The respective first %-value represents the proportion of surveyed employees (n =
20,000), which are often affected by the particular work conditions. The respective
second %-value represents the proportion of surveyed employees, who feel stressed
by the particular work conditions.
One of the main tasks of preventive health protection is the avoidance of back pain
and injuries that can result from lifting of loads without any aids. According to a report
by the ‘Bundesanstalt für Arbeitsschutz‘ und Arbeitsmedizin (BAuA), in 2006 13,3% of
all accepted occupational illnesses were traced back to intervertebral discs problems.
A health risk is particularly common for energetic-effective work forms, especially for
ones in which the handling of loads occurs. The EU Directive 90/269/EWG demands
“preventive measures for the avoidance of risk due to handling of loads, and that
workplaces dealing with load handling evaluate the risks for their employees”.
In ergonomics the ideal-typical extreme forms of human work are referred to as
informational and physical (energetic) work in form of pure information
transformation or else energy transformation. The five specific forms of work
(creative, combinative, reactive, sensorimotor and mechanical) are mixtures of the
two basic forms.
Based on the fact that the work environment indicators of practical working processes
never occur in isolation but in combinations, impact assessments are only permissible
for the entirety of all acting environmental factors in relation with the work specific
stress factors. However, these causal mechanisms haven‘t been investigated
sufficiently until now, so that the examination for each stress factor still takes place
separately. The next step is to identify the specific effect / impact of the work
environmental factors on defined organismic systems. If the same organismic system
is used more than once, possible bottlenecks must be analyzed.
This method proved its effectiveness in the presence of stressful climatic factors
associated with a high energy load of the human, so-called heat work, for instance.
Both stress factors lead to a higher utilization of the cardiovascular system, which in
this case is considered as a system bottleneck. With energy-effector forms of work the
muscles and the cardiovascular system are mainly stressed. In terms of a bottleneck
analysis, different forms of work are differentiated:
• Heavy dynamic work
• One side dynamic work
• Universal dynamic work
• Static work
The energetic aspect of work activities is usually in the mobilization of skeletal
muscles. The work possibilities of a muscle can be distinguished according to two
basic forms: static and dynamic muscular work. Dynamic work is the execution of
movements and is characterized by: (1) change of muscle contraction and relaxation
(recovery), (2) response of the muscle blood flow, (3) adequate oxygen and nutrition
supply for the muscle, and (4) possible activity over longer periods.
Activities with dynamic components are, for example, cycling or activities with
movements that use different portions of the muscle. Static work occurs when objects
are held by muscular action forces, frictional resistance is overcome or when the body
is braced against gravity in particular positions or postures. It is characterized by: (1)
continued contraction of muscles over longer periods, (2) a mismatch between
oxygen demand and oxygen supply to the muscle, because the blood vessels
responsible for the muscle are compressed during the contraction, (3) rapid fatigue
and thus, a continuation of activities over extended periods is impossible, (4) adverse
biomechanical loading conditions of bones, joints and ligaments, resulting in
premature wear, especially of the spine.
The figure shows a mechanical analogous model of a muscle. The length of the
contractile element is denoted with k, while the length of the elastic element is labeled
as s. Using this labels, the lengths of the respective forms of contraction (isometric,
isotonic, and auxotonic) can be compared with and zoned from the non-operating
state. An isometric muscle contraction is present, when a muscle performs solely an
increase in force while the length of the muscles stays thereby constant. As the
distance traveled equals zero, no work is done in a physical sense. The isometric
muscle contraction can therefore merely be regarded as work in the physiological
sense (work = force ∙ duration). If a muscle is contracting isometrically, static work is
present. Static work is required for human body posture, on the other hand for holding
items and tools. Static work should be avoided as it is the unfavorable form of
mechanical work. According to DIN EN 1005-4 it involves high health risks. The
isotonic muscle contraction is marked by a shortening of the muscle length without a
change in force. In doing so, work in a physical sense is accomplished (work= force ∙
displacement). Is the muscle contracting isotonic, one speaks about dynamic work.
Dynamic work is needed when lifting a load, for instance. Dynamic work should be
targeted to lie within a low-frequency range (< 2 load cycles per minute), as this,
according to DIN EN 1005-4, promotes the lowest health risk. The auxotonic
contraction is a combination of isometric and isotonic contraction. Thus, muscle
length and force do change together.
While resting, both demand for blood and blood flow are at a constant low level and
therefore an equilibrium state exists. During heavy dynamic work, however, the
demand for blood and blood flow are at a maximum to provide the muscles
continuously with oxygen, so that glucose can be converted into energy (aerobic
energy). However the muscular system is also in an equilibrium state. As well during
static work the demand for blood and blood flow is higher than in rest, because the
blood circulation is significantly reduced. To avoid a performance hit caused by this
non-equilibrium state, the body adapt to anaerobic energy generation. Here glucose is
converted under the formation of lactate (lactic acid), leading to acidosis of the muscle
on a permanent basis.
To maintain the body position when in an inactive body state (that is without
generation of physical work) muscles need to be tensed. This static muscle work is
energy-related especially inefficient, which is due to the lack of movement. In a non-
moving state the muscle circulation results in a much faster muscle weariness, which
in turn leads to an enhanced circulatory-activity. The higher the holding-force, the
lower the holding period. If not more than 15% of the maximum force is expended for
the holding force, then there is no considerable fatigue for the observed time scale. In
dependency of how high the work load is (that is the coverage rate of the maximum
force), the maximum force that is still available after a certain work duration declines
continuously. For example: If 25% of the maximal force is statically demanded, the
force can, due to the fast occurring muscle fatigue, only be maintained for 4 minutes;
for 50% of the maximum force it is only 1 minute.
Muscle force is a physical strength that works through the activity of the muscles
within the body. As previously explained there is a difference between static and
dynamic muscle force. Static muscle force is the physical strength that occurs without
a change in the length of the muscle during its activity (isometric contraction).
Dynamic muscle force, however, occurs during the change in length of the muscle
during its activity (isotonic contraction).
Inertia force is a physical quantity that acts through the moment of inertia, e.g.
dynamically as accelerating force, force of deceleration, or centrifugal force at mobile
workplaces, or statically as own weight.
Action force is a physical strength that works outward from the body. It results from
inertia force, muscle force, or both. Inertia force and muscle force can reduce or
increase their strength depending on amount and direction.
From the force-releasing body parts the action force is split into e.g. arm, hand, leg or
finger force; from the force direction the action force is split into e.g. vertical or
horizontal force.
With relation to the direction of force, the action force can be differentiated into the
force of attraction and the force of pressure from the sense of direction of force.
Referring to the figure it is important to differentiate between rather basic-oriented
classification schemes of muscle mass and strength (acting in the body system) and
more practical classifications in terms of generated action forces (working from the
body to the outside). The interrelations are important for the work design.
Examples are:
The own weights of the body parts (inertia forces) are compensated by static
muscle forces for maintaining a body posture.
Action forces on body support areas can be composed of gravitational forces of
the body parts and posture forces. This is to be considered e.g. in dimensioning
of the restoring force of a pedal.
Muscle contraction forces are the partial or full cause of driving forces (e.g.
lifting loads).
Muscle extension forces are the partial or full cause of braking forces (e.g. take
down of loads).
Manipulation forces and actuation forces can be applied partially or completely
by the combination of contraction and extension muscle forces (separate
muscle groups) (for example, relocating loads).
The specifications in the above diagram apply to an upright standing body posture
with parallel foot position at a foot distance of 30 cm. The indicated values of the
maximum static action forces were determined at stationary arranged handles during
short-time maximum force exertion by the working person. A cylindrical handle