Table 2-1 : 5 0 ~ Percentile male human fonn mannequin.
Anthropometric Measurements 50"' Percent ile Male Human-Fonn
~ o d e l ' Mannequin
Neck circumference (cm) 40.8 39.5
Acrornial height, sitting (cm) 59.8 61.5
Chest circumference, maximum (cm) 99.1 101.6
Chest circumference, axi llary (cm) 102.3 101.2
Waist circumference, omphalion (cm) 86.2 84.5
Biacromial breadth (cm) 39.7 38
Back length2, C7 to L4L5 (cm) 50.6 45.3
Hip circumference (cm) 89.5
Buttock circumference (cm) 98.4 95.4
1 - Mode1 data were generated by the SafeworkTM Program
2- Back length = (sitting height) - (menton to top of head) - [(waist height) - (buttock
height)]
Figure 2-1 : The Load Distribution Mannequin.
Force Platform The mannequin described above was placed on the force platform, which was
embedded in the floor of the testing facility. By measuring the ground reaction force of
the entire test mannequin, body contact forces below the load ce11 were detennined by
simple subtraction. Subtracting the anterior-posterior force of the upper body from the
net anterior-posterior force reveals the anterior-posterior force acting on the lower body.
Similarly, subtracting the longitudinal force of the upper body fiom the net longitudinal
force reveals the longitudinal force on the lower body. Therefore, by simple subtraction
horizontal and longitudinal body reaction forces could be measured directly and
compared to the model ' s predicted values.
The force platform, AMTITM model LG6-4-lm, consisted of four measurement
gauges at each corner of a 610 mm x 1220 mm metal plate, on which the mannequin was
mounted. The platform simultaneously measured three force components and three
moment cornponents related to the three geometric axes, X, Y, and 2. The measurements
were made by a series of foi1 strain gauges attached to proprietary load cells at the four
corners of the platfonn. The gauges were based on Wheatstone bridges with output
voltages proportional to the force applied to the system. Note: the tnie axis of the
platform is actually located 5.5 cm below the top face (AMTITM, 1989). The LabviewTM
data acquisition system was used to measure the output voltages, apply the calibration
equations and display the force and moment outputs.
In an unpublished report, Potter (1998) was able to show that the force plate has
an average percent emr in vertical force measurements of 0.078% and a standard
deviation of 0.34%. Potter was also able to show a resolution of 0.002 kg, and a linearity
of 0.999 R~. Over a multi-day analysis there was no signifiant difference between data
under the same conditions. Finally, Potter reported a precision of 0.374 mV standard
deviat ions for the vertical force measure.
Test Pack
The test pack consisted of a modified DACMEnl pack board (a 510 mm x 345
mm x 9 mm polyunthane shed), standard 2.54 cm nylon webbing straps, a DACMEN
waist belt, a six-degreeof-feedom load cell, and a variable payload. A 153 mm x 1 53
20
mm cut was made in the bottom-centre of the board and a six-degree-of-freedom load ce11
was mounted in this opening with an alurninum frame. The load cell was onented such
that the Z-axis was perpendicular to and the X-axis was parallel to the long axis of the
pack tiame. Then a 145 mm x 145 mm lumbar pad was fixed to the top of the load cell.
Figure 2-2 shows the modified test pack. The top of the load ceIl protmded 21 mm fiom
the antenor surface of the pack.
Figure 2-2: Modified test pack.
Essentially the lumbar pad was isolated fiom the rest of the pack by the six-
degree-of-fieedom load cell. As a result, the forces transmitted though the lumbar pad
could be directly measured. ~ h e Xsutput of the load cell was a direct measure of the lifi
the lumbar pad transferred to the pack and 2-output of the load cell was a direct measure
of the horizontal Iumbar contact force. This load ceIl was also used to determine the
vertical lift component provided by the waist belt alone. The vertical component of the
lumbar pad was measured directly and the net vertical lift of the pack by the waist belt
and lumbar pad cornplex wss measured by the force plate. Therefore, simple subtraction
revealed the vertical lift provided by the waist belt alone.
Standard 2.54 cm nylon webbing straps were attached to the pack board ta
shoulda straps and load lifter straps. As well, a DACMEm waist belt was fixed
21
mate
to the
user face of the board. A payload of 28.8 kg or 23.8 kg were attached to the back face of
the board, creating total loads 35.0 and 30.0 kg for the test configurations. The centre of
gravity of the pack was determined by reaction board method. The nature of the
D A C W pack board allowed al1 of these elements of the pack (straps, waist belt, and
payload) to be moved, creating unique pack geometry for the validation tests.
The load ceil was the same model as the one embedded in the test mannequin and
the ViewdacTM data acquisition system was used to measure and record the output
voltage, apply the calibration equation, and display the force output of the gauge.
Stevenson et al. (1995) evaluated and validated this system and determined equivalent
linearity, hysteresis, sensitivity, and accuracy.
Strap Tension Probe To measure the tension of the shoulder straps, load lifter straps, and waist belt, a
unique measurement tool was designed. The in-line transducer used by Stevenson et al.
(1995, 1996, 1997, and 1998) was too large for this application. A device that could
measure tension in pack strapping as short as 40 mm was needed.
The strap tension probe that was developed and constmcted with the assistance of
the Clinical Mechanics Group (Kingston, Ontario, Canada) consisted of a pair of surgical
pliers that were modified and instmmented with a foi1 strain gauge. Fixed in CO-linear
alignment to one face of the pliers' head were two parallel stainless steel cylindrical pins
positioned 26 mm apart. Fixed to the second face was an identical single pin, positioned
between and parallel to the other two pins. The single pin was also attached to the pliers
head by rneans of a fiee rotating pin joint so that, throughout the pliers range, the three
pins remained parallel. The modified pliers head can be seen in Figure 2-3 and Figure 2-
4. Fixeâ to one handle of the pliers, 110 mm from the pivot point, was an adjustable
stop-rod to regulate the closed position of the plien. Fixed to the other handle was the
foi1 strain gauge between the pivot and the stop-rod.
Figure 2-3 : Modified surgical pliers.
Figure 2-4: Modified pliers head, open (left view) and closed view (right view).
The foi1 gauge consisted of a half Wheatstone bridge circuit. An excitation
voltage of 6 volts was applied to the bridge and the gauge output voltage was amplified
by an OmegaTM Model 465-115 Bridge sensor. The amplified output voltage was
displayed by a Goldstar OS-9029ATM 20 MHz analog oscilloscope with a significant digit
capability of 10 mV. Al1 output voltages were recorded and entered into a spreadsheet
for analysis.
In the open position, a strap under tension remained in straight alignment between
the two fixed ends, Figure 2-SA. In the closed position, the geometry of the strap
changed as illustrated in Figure 2-58. Using this static equilibrium position and some
geometric simplifications, the change in voltage output of the strain gauge was used to
determine the tension in the suap.
Figure 2-5: Pliers head in open position (A) and closed position (B).
The three geometric simplifications were: 1) The wrap angle of the strap around
the pins was assumed to be negligible becausc of the relatively small diameter of the pins.
This allowed for the three locations where the strap contacted the pins to be considered
point contacts. 2) The thickness of the strap was also considered negligible because it
was also relatively small. This allowed the strap to be treated as a line in space. 3) The
small coefficient of fiction of the stainless steel pins provided a surface around which the
friction could be considered zero. These assumptions produced the system illustrated in
Figure 2 6 and defined by Equations 2-1 through 2-23. Ci and C2 are constants of
proportionality and V is the voltage output of the strain gauge.
Figure 24-A shows that tension in the strap applies a force upward on the centre
pin and a force downward on the two outer pins. The magnitudes of these forces are a
product of the angle q created by geometry and tension in the strap (Equations 2-1 to 2-
3). To maintain this closed position a force equal in magnitude and opposite in direction
(Fi, 4, & F3) must be applied to the three pins. These forces are also defined by the
illustration in Figure 2-6-8. These forces are transmitted to the handles of the pliers
through the pivot.
Figure 2-6-A: Geometrically simplified system.
Figure 26-8: Geometricall y simplified system.
Equation 2- 1 : FI = Tsinq
Equation 2-2: FI = Tsinq
Equation 2-3 : F3 = 2Tsinq
Fr defines the bending moment of the upper handle, which resists the force acting
on the single pin (Fa). F4 defines the bending moment of the lower handle, which resists
the forces acting on the two outer pins (FI and F2). Fs and F, represent the added force
of the hands and the stop rod. Clearly, the user cannot compress the handies with the
exact closure force; these two forces represent the additional force that is involved. Both
forces cancel and are irrelevant to Further calculations. Equations 2-1 to 2-19 completely
define the equilibrium illustrated in Figure 2 6 . The change in voltage in the foi1 gauge
fixed to the upper handle of the pliers i s proportional to the bending moment created by
F6, which in tum is proportional to the tension in the strap.
Equation 2-4: CF = O = FI + FI - F3 + F4 - Fs - Fc + F7
Equation 2-5 :
Equation 2-6: F1+ F2 = F3
Equation 2-7: CF = O = F.4 - Fs - F6 + F7
Equation 2-8: XM = O = F4d2 - Fsd2 + Rd2 - F6d2
Equation 2-9: Fs = F7
Equation 2- 10: CF=O=F4-F6
Equation 2- 1 1 : ZM = O = (FJ - Fr)d2
Equation 2- 12: Fsd2 = Fsdi
Equation 2- 13 : F6 a F3
Equation 2- 1 5:
Equation 2- 16:
Equation 2- 1 7: CiV = 2Tsinq
Equation 2- 18: T=- Lt v 2 sin q
Equation 2- 19: T = C2V
An important result of weaving the strap through the strap tension probe is the
change in îùnctional length of the strap. In the open position shown in Figure 2-7 the
strap, which follows a straight line of length 2d1, is required to span the distance between
the two outer pins. In the closed position, the weave of the strap through the pins
necessitates a strap of length 2h, where h is the hypotenuse of di and dz, to span the two
outer pins. The extra strap needed between the two outer pins is taken from the
remaining swap that runs between the two outer pins and the two fixed ends of the strap.
Figure 2-7: Length change of test strap, open (left view) and closed (right view).
When both ends of the strap are fixed, shortenhg the fiinctional length of the strap
increases tension in the strap. In fact,
The stifhess of such a system can be
kI and k2 at each end, as illustrated
the increase in tension is defined by strap stifiess.
modeled as a strap with tension springs of stiffness
by Figure 2-8A The stiffness ki and kz can be
combined to form constant k at one end of the strap, Figure 2-8B. As a result of
increased tension in the strap due to the geornetry of the closed probe, the tension
detennined by Equation 2-19 (T) is larger than the actual tension of the strap (Ta). Using
simple spring theory Equation 2-22 defines this increase in tension.
Figure 2-8: Stiffness of pack system. A: variable stiffness at both ends of strap under
tension. B: consolidated stiffness of a strap under tension.
The change in length of the spring (Ax) is defined by the geometry (Equation 2-
20) and since di is fixed, it is determined by the amount of closure in the pliers (d2).
Thus, strap tension and stiffness are uniquely defined by Equation 2-23.
An equation in two unknown parameters, Ta and k, is produced. To determine the
two unknown parameters, a unique equation with the same variables was created.
Whenever there is an adjustment to the amount of closure of the pliers, the stoprod, or
changes to the geometry of the system; a unique d2 is created. This unique d2 detemines
the variable Ax, which determines the coefficient of the spring term, k. As well, the
unique dz produces a unique q, which in tum changes the coefficient C2. This produces a
unique equation with which to solve TO and k. In fact, a third closure level was also used
so that a least squares analysis could be used to produce a more accurate method with
which to determine the strap tension.
Equation 2-20: Ax = 2h-21
Equation 2-2 1 :
Equation 2-22:
Equation 2-23 :
Stnp Tension Probe Validation To calibrate the strap tension probe, the constant of proportionality had to be
detemineci for each of the three closure positions, C2,, C22, and CÎ3 respectively. This
ailowed calculation of Axi, Ax2 and Ax3 from the geornetry of the system.
With one end of a strap f i e , as with a weight hung from a fixed end, the spring
constant becomes O and Equation 2-23 is reduced to Ta = T = C2#. To determine C2),
10 known weights were hung three times each in random order and the output voltage of
the tension probe was recorded. Linear regression analyses of the strap tension and the
voltage output were performed. This procedure was repeated to determine Cl2 and C23.
To determine the validity of the strap tension probe, data were collected, recorded, and
anal yzed for resolut ion, accuracy, sensitivity, preci sion, and reliabilit y.
Accu racy
The test setup consisted of a srnall boom crane, an in-line tension strain gauge, an
in-line tension spring, a length of standard 2.54 cm webbing strap, and the strap tension
probe. The dope of the output from an [nstronTM force-displacement measurement unit
was used to detennine the stiffness of the strap-spnng system; the boom crane was
considered to be rigid. One end of the strap was fixed to the lever end of the crane, the
other end was fixed to the in-line strain gauge (Figure 2-9). The strain gauge was fixed to
one end of the spring, which was fixed to the stationary portion of the crane. The crane
was raised to produce tension in the strap (Figure 2-9). The in-line strain gauge used to
rneasure strap tension had an accuracy of S2 N. The tensions selected for analysis were
in the range of tension values reported by Stevenson et al. (1996).
To determine measurement system accuracy, a strap with known tension and
stifhess was measured with the strap tension probed three times, one for each closure
setting, and the output voltages were recorded. The three outputs were used in the system
of equations and a least squares analysis was performed to detemine swap tension and
stifiess. The predicted tension and stifhess were compared to the actual values for
accuracy. The protocol was repeated for various tensions and stifhess and the predicted
values were compared to the actual values using a t-test for the tension and stiffness.
Figure 2-9: Strap tension probe validation setup.
Relia biüty
To determine the reliability of the strap tension probe, the accuracy data were
collected over three different days between which the test set up was disassembled and
reassembled.
Precision
To determine the precision of the strap tension probe, 10 repetitions of the
accuracy data were collected for a low tension, medium tension, and high tension. The
spread of the data points was analyzed for standard deviation and percent error.
In Vivo Analysis
To determine if the strap tension probe was able to masure strap tensions in
pack-like situations and determine if the strap is modeled effectively by a spring-like
system, a series of in viw measures were d e . A standard pack, in which an in-line
strain gauge fit, was set up on the test mannequin. The shouklet strap was set to ten
different tensions and the tension probe was usod to detemine the tensions. The
predicted tensions from the strap tension probe were compared to actual tensions
measured by the in-line gauge using a paired t-test.
Strap Tension Probe Validation Resulk
Calibration
Table 2-2 illustrates the results of the linear regressions of the output voltage and
the strap tensions for each coefficient. Table 2-2 also shows the value determined for
each coefficient and the error associated with these deteminations. The associated Ax
value is also shown.
Table 2-2: Calibration coefftcients and error measures.
Coefficient C2# Value Associated Ax# R-squared Standard Error P-Value c2 I 0.258 2.3 mm 0.99 1.8 4 . 6 ~ 1 ~ " c22 0.360 1.2 mm 0.98 3.1 1.3~10" c23 0.628 0.5 mm 0.96 4.0 1 . 6 ~ 1v8
Accuracy and Reliability
Table 2-3 and 2-4 illustrate the accuracy and reliability of tension and stiffness
measurements respectively. Each table shows the prediction made by the strap tension
probe and the actual value. The average error was 5.4% for tension predictions and
587.7% for stiffness predictions. For tension and stiffness predictions respectively the
average day 1 mors were 5.03% and 689.4%, the average day 2 errors were 5.1% and
473.3%, and the average day 3 errors were 6.1% and 566.5%. A t-test comparing the
predicted tensions with the actual tensions revealed a p value of 0.38. A t-test comparing
the predicted and actual stiffness values revealed a p value of 1.03 * IO*'.
Table 2-3: Accuracy and reliability of the strap tension probe with respect to tension
measures.
Trial # Predicted Tension (N) Actual Tension (N) Percent Enor 1 37 33 12.1 2 56 60 6.7 3 46 47 2.1 4 33 33 O 5 53 60 11.7 6 46 47 2.1 7 40 42 4.8 8 30 3 1 3.2 9 23 21 9.5
Table 2-4: Accuracy and reliability of the strap tension probe with respect to stiffness
rneasures.
Trial Predicted Stiffness ( N h ) Actual Stiffness (Nlrn) Percent Error 1 5246.2 774.5 577.3
Precision
Table 2-5 illustrates the results of the precision analysis. The average prediction
of the 10 samples is shown as well as the standard deviation and percent enor of the
repetit ions.
Table 2-5 : Summary of precision anal ysis results.
Actual Tension (N) Average Predicted Tension (N) Standard Deviation Percent Emt 33 32.8 2.35 0.6 1
In Vivo Analysis
Table 2-6 illustrates the results of the in vivo analysis. The ten measurements
made by the strap tension probe, the actual tensions, and the associated error are shown.
The average error was 5.9%. A paired t-test cornparing the predicted tension to the actual
tension in the strap revealed a p-value of 0.60.
Tabie 2-6: Surnrnary of in vivo analysis results.
Preâicted Tension (N) Actual Tension (N) Percent Error 66 63 4.3 50 50 O 35 38 7.9 23 21 8.0 30 27 11.1 12 15 20.0 65 65 1.5 50 47 5.7 40 40 O 19 19 O
Stra p Tension Probe Validation Discussion
It was possible to measure the tension of a strap using the strap tension probe to
within approximately 6% of tnie tension values. Furthemore, the cornparison between
predicted strap tension and actual strap tension (p = 0.38) revealed that the two sets of
data were not significantly different.
Small standard deviations of a sample of 10 data points, as is shown in Table 2-5,
illustrate the precision of the strap tension probe. Repeated measures of the same strap
setup produced relatively similar values over a number of measures. This indicated thaî
the probe will produce equivalent results over repeated tests.
Similarly, the reliability of the system (strap tension probe and calibration
protocol) was considered acceptable. Not only was the system able to provide precise
readings of a test setup, it was also sufficiently robust to provide equivalent readings
between test setups. Three unique test setups revealed average errors of approximately 5
%, 5 %, and 6 %? for each day, illustrating that the system's accuracy was not dependent
on the test setup being used.
The probe was developed to reach hard-to-masure sites on the pack straps.
Therefore, one of the rnost important tests of the probe was its ability to provide accurate
information in vivo. Average error of approximately 6 % and non-significant differences
between predicted and actual tension reveals that the strap tension probe system was an
accurate representation of the pack system. The approximate error did not exceed the
enor experienced under controlled conditions and the two sets of data (actual and
predicted) were not significantly different.
Since the current model did not attempt to predict system stiffness the ability of
the strap tension probe to measure it is of litt!e immediate importance. However, the next
phase of the model may encompass dynamic conditions in which system stiffness will
become much more relevant. As such, prospective interest in the ability of the strap
tension probe to measure system stiffness exists. While the strap tension probe and
associated system of equations produced accurate estimates of tension, the predictions of
the stiffness were poor. Tables 2-3 and 2-4 illustrate that small changes in the strap
tension were accompanied by relatively large changes in the stiffness, k. This high
sensitivity of k may have been responsible for the inaccurate predictions of stiffness.
Therefore, a small error in tension was accompanied by a large error in k, explaining the
inaccurate predictions of stiffness associated with relatively accurate predictions of
tension.
Strap Tension Probe Validation Conclusions The strap tension probe had clear advantages: it was possible to accurately predict
the strap tension of standard 254 mm straps within the tight confines of pack design.
Results revealed an accuracy of S 5 % dunng calibration and an in vivo accuracy of
M.OO/o. The ability of the strap tension probe to measun strap tensions was considered
acceptable for this study within the range of strap tensions used during actual load
cadage conditions. Although the systmi was "ill-conditioned" for measurement of strap
stifhess future work may be able to exploit the strap tension probe to provide insight
into the stiffness of the pack-person intedace in order to improve Our understanding of
pack design.
Chapter 3
Biomechanical Model of a Personal Load Carnage System
Introduction In its simplest form, a biomechanical mode1 is a set of mathematical expressions
that define the physical relationship of forces and moments in a biological andor
mechanical system. In this case, the system is the pack-person interface. The goal of a
model is to predict values of the system that could not otherwise be measured and, in
general, allow experimentation in a controlled environment.
In a design setting, a biomechanical model can be used as an effective tool. The
model can provide insight into the relationships between variables and it can also be used
as an objective method of evaluating potential designs. As was outlined in Chapter 1,
Stevenson et al. (1996) identified a number of pack-person interface variables that have a
significant impact on the user's perception of a pack. If a biomechanical model could
predict these variables based on the physical characteristics of a design, much time and
money could be saved. Furthermore, work done by Pelot et al., (1998) used a previous
version of this biomechanical mode1 in an optimization routine. The routine was
designed to optimize pack geometry and placement of kit in that pack. The usefùlness of
their optimization routine is dependent on the effectiveness of the biomechanical model
on which it is based. lmprovement of the personal load caniage system biomechanical
model, therefore, enhances potential design tools and strengthens the basis for future
optimizat ion routines.
A model's effectiveness is bounded by the researcher's understanding of the
system the model represents. In this case, that understanding cornes from the two
previously validated models, experimental data from the Load Carriage Simulator,
experimental data fiom the Load Distribution Mannequin, and subjective input from
experienced pack users. Furthermore, a number of assumptions and simplifications are
made during the modeling process. While these assumptions and simplifications are
necessary to develop a determinate model, they lead to limitations in the model itself.
Not only must these limitations be understood but they must also be accepted. The model
description, information used to develop the model, assumptions and simplifications are
detailed in this chapter.
First, a general outline of the pack-person interface is described including al1
relevant notation. A free body diagram is presented detailing the static equilibrium of the
pack. Then, each subsection of the pack is broken down into specific subcomponents of
the model: shoulders, waist, and lumbar regions. Finally, the system of equations
generated is solved to provide a statically deteminate solution.
The Model
General Pack Model
A photograph of a typical commercial pack is shown in Figure 3-1. Each of the
three shoulder strap segments combine to create a suspension system for the pack over
the shoulder of the user. These straps have been designated, fiom bottom to top, the
lower shoulder strap, upper shoulder strap, and load lifter strap. It is the tension in these
straps around the shoulder that supports a portion of the pack's weight. The effect of
these tensions in the free body diagram are twice the recorded value as the Iefi and right
hand sides have been combined due to assumed symmetry of the system.
The waist belt and the lumbar pad elements of the pack are also important as they
help to maintain the pack's equilibrium position on the user by providing a lifi force and
horizontal reaction force at the point of contact with the lower back (Stevenson, 1995,
1996, 1997, 1998). It is assumed that both the lumbar pad and the waist belt act on the
pack at the same attachment point and provide cumulative suspension to the system.
Previous studies (Stevenson et al., 1995; MacNeil, 1996) have shown that a reaction force
exists perpendicular to the pack at the lumbar region. This force counteracts the moment
generated about the centre of gravity of the pack by the shoulder straps and the vertical
lie component of the waist belt and lumbar pad.
Figure 3- 1 : Elements of a standard commercial pack and the biomechanical model.
A fkee body diagram of the backpack is shown in Figure 3-2 and the notations
used are defined within this chapter. The suspension system elements have been
numbered fiom the top down in previous generations of the model. As new elements
were added they were numbered sequentially. The subscripts have been grouped
regionally for convenience. For example, the upper shoulder strap's location is noted di,
its tension Ti and its angle fiom the vector normal to the pack, 0,. The subscript 2 refers
to the lower shoulder strap, subscript 3 to the waist belt and lumbar cornplex, and
subscript 4 to the load lifter straps. Please note that the entire figure and the reference
coordinates have been angled at P degrees fiom the vertical to reflect the normal body
lean which occurs under heavy loading conditions (Stevenson et al., 1995). This leads to
a pack-based courdinate system angled at B from a global reference system.
Inputs to the model include: mass of the pack and its contents, position of the
centre of gravity, geometry of the pack, and tension in the straps that the users c m control
(the waist belt, the lower shoulder strap and the load lifter strap). The other variables that
are not controlled by either the user or the designer are leA as outcome masures of the
model. Al1 variables are identifiai as either inputs or outputs in the notation section.
Figure 3-21 Biomechanical mode1 of a personal load carriage system.
The pack static equilibrium equations for the force in the X-direction, force in the
2-directions and the moments about the centre of gravity of the pack can now be
simplifiecl accordingly. These three equations are presented in tems of multiple
unknown values. The remaining sections of this chapter are devoted to modeling the
specific regions of the suspension system thereby expressing unknown variables in tems
of known quantities.
Equation 3- 1 : Equilibriurn expression for forces in the X-direction:
Equation 3-2: Equilibrium expression for forces in the 2-direction:
Equation 3-3 : Equilibrium expression for moments about the pack's centre of gravity:
Notation
Following are detailed descriptions of al 1 the parameters il lustrated by the general
pack mode1 and associated regional models (shoulder, waist, and lumbar region). For
ease of location this notation is also repeated in Appendix 1.
Orienration:
X coordinate dong pack depth
2 coordinate dong pack height
Pack Container:
W the force of the mass of the pack (input)
vx horizontal position of t he centre of mass fiom the back of the pack (input)
vz vertical position of the centre ofmass fiom the bottom of the pack(input)
h horizontal dimensions of the pack container (input)
hz vertical dimensions of the pack container (input)
Bearer:
d3 distance from bottom of pack to lumbar pad contact centre (input)
d5 distance fkom lumbar pad contact centre to shoulder centre (input)
d6 distance from pack to centre of shoulder (input)
r average radius of shoulder (input)
r~ average radius of hips (input)
P body lean angle (input)
YL anatomical lower back angle fiom vertical (input)
YB anaiornical hip angle from vertical (input)
Shoitider sïraps:
tension in upper shoulder straps (LHS and RHS summed) (output)
tension in lower shoulder straps (LHS and RHS summed) (input)
tension in load lifter straps (LHS and RHS summed) (input)
distance from lumbar pad centre to attachment of upper shoulder strap (input)
distance from lumbar pad centre to attachment of lower shoulder strap (input)
distance fiom lumbar pad centre to attachment point of load lifter straps (input)
upper shoulder strap angle from the vector normal to the pack (output)
lower shoulder strap angle from the vector normal to the pack (output)
load lifter strap angle C.C.W. from the vector normal to the pack (output)
upper shoulder strap wrap angle around the shoulder (output)
load lifter strap wrap angle around the shoulder (output)
angle at which sN acts h m pack normal (output)
coefficient of fiction of strap on shoulder (input)
net force of shoulder straps acting though the centre of the shoulder (output)
X-component of sN (output)
2-component of sN (output)
force of fiction around the shoulder (output)
WQist M t :
Ts tension in waist belt (input)
d3 distance to lumbar pack centre fkom bottorn of pack (input)
TX compressive force that T3 applies around the hips (output)
TG normal force component of T3c (output)
Tm the force of fnction due to ~3~~ (output)
FBz lifl provided by the waist belt resting on hips (output)
coefficient of friction of waist belt on hips (input)
Lirm bar region:
Fx reaction force of lower back on pack in X-direction (output)
Fsn the component of Fx normal to the lower back (output)
Fxr the force of fiction due to Fx (output)
FLz lifi on the pack fiom friction and angle at lower back (output)
coefficient of friction of lumbar pad on lower back (input)
Fz total lift force at lumbar contact point of pack (output)
The Shoulder and Related Pack Elements
The Shoulder
In previous iterations of personal load carriage system biomechanical models, the
shoulder has been simplified to a cylindrical shape oriented such that the cross sectional
circumference was in the sagittal plane. MacNeil ( 1996) showed that this simplification
was appropriate for purposes of developing a biomechanical model. Therefore, this
generation of the model will continue to use the simple cylinder shape to represent the
shoulder.
Gcometric Calcu1ations
Tt is important to determine the cornplete geometric propenies of the pack system.
Figure 3-3 and Equations 3-41 though 3-8 outline this process. Frorn the input
dimensions of the pack and user, the angles of action of the thtee shoulder straps (€II,&,
and 8 4 ) were determineci. As well, the wrap angles of the upper shoulder straps (ai) and
the load litter straps (a) were detennined.
42
Figure 3-3: Determination of €Il ,&, 84, al, and ~ 4 .
The following series of 3-4 equations express the calculations needed for 81, the angle of
the upper shoulder strap relative to a normal vector to the pack.
r Equation 3 4 . 1 : cos(81) = - 4
(4 -d,)+e, - JR
Equation 3-4.2:
Equation 3-4.3:
The following series of 3-5 equations express the calculations needed for 82, the angle of
the lower shoulder strap relative to a normal vector to the pack.
r Equation 3 -5.1 : cos(&) = - - d6
( d , - d d - e , Jm~
Equation 3-5.2:
Equation 3 -5.3 : 02 = arctan (%)
The following series of 3-6 equations express the calculations needed for 04. the angle of
the load lifter strap relative to a normal vector to the pack.
Equation 3-6.1 :
Equation 3-6.2:
Equation 3-6.3 :
Equations 3-7 and 3-8 determine, through simple subtraction the wrap angles of the lower
shoulder strap to the upper shoulder swap and the lower shoulder strap to the load lifter
swap respect ive1 y.
Equation 3-7:
Equation 3-8:
The Shoulder Model
In the previous two versions of the model, the shoulder strap suspension of the
pack was modeled as a simply pulley with fnction. The pulleys represented the
cylindrical shaped shoulders and the straps were ropes that feed around the pulley.
MacNeil (1996) showed that the simple pulley equation (Equation 3-9.1) provided a
relationship between the lower shoulder strap tension (TI) and the upper shoulder strap
tension (Ti) such that the upper shoulder strap tension (Ti) was higher. The higher
tension in the upper shoulder strap indicated that the force of fnction around the shoulder
(FS) was directed down over the shoulder surface. However, during preliminary work on
the current model it was found that the direction of the shoulder frictional force (FR) was
actually a product of how the straps were tightened. If the lower shoulder strap was
tightened last, the force of friction (Fis) moved up over the shoulder surface producing a
relationship between the shoulder strap tensions defined by Equation 3-9.2. In other
words the method of d o ~ i n g or adjusting the pack dictates the direction of the force of
fiction.
Equation 3-9.1 :
Equation 3-9.2:
In this phase of the model, the load lifter strap was added to the shoulder complex
and its tension (T4) became another input variable because the user can control the
tension in the strap. Previous work by Stevenson et al., (1 996, 1997, and 1998) discussed
the fhnctional significance of the load lifter straps. They were able to show that the upper
and lower shoulder straps function almost solely as a suspension system for the pack
about the user's shoulder whereas the load Mer straps were considered to be geometnc
manipulators. Stevenson (1996) and her colleagues suggested that the load lifter straps
served to change the angle of pull and point of application of the upper shoulder strap
tension (Tl). Stevenson et al. (1996) aîso discovered that, for the most part, the upper
shoulder strap and the load lifter strap were not joined to form the lower shoulder strap
until both were r u ~ i n g tangent to the shoulder's surface. Therefore, the upper shoulder
strap tension (Ti) and the load lifter strap tension (TI) could be assumed to have sorne
type of additive effect on the lower shoulder strap tension (TI). In fact, if the pulley
analogy of the upper and lower shoulder straps was extended to the load lifter strap and
the pulley was considered fnctionless, the sum of the tensions of the load lifter strap (T4)
and the upper shoulder strap (Tl) would be equal to the lower shoulder strap (T2)
(Equation 3-10). Assuming that the load lifter strap had a similar relationship to the
lower shoulder strap as the upper shoulder strap, a unique relationship between the three
tensions was determined.
Suppose two separate pulleys existed: one for the upper and lower shoulder strap
and one for the load lifter strap and the lower shoulder strap. When donning a pack, the
lower shoulder strap is tightened and then the load lifter strap is tightened (Stevenson et
al., 1995). Since the lower shoulder strap is tightened afler the upper shoulder strap,
Equation 3-1 1.1 would define their relationship. Similarly, the load lifter strap is
tightened aAer the lower shoulder strap and Equation 3-11.2 would define their
relationshi p. Superposit ioning these equations (3- 1 1.1 and 3- 1 1.2) Equation 3- 1 1.3 was
created, producing the relationship between the three strap tensions.
Equation 3- 10: T2=T1 +T4
Equation 3 - 1 1.1 : T*' = T~. e ~ 4
Equation 3- 1 1.2: T ~ * ~ = T*/e"l
Equation 3 - 1 1.3 : Tz = TI' + T2"
Equation 3- 1 1.4: T4 T2 = Ti .ehal +- e Pta4
Since the three shoulder strap tensions are able to be expressed in terms of each
other and the straps are at known orientations to the shoulder, a net shoulder strap effect
(sN) on the shoulder was deduced and is illustrated in Figure 3-4 (Equations 3-12 through
3-15). In fact, because the shoulder is modeled as a pulley, the net shoulder strap
reaction force must run though the centre of the shoulder at a specific angle (4).
Figure 34 : Net shoulder contact force, s'.
Equation 3-12:
Equation 3-13:
Equation 3-14:
Equation 3- 1 5 :
The Waist and Related Pack Elements
The Waist
To mode1 a waist belt, an understanding and simplification of the anatomical
waist had to be achieved. Figure 3-5 shows a transverse section of the simplified waist
used by Rigby (1997) in Phase II. The major assurnptions were that the lower back and
abdominal areas were flat-faced sections in the transverse plane and the hips were two
semicircles. Although these assumptions seem to be extreme when cornpared to a typical
transverse human section, similarities can easily be drawn and such simplifications were
necessary for future calculations.
In the frontal and sagittal planes, human hips lie at a slight angle fiom vertical and
were modeled using the wedge shape illustrated in Figure 3-6. In three dimensions, this
wedge shape takes on the geometry of the midsection of a half cone.
Lumbar Region
Abdomen
Figure 3-5: Anatomical simplification of a transverse waist section (Rigby, 1997).
4%
Figure 3-6: Wedge shape of hips in frontal and sagittal planes (Rigby, 1997).
The transverse simplification of the waist (including the abdomen and lurnbar
region) was then combined with the half cone shaped hip section. The result was a cube
shaped midsection representing the abdomen and lumbar region flanked on either end by
semicircular half cones. The final model is illustrated in Figure 3-7. The exact size and
ultimate shape of the geometric simplifications are based on a 50' percentile male human
form, which compared reasonably to the SafeworkTM software 50' percentile model
outlined in Chapter 2.
.(,....O...@@ m..... m.*.. b.
Figure 3-7: Cornplete anatomical simplification of the hip region (Rigby, 1997).
Tbe Waist Btlt
The waist belt accomplishes the task of supporting the pack mass through
compression of the waist. Over the hip area the waist belt applies a compressive force
(Tx) that transmits lia to the pack through friction and body reaction force at the angle
described by the hips. The friction resists the pack fiom sliding down the body
(Equations 3-16 and 3-17) and the angle of inclination of the hips allows for a portion of
the body reaction force to transmit directly upward (Equations 3- 18 and 3- 19).
Equation 3- 1 6: F Z ~ = T=COS(YB) + sin(^^)
Figure 3-8 describes how the current model represents the waist belt. Rigby
(1997) modeled the waist belt using hoop stress. While this was effective for the specific
condition of a continuous waist belt of constant tension around the entire circumference
of the hips that applied a uniform pressure, it failed to represent a more general situation.
The current model attempts to represent a more generalized case.
The compressive force of the waist belt (Tx) is a product of the pressure and area
between the waist belt and the waist. While it is obvious the waist belt provides lif l to the
pack (F~Z), it was not initially clear how the vector sum of the compressive forces (which
is zero for a complete circle) leads to the lia. Despite the fact that the direction of the
forces lads to a vector sum in the transverse plane of zero, the magnitude of the forces
still acts perpendicular to the hips. Hence, it is the net magnitude acting perpendicular to
the hips that generates lie. Therefore, one might cal1 this vector a pseudo-vector where it
is only important that the component of the vector normal to the hips be considered in
model calculations.
To determine the relationship between the tension in the belt (Ta) and the
magnitude of the compressive force (Tx), the belt model was split into four equal
quadrants. Each quadrant is the largest sized section that will not result in the belt's
compressive forces canceling each ot her based on their direction. Furthemore, this split
makes the intemal force of the belt (tension) an extemal force that can be related to the
compressive force. Figure 3-9 illustrates a typical quarter section and Equations 3-18
through 3-23.3 detail the equilibrium expressions. This model need only assume that the
tension in the belt (Ta) be constant throughout its length and that the four quadrants are
equivalent. The point of application of the force is over the entire surface of the hips in
the fonn of pressure. This pressure is simplified to four pseudo-vectors normal to the
hips. As noted above, the global direction of the compressive forces are not important,
only their direction relative to the hipq which are four normal forces that impact the lift
of the waist belt.
Pressure between
a
1 1 2 ~ 2
Figure 3-8: Lift capability of the waist belt.
Figure 3-9: Transverse view of a typical quarter of a waist belt in tension over the hips.
Let P = the pressure between the belt and the hips for one quarter segment
Let A = the area of contact between the belt and the hips for one quarter segment
Eguation 3- 18: Fo = P-A
Equation 3-19.2: T3 = Fox
Equation 3-20.1 : DY = 0 = T3 - FOY
Equation 3-20.2: T3 = FOY
Equation 3-2 1 :
Equation 3 -22:
Equation 3-23.1 :
Equation 3-23.2
Equation 3 -23.3 :
To model the belt over the abdomen and lower back, a further simplification was
required. The waist belt was assumed to exit the pack parallel to the lumbar surface.
Since the abdomen and the low back are assumed to be flat-faced surfaces and the waist
belt runs tangent to these surfaces, no compression of the abdomen or lumbar area exists.
Therefore, they were ignored for the waist belt model. Due to the simplifications made,
the net force applied to the belt (and therefore the pack) in the transverse plane was zero,
which was important in determination of the lumbar reaction force (Fx) variable and any
media1 lateral forces (Y-axis). Finally, the belt was assumed to connect to the pack by
means of a pin joint so that no net moment is conducted to the pack systern. This is a
reasonable assumption due to the relatively sofi nature of the waist belt connections
available in current packs. Figure 3-2 illustrates how the lia of the waist belt is
transferred to the pack by this pin joint.
The Lum bar Region and Related. Pack Elements
The Lumbar Region
The lumbar region model was taken from Rigby (1997). Similar to modeling the
waist belt, modeling the lumbar pad required understanding and simplification of the
lower back anatomy. During modeling of the waist belt the lumbar region was simplified
as a fiat-faced surface. However, like the hips, the low back actually lies at a slight angle
from vertical (Figure 3- 10) and the lumbar pad i s designed to exploit this angle. Since
the waist belt forces are tangent to this surface, the waist belt forces have no effect in this
region.
Figure 3-10: Anatomical simplification of the lurnbar region, sagittal view (Rigby, 1997).
The Lumbar Pad
Although the waist belt was assumed not to contribute horizontal force to the
pack, a force is transmitted perpendicular to the low back through the lumbar pad to
maintain pack equilibrium; denoted Fx. Like the waist belt the body reaction force to FX
applies a vertical lie at the noted angle of inclination of the back. As well, friction resists
the pack nom sliding down the surface of the lumbar region. Figure 3-1 1 outlines the
lumbar pad model. Since the waist belt and lumbar pad support the volume of the pack at
the same point, the net lift provided by the lumbar pad - waist belt complex is simply a
sum of the two independent lias. This relationship is illustrated by Equation 3-25 and is
represented by Fz on the general pack model.
Finally, the sum of the waist belt lift force @zB) and lumbar pad lie force (fiL) creates the net lifi force of the complex (Fz) (Equation 3-25).
Figure 3-1 1 : Lumbar region - lumbar pad model (Rigby, 1997).
Equation 3-24.2: Fxr = F X ~ - C ~ L
Equation 3-24.2: F~ = asin( in(^^) + Fxr cos(yL)
Equation 3-25 : Fz = F~~ + F~'
The Role of Friction
Unique solutions for the model are determined based on the idea that each
element of fnction in the model finctions at its maximum value. However, this may not
necessarily be the case.
Suppose a block of known mass sits on a surface with a coefficient of static
fiction p and a tension of force T pulls at the block (Figure 3-12). It is clear that the
maximum force of fnction is the product of the normal force and the coefticient of
fiction. It is also clear that under static equilibrium conditions, the force of fiiction is
equivalent to the tension. If the tension exceeds the product of the normal force and the
coefficient of friction, the block will move. However, it is also true that as the tension
drops below the product of the nomal force and the coefficient of fiction the block will
not move in the opposite direction. In other words, the force of friction is a fùnction
ranging fiom a minimal value of zero when the opposing force is zero to a maximal value
of the product of the normal force and the coefficient of friction when the opposing force
is high.
Fr
Figure 3-12: The role of friction.
Consider this principle in the context of the personal load carriage system
biomechanical model. The fnctional forces that serve to provide lie to the pack will only
be as large as the net force tending to move the pack (down the back). For example,
suppose the pack is in equilibrium on the user with the fnctional components acting at
their maximum values. The user then tightens the shoulder straps forcing the shoulders
to bear more of the load. The result then (to keep the pack fiom moving up the user's
back) is for the hips and lower back to becorne unloaded. This unloading is a result of
the frictional force decreasing. The specific values that counter the tendency to move the
pack down the user's back are reduced. In fact, the shoulder straps can be tightened to
the point that the fnctional forces are reduced to zero.
The fact that al1 the frictional forces operate over a range makes the system of
equilibrium equations indeterminate. However, with an understanding of how the
Grictional forces Vary within those ranges and how the three frictional forces (shoulder
swaps, lumbar pad lift and waist belt lia) react with respect to one another, the model can
be predictive of the system. The model expressions that use fiictional forces can be
rewritten by including notation that represents the range of possible values for the forces
of friction (Equations 3-26 through 3-28).
Equation 3-26, from Equation 3-23.2:
Equation 3-27, fiom Equation 3-24.2:
Equation 3-28, fiom Equation 3- 1 1 :
The system of equations that represent the static pack model can now be simplified.
Equation 3- 1 becomes Equation 3-29:
Equilibrium expression for forces in the X-direction:
Equation 3-2 becomes Equation 3-30:
Equilibrium expression for forces in the 2-direction:
Equation 3-3 becomes 3-3 1 :
Equilibrium expression for moments about the pack's centre of gravity:
Chapter 4
Biomechanical Model Validation Methods
Introduction It was necessary to validate the persona1 load carriage system biomechanical
model presented in the previous chapter. Such a model would be of little use without
substantial proof that the model does, in fact, represent the pack-person the system.
Values predicted by the model had to be compared to the actual forces. If the differences
between these values were acceptably small, the model would be considered accurate and
thus representative of the pack-person interface.
The goal of this chapter is to describe the protocol for validating the model. The
techniques used to measure each of the relevant forces are explained, methods used to
collect the data are discussed, methods used to conduct a sensitivity analysis are detailed,
and the statistical analyses necessary to evaluate the data are presented.
Validation Procedure
The Model
Model inputs, which are detailed in Appendix 1, were gathered and recorded.
Geometric inputs were measured fiom the test pack.
The position of the centre of gravity was determined by reaction board method.
The mass of the pack was determined by weight on a hanging scale.
The strap tensions that the user controls (waist belt [Ts], lower shoulder strap [T2],
and load lifter strap [T*]), as inputs to the model, were gathered in a randomized
fashion. The test sehips are detailed on the test setup configuration chart (Table 4-2).
Note that the tensions listed in the Table 4-2 for shoulder straps were the tension of a
single strap. Men entered into the model the tensions were multiplied by two to
represent the combination of the le&-hand-side and the right-hand-side.
The coefficient of friction around the shoulder (ps) was determined by the rnethod
describeci by MacNeil(1997). A shoulder strap with two ftee ends was wrapped over
the shoulder of the load distribution mannequin at a wrap angle of 180 degrees. One
fkee end of the strap was fixed to the floor and fkom the other end three different
known masses were hung. The strap tension probe was used to masure the tension
of both the fixed end and the &ee end of the strap and. by subtraction, the force of
fnction around the shoulder was determined. The force of fnction and the pulley
equation were used to solve for the coefficient of fnction around the shoulder. Three
different masses were used to estimate three values for the coefficient of fiction and
an average value was used as an input to the model.
The coefficient of fiiction for the lumbar region (pL) was determined by placing three
loads of known value on an inclined surface. The load was covered with the same
material as the lumbar pad of the test pack and the inclined surface was covered with
BockliteTM to replicate the interface between the lumbar pad and the lumbar region of
the test mannequin. The surface was gradually inclined until slippage of the load
occurred. The tangent of the angle was used io detennine the coefficient of friction.
Three loads were used to determine three coefficients and an average value was used.
A similar method to the lurnbar region was used to determine the coefficient of
friction for the waist belt (pB).
The previous chapter outlined the role of fnction and how the range of fnctional
forces could Vary, effecting model outputs. Table 4-1 outlines the various permutations
of the forces of fiiction over the shoulder (FR), waist belt (T& and lumbar pad (Fsr).
Each of these combinations was irnposed (along with al1 other inputs to the
biomechanical model) on Equations 3-29, 3-30, and 3-31. As a result, eight different
model output combinations were calculated for each test setup. The maximum and
minimum values of a11 eight of the modei output permutations were recorded as the range
of output for the regional model and saved for analyses.
Table 4-1 : Permutations of fictional force values for the three anatomical regions.
Model Output Shoulder Region Waist Belt Region Lumbar Pad Region
1 Max. Max. Max.
2 Max. Max. O
3 Max. O Max.
4 O Max. Max.
5 O O Max.
6 O Max. O
7 Max. O O
8 O O O
The Measurement System
The test mannequin was setup on the force platform such that the centre of
rotation of the vice was collinear with the axis of the force platform. The rotating vice
was set at 5 degrees of forward lean (P), which was based on human trunk postures
assessed during load carriage (Stevenson et al., 1995). The force platfonn, load cell in
the test pack, and load cell at the waist of the test mannequin were zeroed so that all
measurements made were a result of the pack-person interfice only.
The first test pack configuration was setup and the pack was mounted on the test
mannequin according to methods described by experienced users (Stevenson et al., 1995).
The strap tension probe was used to ensure that the input strap tensions were set
correctly. Data were then collected in the following order: forces and moments fiom the
force platform, forces and moments fiom the load ce11 in the mannequin, forces and
moments fkom the load ceIl in the test pack, and tensions in the straps (waist belt, lower
shoulder strap, upper shoulder strap, and load lifter strap). Shoulder wrap and strap
angles were measured and rewrded using a protractor and a flexible spline rule. The
pack was removed and the procedure was repeated bvice more for a total of three trials
per pack configuration. The variables were averaged and recordeci in spreadsheet fonn
for fiirther analyses.
Table 4-2 outlines the test pack contiigurations. Configurations consisted of al1 16
permutations of pack mass (30 kg and 35 kg) and geometry (location of shoulder strap
and load lifter strap attachment points). The pack masses were show to be typical loads
in previous research (Patton et al., 1991; Iverson, 1987) and pack geometry was
constrained by the DACMETM pack-board. Three different strap tension input
combinations were then generated at randorn for each pack configuration creating a total
of 48 test configurations. The strap tension inputs, in 10 N increments per strap, were
randomized with group averages of 60 N for the lower shoulder strap, 60 N for the load
lifter strap, and 40 N for the waist belt. Stevenson et al. (1996) showed that these
tensions were the average selections of experienced pack users.
Note: the upper shoulder strap tension was measured on both the right and leA-
hand-side. The values were recorded and then summed and recorded as the single
tension Ti. The right and Ieft-hand-sides were compared using a paired t-test of the two
sets of data. Similarly, the right and lefi-hand-side geometry was also recorded
separately. The two values for shoulder strap angles (el, Cl2, and 84) and the shoulder
wrap angles (al and a) were averaged to detennine a single value for each. The right
and left-hand-side geometric measures were also compared using a series of paired t-
tests.
Table 4-2: Test setup configurations.
Sensitivity Analysis
To detemine the effect that each input variable had on the biomechanical model
output variables a sensitivity analysis was conducted. The results indicated how the
model outputs were affected by changes in inputs thus providing insight into which input
variables were of importance and which output variables were particularly sensitive to
errors.
A typical test pack setup was selected from the validation analysis and the model
outputs were m r d e d and denoted "original". Then, as each input variable was
systematically and independently increased by IV!, the model outputs were recorded.
The absolute changes in the model outputs were calculated for each model input increase
by subtracting the new output value from the original output. These absolute increases
were tabulated. The absolute changes were then normalized to determine the absolute
fractional change in the output variables as a result of systematic independent increases in
the model inputs.
The results were tabulated and outputs that varied greater than 3% (the mean
change in output values generated by 10% increases in inputs) were highlighted. The
results were then summarized to illustrate the most sensitive elements of the pack-person
intefiace.
Sta tistical Analysis
Statistical analysis needed to validate a biomechanical model was a cornparison of
predicted values from the model and actual values measured using the validation
materials (the test mannequin, test pack, load platform, and the strap tension probe). If
there were no statistical difference between the actual values and the predicted values, the
model would have been considered valid.
The equilibrium expression predictions and the geometric predictions were
compared to the matching measured values using paired t-tests. There were seven
equilibrium expression comparisons and five geometric comparisons, one for each
variable. Under normal conditions multiple paired t-tests is inappropriate. However,
because only the ability of a single prediction to represent the related measured value was
important, and not the interaction of the variables, muhi pie paired t-tests were acceptable.
The nul1 hypothesis of this analysis was that no significant difference existed between the
sample means of the two groups (predicted and measured). By standard convention, the
analysis was performed at a significant a level of 0.05. Failure to reject the nul1
hypothesis would indicate that the predicted pack-person reaction forces were not
different from those measured by the test setup, indicating that the mode1 was effective in
predicting the pack-body contact forces.
To determine the sample size, or number of packs to be tested, a power
calculation was performed. By convention, the test statistic was calculated based on 95%
confidence level, with repetition power of 80%. The most conservative estimate was
required. The force with the largest variance estimate was used to calculate the number
of pack geometries necessary. The pilot study by Rigby (1997), while using inputs of 60
N at the shoulders and 40 N at the waist, revealed that the lumbar lift force provided the
most conservative estimate of sample size, or largest variance. The population variance
was (S~,,,,,,~ lin = 17.2), grand sample mean (pg = 1 12.6), and treatment means of ( ~ i , =
116.9, p,,, = 108.2). Calculations for the number of pack test configurations were derived
from Glanz (1997). The sample size was estimated and noncentrality parameter was
calculated. The power of this study was then detennined from standard power tables. At
a power of 80 % Vd = 15 (fiom power table), therefore 14 pack iterations were required.
Since the protocoi generated a population size (different pack geometries) of 48, a power
of greater than 80% was easil y assured.
As was mentioned above the equilibrium expression predictions of the output
values required direct comparisons. The regional models, on the other hand, were only
capable of predicting their respective output forces and moments within a range
determined in part by the force of friction in these regions. Since a paired t-test only
compares the mean of the predicted and measured pack-person variables and this statistic
is not capable of comparing the measured values to the predicted ranges, the regional
models were evaiuated by inspection. The number of measured pack-person interface
variables that feil within the regional models range predictions were counted and a
percentage of the maures bounded by the predictions were calculated.
Chapter 5:
Results
Coefficients of Friction
Table 5-1 shows the average coefficient of friction and the range of the
rneasurements for the shoulder (ps), waist (pe), and lumbar regions (pL) of the pack-
person interface.
Table 5- 1 : Coefficient of fiction determination.
Area Met hod Average C( Range of p
Shoulder Equation 2-9.2, pulley equation 0.39 0.0 1
Waist Tangent of angle of inclination that created 0.32 0.05
impending slip
Lower back Tangent of angle of inclination that created 0.35 0.06
impending slip
Mode1 Predictions
Geometric Preâictions
Table 5-2 shows the strap and wrap angles of the shoulder straps predicted by the
model, the actual measured equivdents, and the associated p-values fiom sets of paired t-
tests. For example, the model prediction of the angle of the upper shoulder strap (81) was
45.3 degrees and the measured value was 44.9 degrees. Comparison of dl 16 pairs of
upper shoulder strap angles (81) (predicted and meawred) revealed a t-statistic p-value of
0.97. Comparison of the tight and lefk-hand-side of measures revealed average p-values of
0.79. The complete data set can be seen in Appendix 2.
Table 5-2: Shoulder geometty mode1 predictions compared to actual measured values,
C O
.r( Ci
Yi % bu
1
2
3
4
5
6
7
8
9
10
I l
12
13
14
15
16
Avg.
Std. Dev.
expressed in degrees. Paired 1-test comparisons are noted.
Shouldtr Modtl
Table 5-3 shows the measured tension in the upper shoulder strap (Ti) compared
to the equilibrium expression prediction of that value. A paired t-test of the measured
data and the predicted data pmduced a p-value of 3.16* 10''~. Table 5-3 also shows the
range of strap tension (Tl) predicted by the regional mode1 (Equations 3-10 and 3-1 1).
Forty-four of the 48 measures fa11 within the predicted range. Cornparison of the nght and
lefi-hand-side measures of the upper shoulder strap tension (Ti) revealed a p-value of
0.93. The complete data set can be seen in Appendix 2.
Tables 5-4, 5-5, and 5-6 show the measured shoulder contact forces (sN, sNx, and
sNZ) and equilibrium expression prediction of those forces. Each of the three predictions
(s" sNX, and sNz) were compared to the measured value by paired t-tests resulting in p-
values of 2.16 * 1 O-", l . 9 l * 1 O*", and 2.70' 1 respectively. Tables 5-4, 5-5, and 5-6 also
show the range of these forces predicted by the regional model (Equations 3-12 through
3-15). For s', s\, and sNz, 20,3 1, and 34 of the 48 measures respectively fall within the
predicted range. The complete data set can be seen in Appendix 2.
Table 5-7 shows measured shoulder fiction force (FR) compared to the
equilibrium expression prediction of that force. A paired t-test of the predicted and
measured values produced a p-value of 7.53*10? Table 5-7 aiw shows the range
predicted by the regional model (Equation 3-9 through 3-10). Forty-four of the 48
rneasures fa11 within the predicted range. The complete data set can be seen in Appendix
2.
Table 5-3: Upper shoulder strap tension (Tl) mode1 validation results (in Newtons).
Test Condition Measlïred Equilibrium Regionai Mode1 Range Prediction Prediction Minimum Maximum
1-1 71.33 19.94 27.10 80.00
Table 5-4: Shoulder contact force (sN) mode1 validation results (in Newtons).
Test Condition Measured Equilibrium Regionai Mode1 Range Preâiction Predict ion Minimum Maximum
1-1 174.63 1 14.28 121.1 0 173.96 1-2 227.41 f 54.43 156.96 239.40 1-3 148.93 77.21 111.86 131.21 2- 1 182.71 87.89 1 14.60 150.78 2-2 108.72 64.68 89.96 95.99 2-3 214.28 98.21 159.55 206.82 3- 1 t 76.25 80.1 3 131.23 147.86 3 -2 289.22 i 92.69 206.48 31 9.97 3-3 196.39 1 03.95 136.76 1 87.62 4-1 233.36 t 74.05 180,67 279.97 4-2 159.87 86.78 112.12 148.62
Table 5-5: Shoulder contact force (sN*) model validation results (in Newtons).
Test Condition Measured Equil ibrium Regional Model Range Predicîion Prediction Minimum Maximum
1-1 1 17.67 83.78 88.82 126.03
Table 5-6: Shoulder contact force (sNz) mode1 validation results (in Newtons).
Test Condition Measuted Equilibrium Regional Mode1 Range Prediction Prediction Minimum Maximum
1-1 129.00 77.72 82.32 1 19.91
Table 5-7: Shoulder fiction force (FR) mode1 validation resuits (in Newtons).
Test Condition Measured Equilibrium Regional Mode1 Range Prediction Prediction Minimum Maximum
1-1 8.67 60.06 0.00 52.90 1-2 10.00 85.31 0.00 82.76 1-3 7.67 60.14 0.00 20.70 2- 1 7.33 04.37 0.00 36.83 2-2 8.67 39.1 3 0.00 6.97
Lumbar Pad Model
Table 5-8 shows the measured lumbar contact force (Fx) compared to the
equilibrium expression prediction of that force. A paired t-test revealed a p-value of
3.20*10". Table 5-8 also shows the range of Fx predicted by the regional model
(Equations 3-24 through 3-25). Seventeen of the 48 measures fall within the predicted
range. The complete data set can be seen in Appendix 2.
Table 5-9 shows the measured lifi force of the lumbar pad ( F Z ~ ) compared to the
range predicted by the regional model (Equation 3-24.2). Twenty-four of the 48 measures
fa11 within the predicted range. The complete data set can be seen in Appendix 2.
Remember that there is no equilibrium expression representation of the lumbar pad lie
force ( F ~ ~ ) . Hence only a range predicted by the regional model exists.
Table 5-8: Lumbar contact force (Fx) mode1 validation results (in Newtons).
Test Condition Measu red Equilibrium Regional Mode1 Range Ptediction Ptediction Minimum Maximum
1-1 122.33 1 13.71 88.82 128.03
Table 5-9: Lumbar pad lift force ( F ~ ~ ) mode1 validation results (in Newtons).
Test Condition Measured Regional Mode1 Range Prediction Minimum Ma-ximum
1-1 40.33 10.91 59.58
Waist Belt Mode1
Table 5-10 shows the measured waist belt lia force (FzB) cornparecl to the range
predicted by the regional model (Equations 3- 16 through 3-23). Forty-six of the 48
measures faIl within the predicted range. The complete data set can be seen in Appendix
2. Remember that there is no equilibrium expression representation of the waist belt lie
force (IJzB). Hence only a range predicted by the regional model exists.
Table 5-1 1 shows the measured lift force of the waist belt and lumbar complex
(Fz) compared to the equilibrium expression prediction. A paired t-test produced a p-
value of 3.99* l O-'. Table 5-1 l a l s ~ shows the range of Fz predicted by the regional model
(Equations 3-16 through 3-23). Forty-eight of the 48 measures fa11 within the predicted
range. The complete data set can be seen in Appendix 2.
Table 5- 10: Waist belt lifi force ('FZB) model validation results (in Newtons).
Test Condition Measuted Regional Mode1 Range Prediction Minimum &ximwn
1-1 163.36 39.89 200.1 5
Table 5-1 1 : Net lifi force (Fz) mode1 validation results (in Newtons).
Tesi Condition Measured Equilibrium Regional Mode1 Range Prediction Prediction Minimum Maximum
1-1 21 2.69 264.82 50.80 258.73
Sensitivity Amlysis
TaMe 5-12, Table 5-13, and Table 5-14 summanze the results of the sensitivity
analysis for the geometric outputs, regionai model outputs, and equilibrium expression
outputs respectively. The detailed sensitivity analysis results can be seen in Appendix 3.
The table shows the fraction of change in the output variables (rows) as the input variables
(columns) were systernatically and independently increased by 10%. Cells only contain
values if the change in output variables were greater than or equal io the 50' decile. The
most sensitive model output was clearly the equilibrium expression prediction of the upper
shoulder strap tension (Ti). One can see from Table 5-14 that small changes in model
inputs (lm) led to consistently large changes in the equilibrium predictions of the upper
shoulder strap tension (Ti). It might appear as though the shoulder reaction forces (!SN,
s\, !SNZ), and the force of friction around the shoulder (FR) were also highly sensitive.
However, the upper shoulder strap tension (Ti) was used to calculate these variables
directly, so sensitivity of shoulder reaction forces (sN, s\, and sNz) was a product of the
sensitivity of Ta. Conversely, the lumbar reaction force (Fx) and vertical lifi of the waist
belt - lumbar pad complex (FZ) were relatively insensitive to changes in the inputs.
Table 5- 12: Summary of sensitivity analysis for geometric outputs, expressed in percent
change.
Model
outputs
01
02
0 4
a1
a
Model inputs increased by 10 %
ds (mm) ds (mm) r (mm) dl (mm) 6 (mm) O. 12 0.08 0.05 0.20
0.07 0.04
0.32 0.09 O, 16
0.05
0.1 1 0.08
Table 5- 14: Summary of sensitivity analysis for equilibrium expression outputs, expresseci
in percent change.
Mode1 inputs increased by 10 %
Cbapter 6
Discussion
Coefficients of Friction Determination of the coefficients of friction about the thtee anatomical regions
(shoulder, waist, and lumbar region) revealed values of 0.39, 0.32, and 0.35 respectively.
Of note was the dramatic increase in these values fiom MacNeil's (19%) measurements.
MacNei1 (1996) used a rnilitary combat shirt, over Tekscanm pressure sensing devices,
which were mounted on the mannequin. The current work used only the mannequin.
These differences accounted for the variation in coefficients of fiction.
Geometric Predictions
Table 5-2 clearly illustrates the ability of the mode1 to accurately predict the test
pack's geometry. The predicted values and the measured values of strap geometry (el, Oz, 04, ai, and a) were not significantly different (average p-value = 0.94) and thus Rom
the same sample.
The ability of the model to accurately predict the test pack's gecmetry cannot
only be attributed to the model itself, but also the repeatability of the validation protocol.
The mannequin was marked and labeled so pack elements could be placed and replaced
in an accurate manner. Furthermore, input data corn the test pack and mannequin
geometry were accurately measured and remained unchanged between trials.
While predictions of al1 the geometric masures were very accurate, the validation
protocol revealed that the vertical stays supponing the attachment points for the load
lifter straps were quite compliant. When the load lifter straps were tightened the vertical
stays flexed, altering the geometnc relationship between the straps and the stays and the
stays and the pack body. However, angles of the load lifter straps (Or) were measured
relative to the main body of the pack itself and the compliance of the stays did not
significantly effect the geometric reiationship between the straps and the test pack body.
While this may have had a signiticant effect on other elements of the pack-person
interface, such as strap tension, it is of littie importance to the geometric predictions.
Since the relative angle of the strap to the pack was not significantly affected the factor
can be considered inconsequential. However, the cornpliance of the vertical stays is
addressed fbrt her below.
Comparison of the geometric measures on the right and left-hand-sides of the test
pack indicated that the right and lefl shoulder strap angles (81, &, and Br) and shoulder
wrap angles (al and a) (p = 0.67, 0.85. 0.92, 0.71, and 0.80 respectively) were not
significantly different. This indicated that the right-lefi symmetry assumption was
accurate for pack geometry.
Equilibrium Predictions
In general the equilibrium predictions of upper shoulder strap tensions (Tl),
lumbar reaction forces (Fs), waist belt - lumbar pad complex lift forces (Fz), shoulder
reaction forces (sN, s\, sNZ), and the forces of friction around the shoulder (FB) were
poor as evidenced by the lack of statistical relationship (p < 0.001) for al1 seven outcome
variables. However, because upper shoulder strap tensions Tl, which were poorly
predicted, were used to calculate the shoulder reaction forces (sN, sNx, and sNz) and the
forces of friction around the shoulder (FB) it was not surprising that the later predictions
were also poor. It is not entirely understood why the equilibrium expressions were so
poor in predicting upper shoulder strap tensions (TI), lumbar reaction forces (Fx), and
waist belt - lurnbar pad complex lift forces (Fz). The author hypothesizes that the
sensitivity of the equilibrium system and the accuracy of the equilibrium expressions
themselves may both have had a detrimental effect.
This hypothesis was examined by conducting a sensitivity analysis to determine
the effect that each input variable had on the biomechanical model output variables. It
was apparent from the sensitivity analysis that upper shoulder strap tensions (Ti) were
highly sensitive to changes in the model inputs, whereas lumbar reaction forces (Fx) and
waist belt - lumbar pad complex lie forces (Fz) were noticeably less sensitive. This
discrepancy in sensitivity created an "ill-conditioneâ" system. The equilibnum
expressions were not capable of depicting the pack-person interface. The details of this
hypothesis are outlined below during discussion of the biomechanical rnodel's sensitivity
anal ysis.
The second possible explanation for poor predictive power of interface variables
could be incorrect modeling. This would occur if a kinetic element that existed in the
physical equilibrium was not included in the rnodel or kinetic elements that were both a
part of the physical system and the model were misrepresented. Since previous, simpler
phases of the model were more accurate predictors (MacNeil, 1996; Rigby, 1997), the
author feels that elements added to this phase of the model were rnisrepresented. One
such element was the load lifter strap tension (T4). Inspection of the model output
surnmary table in Appendix 2 revealed that when load lifter strap tensions (T4) were
large, the prediction of upper shoulder strap tensions (Ti) became less accurate and often
becarne negative, a physical impossibility for tension. Furthermore, when load lifter
strap tensions (T4) were eliminated from the equilibrium expressions (a variable value of
O N) the equilibrium expression's predictions of upper shoulder strap tensions (TI) were
much more accurate. For example, the two closest predictions occurred on test setup 15-
3 and 16-2, both of which had T4 inputs of zero.
The author postulates that the vertical component of the load lifter strap was given
too much weight in the equilibrium expressions. The current model suggested that the
entire vertical component of the load lifter strap, the cosine of the strap tensions (T,),
acted on the pack-body by pulling the pack down. This condition would suggest that, as
the load lifter strap was tightened, the pack would effectively have a greater downward
force and thus lessen the vertical lift created by the increase in lower shoulder strap
tensions (TI). Practically, however, a wearer of the pack would not feel an increase in
"weight" of the pack when the load lifter straps were tightened. m i l e it was clear that
load lifter strap tensions (T4) were an important element of the load carriage system that
exerted some force on the pack and user, it was not entirely understwd how this
occurred.
Future work is needed to fùrther explore how the load lifter straps transfer forces
between the pack, trunk, and shoulders. One possibility is to assume that the load lifter
straps contributed only anterior-posterior force to the system and not downward force.
The downward forces that would be generated by the load lifter strap geometry may not
be transferred to the pack because the shoulders are unable to transfer the reaction force
to the load lifter straps as the shoulders do to the upper and lower shoulder straps. The
upper and lower shoulder straps wrap around the shoulders at approximately 180 degrees
and the reaction force of the shoulders can oppose the tension in the straps directly,
allowing the straps to act on the pack itself Whereas the load lifter straps wrap angle
was much smaller and the contact with the shoulder was almost entirely on the anterior
surface of the shoulder thus the reaction forces of the load lifter straps may not act in the
superior-inferior direction.
If a system of equations is "ill-conditioned" by an incorrectly represented
element, such as load lifter strap tension (T4), solutions for the other variables will be
incorrect also. The author feels that this is the case for equilibrium predictions of lumbar
reaction force (Fx) and waist belt - lumbar pad complex lifi force (Fz). Changing the
shoulder suspension model will have a significant effect on the remainder of the
equilibrium expressions. Once the representation of load lifter strap tension (TI) is
improved, the remainder of the equilibrium expression predictions will improve also.
Furthemore, because the shoulder reaction forces (sN, sNx, and sNz) and the force of
fiction (FR) were calculated directly fiom upper shoulder strap tension (TI),
improvements in the shoulder suspension model will improve predictions of the former
variables as well.
Comparison of the upper shoulder strap tensions on the tight and lefi-hand-sides
of the test pack indicated that the right and left upper shoulder strap tensions (Ti) were
not significantly different (p = 0.93). This indicated that the assumption of right-left
symmetry was accurate for strap tensions.
Regional Mode1 Predictions
The most notable outcome of the regional model predictions was the size of the
predictive ranges. Chapter 3 outlined the variable contribution of fiction throughout the
model and how these variations led to predictive ranges rather than predictive values.
Inspection of Tables 5-2 through 5- 1 1 illustrated that these ranges were quite large; on
average 152% of the minimum prediction and 56% of the maximum prediction.
Furthermore, large coefficients of fiction at the shoulder (b), the waist De), and lumbar
region (pL) the sudaces in question exacerbated this situation. Most imponantly, while
the regional model predictions encompassed most of the validation outputs, suggesting
the regional models were able to accuratel y predict pack-person interface variables, the
size of the ranges made this statement questionable.
A better understanding of key factors affecting the model could be gleaned by
eliminating the fictional contribution of the lumbar pad, waist belt, and shoulder. Since
the precision of the model was so pwr, it was dificult to access its accuracy. While
most predictions fell within the regional model ranges, the ranges resulting from
frictional components were too large to be accepted as an effective model. It is
recommended that future validations alter the test mannequin andlor test pack to ensure
that smaller ranges for the fictional forces are determined. One suggestion is that the
mannequin be covered with TenonTM sheeting, reducing the coefficients of friction and
thus the ranges of the frictional forces. While this does not provide much information
about the contribution of frictioii, it will help researchers better understand the pack-
person reaction forces.
Inspection of Tables 5-5 and 5-8 reveals that the regional model predictions of
lurnbar contact forces (Fs) were consistently high, while predictions of shoulder reaction
forces (sNX) were more accurate. Since lumbar reaction force (Fx) and shoulder reaction
forces (sNX) were in direct opposition and were modeled as counter forces to each other
they should be equivalent. However, consistently high lumbar reaction force (Fs) values
led the author to believe that, while it was not apparent during testing, the waist belt
contributed some anterior-posterior force to the pack. This would challenge the
assumption that the waist belt did not contribute to anterior-posterior forces.
Future work should attempt to either eli minate any anterior-posterior force
generated by the waist belt or include these forces into the regional model. The . attachment of the waist belt to the pack could be changed to one that allowed free
movement in the antenor-posterior plane. If the waist belt was comected to a rod that
was permitted to slide within a chamber attached to the pack-person and oriented in the
anterior-posterior plane, 2-axis, and Y-axis forces would still be transmitted and X-axis
forces would be eliminated. The other option would be to account for the anterior-
posterior force in the model by calculating it as twice the product of the belt tension and
the cosine of the transverse angle of the belt attachent point.
Despite the two factors noted above, the regional rnodel predictions encompassed
71.3% of the measured values. The most accurate predictions were of waist belt - lumbar pad complex lift force (Fz) whose predictions encompassed 10W of the
measured values while the lumbar reaction force (Fx) predictions were the least accurate,
only encompassing 35.4% of the measured values. The relatively low accuracy of the
latter was best explained by the fact that the waist belt generated some anterior-posterior
force. Other poor predictions included shoulder reaction forces (sN) and lumbar lift
forces ( F ~ ~ ) . Because net shoulder reaction forces (sN) were calculated fiom the root of
the sum of the squares of horizontal (sNx) and vertical (sNz) shoulder reaction forces, the
inherent error in the later two cornbineci to increase the enor in the former. Similarly,
lumbar lift forces ( F ~ ~ ) were derived from lumbar reaction force (Fx) predictions, which
as noted above, were relatively poor.
Sensi tivity Analysis
The sensitivity of upper shoulder strap tensions (Ti) suggested that the
mathematical relationships generated by the equilibrium expressions were "ill-
conditioned" toward upper shoulder strap tensions (Ti). Because the mathematical
relationship disproportionately weighted upper shoulder strap tensions (TI), the physical
system could not be well represented without an improved method of assessrnent for
upper shoulder strap tensions (Ti). Small errors in input values significantly altered the
output value of upper shoulder strap tensions (TI) thus leading to poor predictability.
Alternatively, insensitivity of the lumbar contact forces and waist belt - lumbar pad
complex lie forces (Fs and Fz) led to similar problems for opposite reasons. To register
change in lumbar reaction forces (Fs) or waist belt - lumbar pad complex lifi forces (Fz),
the input values must be changed drastically. It would seem that lumbar reaction forces
(Fx) and waist belt - lumbar pad complex lia forces (Fz) were more responsive to load
factors than pack geometry. The physicai system required that model predictions be
more accurately measured because of the high sensitivity of some model outputs and
relatively low sensitivity of others.
The sensitivity analysis provided support for logical model relationships:
Geometric outputs were sensitive to changes in geometric inputs. Upper shoulder swap
angles (el), lower shoulder strap angles (&), and load lifter strap angles (e4) were al1
sensitive to changes in: the lumbar contact point to shoulder center (d5), pack body to
shoulder center (&), shoulder radius (r), lumbar center to load lifter strap attachment
points (&), and lumbar center to upper shoulder strap attachment points (di) dimensions.
The shoulder wrap angles (al and a4) were also sensitive to changes in similar inputs.
Considering that upper shoulder strap tensions (Tl) were related to lower shoulder strap
tensions (TI) by the modified pulley equation, it was not surprising that upper shoulder
strap tensions (Ti) were sensitive to changes in lower shoulder strap tensions (4). The
same relationship dictated that upper shoulder strap tensions (Ti) and frictional forces
around the shoulder (FR) were sensitive to changes in the coefficient of friction around
the shoulder ($1. It was also logical that shoulder strap tensions (Ti, Tt, and T4) had
significant effects on the shoulder reaction forces (sN, sNX and sNZ). Because forces of
fiction around the shoulder (FR) were derived fiom the difference between the lower
shoulder strap tensions (TI) and the combination of the upper shoulder strap tensions (Tl)
and the load lifter strap tensions (TJ), it was not surprising that the forces of friction
around the shoulder (Fa) were sensitive to shoulder strap tensions. The lifl forces of the
waist belt (F~z) and lumbar pad ( F ~ z ) were sensitive to changes in anatomical angles (ye
and yL) or the hip and lumbar region. Waist belt forces, such as the compressive forces
(Tx and TX~) , were sensitive to increases in waist belt tension (T3) And finally, the lie forces due to fiction at the waist (Tm) and lumbar regions (Fsr) were sensitive to
changes in the coefficients of fiction in these regions (pe and pt).
Some input-output relationships were less obvious. Increasing the rnass of the
pack (W) while not increasing the shoulder strap tensions (Ti, T2, and T4), which
occuned in the sensitivity analysiq required that the extra mass be bom elsewhere.
Therefore, waist belt - lurnbar pad complex lie forces (Fz), which was essentially the
difference between the vertical shoulder suspension forces (sNX) and the mass of the pack N N (W), was sensitive to changes in pack mass. Shoulder reaction forces (S , S X, and sNz)
and lumbar contact forces (Fx) were highly sensitive to changes in the lumbar contact to
shoulder center (d5) and pack body to shoulder center (&) dimensions. More accurately, N N however, the shoulder reaction forces (S . S X, and sNz) were sensitive to the geometric
outputs, which were in tum sensitive to the geometric inputs. Changes in the angle of
pull of the shoulder straps (01, O*, and 04) would certainly e f f ~ t the distribution of force
about the shoulder. As well, because the lumbar contact forces (Fx) counteracted the
shoulder reaction forces (sNx) in the anterior-posterior plane, the lum bar contact forces
(Fx) were highly sensitive to shoulder reaction forces (sNX). Altering the tensions in the
lower shoulder strap (T2) also created the same chain reaction effect of sensitivity. The
lumbar contact forces (Fx and Fz) were highly sensitive to lower shoulder strap tensions
(Ta) because lower shoulder strap tension (T2) significantly effected the shoulder reaction
forces (sN, sNx, and s'z), which in tum effected the lumbar contact forces (FA and Fz).
The shoulder Contact forces (sN, sNy, and sN2) and lumbar contact forces (Fx and Fz)
were also sensitive to variations in the lumbar centre to upper shoulder straps (di) and
lumbar centre to load Mer stnps (6) dimensions as a result of a similar chain reaction.
Methods and Materials In validating the system, it is important to have an accurate benchmark pack as a
reliable measurement tooi. The test pack was a simplified extemal frame system with
most features of the pack-person interface sirnilar to a standard pack. It is clear that the
pack was representative of a stiff, extemal frame style pack. The results of the study
cannot currently be extended to sofier interna1 frame style packs. Further investigation
into how the mode1 represents these packs would be necessary. The main body of the test
pack was relatively rigid. However, as mentioned earlier, the vertical stays chat served as
the load lifter anachment points were much less rigid than the remainder of the pack.
While this did not effect the model's ability to predict geometry, it may have had some
effect on the force transmitted between the load lifter straps and the pack itself Under
normal static conditions a more compliant connection would not Iead to lower force
transmission. However, the relative sensitivity of the system to extraneous factors
suggested that alteration in any element of the pack-person interface test setup may have
had significant effects on output masures. Furthemore, a dynamic mode1 has been
suggested as a future goal of the research team thus making test pack cornpliance a much
more critical variable.
It is recommended that the vertical stays on the test pack be changed. Altering the
stays so that a uniform stifiess is achieved throughout the pack will reduce the
extraneous factors that may tend to effect measurements. Specifically, because the effect
of varied cornpliance is not well understood, future work should strive to eliminate such
concems. One recommendation is to mn aluminum stays along the entire height of the
pack so that al1 shoulder straps are attached to the pack through the same medium.
Another concem with respect to pack stiffness was the assumption that the waist
belt transmitted forces to the pack body by a pin joint. While this assumption was made
it was known that the connection was not actually a pin joint and a small error was
expected. However, since it has become apparent that the pack-person interface outputs
are quite sensitive to extraneous factors this assumption has become suspect. It is not
ent ire1 y understood what effects t his assumpt ion may have had on the pack-person
interface measures. However, it is believed that the assumption may have had more of an
effect than previously anticipated and moments at the waist k l t may have been
transfened to the pack. The actual rotational equilibrium of the pack would be affected
and thus the test pack may not be indicative of a standard pack in this respect.
Future endeavors should strive to consolidate the pin joint concem. Either by
gaining an understanding of how the stifhess of the waist belt effects the system or
changing the connection of the waist belt to the pack to an actual pin, this concern can be
eliminated. The most effective solution may be to reattach the waist belt to the pack with
a simple bal1 and socket style connection. This would ensure that a pin joint connection
exists and no assumptions need be made.
As was mentioned in Chapter 3, the order of tightening pack straps affected the
direction of the force of friction around the shoulder (FR). It was also discovered during
the initial stages of a broader military pack investigation that the method of donning and
dofing the pack had a significant effect on pack-person interface variables. For example,
doming the pack by tightening the shoulder straps and then the waist belt would result in
a different set of measured outputs than donning the pack by tightening the waist belt and
then the shoulder straps. The doming factor was accounted for during the validation
study by employing the same doming technique throughout the study (taken fiom
protocol employed by experience pack users). However, this result indicates the
sensitivity of the system to extraneous factors and suggests that Gare must be taken during
ftture validation studies or interpretation of results.
As was discussed above, the method used to don the pack significantly affected
the outcome measures. Whiie the same method could be used consistently throughout
future validation studies it may be more important to gain an understanding of how
donning methods affect outcornes. The validation protocol could be repeated using the
various donning procedures outlined in the literature. The outcome measures could then
be compared using a paired analysis and thus gain an understanding of the effects of
doming techniques. Not only can this information be used to better validate the
biomechanical model, but it rnay also provide more insight into the pack-person interface.
The assumption that contact between the pack body and the users' back only
occurs in the lumbar region rnay not be entirely accurate. During two test setups (3-2 and
7-3), the pack-body came into contact with the upper back. The author believes that the
contact was the result of extreme shoulder strap tensions. These two test condition setups
(Table 4-2) required strap tensions that were higher than the predicted equilibrium could
provide. The extra shoulder force pulled the pack against the upper back of the test
mannequin creating another counter force to the shoulder force. The sum of the lumbar
contact forces (Fx) and the new upper back contact force counteracted the excessive
anterior-posterior tensions of the shoulder straps (TI, TI, and T4), thereby permitting the
required tensions.
Previous literature reviews and investigations at Queen's University suggested
that experienced users oflen tighten the shoulder straps beyond the minimum necessary
tension (Pelot et al., 1995). This tension draws the pack into contact with not only the
lumbar region, but also the upper back as well. It is not surprising that this occurred
during two test setups. Therefore, it is suggested that future work attempts to understand
what conditions produce contact with the upper back and how this contact affects
equilibrium conditions. With intemal fkame packs, this issue would be exacerbated.
Some concem existed with the strap tension probe. The force transducer of the
strap tension probe was louited between the pivot and the handles. In this configuration,
the strap tension probe was sensitive to the location of the force applied to the handles. If
the force wes not applied directly over the stop rod, a bending moment was generated
about the stop rod thus altering the output of the force transducer. In this controlled
laboratory setting, a grip that concentrateci the force directly over the stop rod was
employed and was considered reliable and repeatable. However, for future field studies,
the probe should be modified so variable grips on the pliers do not affect the probe's
readings. It is suggested that the force transducer be move to the shaft of the pliers
between the pivot and the strap prongs and the pliers be re-calibrated and re-validated.
With the transducer located between the pivot and prongs no extemal bending moment
would be applied. This would ensure that any gripping technique employed would
provide accurate outputs.
Chapter 7
Future Directions and Conclusions
Future Directions Many concems were expressed and recommendations made about the model and
testing materials in the previous chapter. The concems specitic to this wotk should be
examined and decisions made to consolidate this model before significant model
applications or funher model advances are attempted. Specifically, the following
concerns and recommendations should ail be investigated fbrther:
The stiffness of the vertical stays for anachment of the load lifter straps to the pack
should be altered so that the connection of al1 the shwlder straps provide consistent
cornpliance;
Re-evaluate certain elements of the model, specifically the way in which the load
lifter strap tensions (T4) transfers forces between the shoulder and the pack body. In
fact, the waist-lumbar region should be evaluated in isolation fiom the shoulder
model. Once both elements are accurately represented independently, a combined
comprehensive model, such as the curent attempt can be realized.
The size of the predictive ranges of the regional models should be reduced and the
mode1 revalidated to better understand the pack-person reaction forces;
Eliminate the antenor-posterior force generated by the waist belt on the test pack or
include this force in the model;
Re-evaluate contact between the pack and the user's (or test mannequin's) upper
back;
Detennine the efTects of different donning methods for the test pack;
Improve the design of the strap tension probe so that it may be used in various
situations and be considered more reliable;
Improve the swap tension probe to be insensitive to gripping techniques.
Once this work is improved through consolidation of the model and associateci
validation, researchers would be better able to undertake the following two related
94
investigations: further development of the current optimization routine and development
of a dynamic personal load carriage system biomechanical model.
Previous research (Pelot et ai., 1998) outlined an optimization routine that used a
biomechanical model as its basis. The routine was designed to optimize the pack
geometry, pack materials, kit selection, and kit placement in a pack to minimize negative
effects in the pack-person interface. Currently, the optimization routine uses previous
phases of this model as a base. Once this phase of the model is improved and
revalidated, it can be incorporated in an attempt to funher the optimization routine. The
routine can only be advanced as the biomechanical mode1 is advanced.
The ultimate goal of any model is to provide a perfect representation of a system.
Since the personal load carriage system model represents a dynamic situation, eventually
a dynamic load carriage model needs to be developed. Furthemore, because this phase
of the model has begun to deal with dynamic elements, such as pack stiffness, it seems
that the next phase should attempt such an undertaking. The author suggests that the
biomechanical model be investigated in a cyclical motion, such as the vertical oscillations
associated with gait. Under these conditions, peak forces, moments and pressures could
be studied and the stiffness of certain pack elements evaluated.
Conclusions The main objective of this work was to develop and validate the next phase in a
series of pack-person interface biornechanical models. It was also expected that four sub-
objectives would also be met:
1. The biomechanical model would be the basis for the persona1 load camage system
design tool outlined in Chapter 1;
2. The equilibrium expressions and the regional models contained within the personal
load camage system model would provide researchers and designers alike with more
information about the pack-person interface;
3. The process of developing and validating the biomechanical model would provide
more insight into the pack-person interface;
4. And the load cadage system measumnent tools would be improved through the
addition of a strap tension probe and test pack.
The current phase of the biomechanicai model is not as representative of the
physical pack-person interface as the author had hypothesized. The equilibrium
predictions of the pack-person interface variables were quite poor. A number of
approaches should be taken to improve this phase of the model before it is considered
accurate and appropriate for xientific investigations of design work. The regional
models did a reasonable job at predicting the range in which the physical variables will
reside. However, the ranges were too large to be conclusive about iheir accuracy. The
author also believes that the current representation of the load IiAer strap is quite poor
and it had major impact on many other output variables in the model.
Despite the poor ability of the current model to predict pack-person contact
forces, the model proved to be an excellent predictor of pack geometry. The relative
angles of the three shoulder draps to the pack and the wrap angles of these straps were al1
accurately predicted. This aspect of the rnodel can be used in future investigations.
The biomechanical model could not considered a scientifically valid
representation of the physical pack-person system and should not yet be used to conduct
pack performance evaluations of current or prototype pack designs. However, despite the
fact that the model did not stand up to a rigorous scientific validation, many elements of
the model can provide qualitative information about pack-person interface variables.
Designers may be able to gain some insight into trends or descriptive relationships
between variables, helping the design process.
Development and validation of the current biomechanical model has improved the
understanding of the pack-person interface. Results of this study has illustrated the
following points:
The method of donning a pack can have a significant effect on the pack-person
interface variables;
The direction of the force of fiction is determined by the order in which the shoulder
straps are tightened;
The cornpliance of the pack may have a significant effect on pack-person contact
forces;
The sensitivity analysis revealed that the pack-person intertace might be quite
sensitive to design features;
5. The sensitivity analysis also illustrated that changes in most model outputs were the
logical result of changes to model inputs, illustrating some legitimacy within the
current biomechanical model.
6. Friction within the pack-person interface is a very important characteristic that must
be fiilly understood for future designs.
It is interesting to note that the complex pack-person interface may not be best
represented with an equally complex model, since previous versions of the model seemed
to better represent the system than the current model.
The strap tension probe and the validation test pack make excellent additions to
the Queen's Ergonomics Research Grou p' s battery of persona1 load carriage system
measurement tools. Validation of these tools illustrated that the devices were an effective
means of measuring strap tensions and numerous pack-person contact forces. The
current validation test pack could be used to conduct future pack-person interface
analyses. However, if the stiffness of the pack is required to be uniform, the vertical
stays that serve as the attachment point for the load lifter straps must be improved. Also,
if the waist belt connection is to be assumed a pin joint, the rigidity of that connection
must be reduced. The strap tension probe also requires some fiirther investigation before
it can be used in fùture analyses. Despite the fact that the probe was considered valid for
this study, it should be modified to be less sensitive to grip location and thus be more
versatile for field use.
In general the author feels that this work has contnbuted to the larger military
load carriage project. While the current model cannot be used as a robust scientific tool,
much information has be learned and added to the load carriage system knowledge base.
Furthermore, this work provides an excellent beginning for improvements to the current
model and the next iteration of a dynamic biomechanical model. Although the model
cannot be used as the basis for a valid design tool, qualitative information can be gleaned
€rom the model for design purposes. In addition, two new measurement tools have been
added to the load carriage system measurement battery and can be used for both fùture
validation studies as well as investigations in a larger scientific scope. Finally, the tiiture
directions outlined above detail the strategy by which the current model and measurement
tools can be used as a stepping stone to tiiture scientific work.
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Appendix 1
Biomechanical Model Notation
Notation . . .. ... .. .. .. ...... .... .... ..... . ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Notation
Following are detaiIed descriptions of al1 the parameters illustrated by the general
pack mode1 and associated regional models (shoulder, waist, and lumbar region).
Orientation:
X coordinate along pack depth
Z coordinate along pack height
Pack Container:
W the force of the mass of the pack (input)
vx horizontal position of the centre of mass (input)
vz vertical position of the centre of mass (input)
hx horizontal dimensions of the pack container (input)
hz vertical dimensions of the pack container (input)
Bearer:
d3 distance from bottom of pack to lumbar pad contact centre (input)
d5 distance from lumbar pad contact centre to shoulder centre (input)
d6 distance fiom pack to centre of shoulder (input)
r average radius of shoulder (input)
r~ average radius of hips (input)
P body lean angle (input)
y~ anatomical lower back angle fiom vertical (input)
YB anatomical hip angle from vertical (input)
Shmdder strqs:
TI tension in upper shoulder straps (LHS and RHS summed) (output)
T2 tension in lower shoulder straps (LHS and RHS summed) (input)
T4 tension in load lifter straps (LHS and RHS summed) (input)
di distance fiom lumbar pad centre to attachment ofupper shoulder strap (input)
d2 distance from lumbar pad centre to attachment of lower shoulder strap (input)
4 distance fiom lumbar pad centre to attachment point of load lifter straps (input)
81 upper shoulder strap angle fiom the vector normal to the pack (output)
& lower shoulder strap angle Grom the vector normal to the pack (output)
8 4 load lifiet strap angle C.C.W. fiom the vector n o d to the pack (output)
al upper shoulder strap wrap angle around the shoulder (output)
a4 load lifter strap wrap angle around the shoulder (output)
4 angle at which sN acts from pack normal (output)
PS coefficient of fiiction of strap on shoulder (input)
sN net force of shoulder straps acting though the centre of the shoulder (output)
sNx X-component of sN (output)
sNz 2-component of sN (output)
Fm force of fnction around the shoulder (output)
Waist belt:
T3 tension in waist belt (input)
d3 distance to lumbar pack centre tiom bottom of pack (input)
TX compressive force that T3 applies around the hips (output)
T X ~ normal force component of Tu: (output)
T x ~ the force of friction due to T ~ C * (output)
~~z lifl provided by the waist belt resting on hips (output)
coefficient of fiction of waist belt on hips (input)
Ltcmbar region:
Fx reaction force of lower back on pack in X-direction (output)
F X ~ the component of Fx normal to the lower back (output)
Fxr the force of friction due to Fx (output)
FLz lift on the pack fiom fiiction and angle at lower back (output)
PL coefficient of tnction of lumbar pad on lower back (input)
Fz total l i e force at lumbar contact point of pack (output)
Appendix 2
Summary Data Sets
....................................................................................... Geornetric measurements 1 07
......... Geometric mode1 predictions and validation measurements: Group averages 1 10
....................................................... Mode1 predictions by equilibrium expressions 1 11
Regional model predictions ................................................................................... 1 12 . ....................................................................................... Validation measurements 1 14
............................................................ Validation measurements: Group averages 1 17
..................................................................... Sample data set of model predictions 1 18
Minhum Mpdmum MWmun Markmim Minhum Mixlmum Minimum W m u m Minimum 27.10 37.24 1934 23.1 7
13.03
31.6s
20.14 46.50
27.51
40.60
21 .?O
23.03
4-43
21 .O4
13.05
27.62 15.29
4.43
1 7.35
22.28
49.65
32.23
2 7.29 33.50
12.74
40.06
26.m 20.86 25.S 34.87
16.07
35.66 18.1 7
31.0t
26.20 39.8s 46.06 38.06
nat 22.44 17.57
13.66
20.1 1
40.22
29.72 18.73
n.n
X s*, f, Pt Pz Modmum Mlnimum Msximum Minimum Mprknum Mnknum W m w n Minimum Msximurn
irwu of pck input, Symbd M a Unib W 33.00 kg 1 up9ll8h. Stnp q k from n m .
V W t l c r l p o J t k n o f ~ o f m r u h o i f i o r i t o l ~ o f ~ o f m r u wrtkrl dimemion of pack horimtal dl- of prck lumbr contrct to di. C m pack to rti. Center dius of rh0Uw.r fadiw of h i p body ban angb low b c k angk hip angle bwer stl. Stmp tsmkn laid lifter tendon lumbar Mer to upper sh. Stmp lumtmr Wdl to kwr sh. Strsp lumôar Ebntw to kad iiitff drap eoelflcknt of friction iround 8h. Waia belt tam&n lumbar psd mter to bottom of prck coefficht of fricth of wrht bsn coeflicknt of frktbn of lumôar pnâ
force of fricth wwnd diouidar
c o m p r ~ v e r01C-e of WaM belt normal fana componsnt of T3C force of friction due to f X
Fx 115.09 N Fz 236.63 N TI 26.11 N lift on pack from lumbar cornpiex
Statk Equilikium Output8 [~meripcim ~yrnimi ~ a t a Units
force of friction due Co fXN
-ci0 min )LS
~ ) i ô
min Ps Ps
min Cis lfux Ci6 min )rs - PB min PB
lfuxcie min W c i e min )y
m)ie
min )is
WIis min Ps
Cie min )is mmt Cis min Ci8
m)is
min Ps Ci6
min
-)is
min r s
min Mi -Cis -)ib
M C i e fnin P?l
inhl Pa
Appendix 3
Cornpiete Model Sensitivity Analysis Results
Unique variable outputs as input variables increase by 1 û% ................................... 121
N o m l i z e d increase in output variables as input variables increase by 10% .......... 123
Appendix 4
Mode1 Validation Protocol
Mode1 validation protocol ... .. . . .. .... .. . . . . ... ... . ... . .. ... . ......... ........ .. . ... . . . .. ... .... .. . . . .. .. . . . .-. 126
Mode1 Validation Protocol
refer to Table 4-2 and setup the test pack according to the designated test condition
(strap position and pack mass)
using the reaction board method, measure and record the centre of gravity of the test
pack in the current setup
position test mannequin on force platform and adjust forward lean to required body
lean angle
zero the force platform and the test mannequin load ceIl
mount the test pack on the mannequin such that the centre of the lumbar pad contacts
the mannequin at the centre of the lumbar region (marked with an asterisk)
tighten the waist belt
feed the shoulder straps over the shoulder and tighten the lower shoulder swap
ensuring that the straps contact the shoulder along the path marked with black lines
tighten the load lifter strap
using the strap tension probe and a zeroed digital volt meter, measure the change in
voltage for the three closure positions (noted in Chapter 2)
10. using a least squares method (typical spreadsheet fùnction) and the coefficients
provided in Chapter 2, determine the strap tension for the waist belt and the right and
lefi lower shoulder strap and load lifter strap
1 1. adjust the pack straps in order: waist belt, lower shoulder strap, and load lifter strap
12. repeat strap tension measurement and strap adjustment until the input strap tensions
are within 1 N of the input tensions noted in Table 4-2
13. using a protractor measure the angle between the test pack main board fiame and the
right and left lower shoulder straps, upper shoulder straps and the load lifter straps
14. feed a sheet of paper between the lower shoulder strap and the test mannequin and
mark the point of resistance of the paper (the point at which the strap contacts the
mannequin)
15. repeat between the upper shoulder swap and the mannequin and the upper shoulder
strap and the load lifter strap
16. using a flexible spline rule, measure the wrap angle of the upper shoulder straps and
the load lifter straps on both the right and left sides
17. record the force data from the force platform
18. record the force data from the test mannequin load ceIl
19. record the force data tom the test pack load ceIl
20. using the strap tension probe measure and record the strap tension of the upper
shoulder strap
21. remove the test pack and randomly select a new test condition fiom Table 4-2
22. repeat data collection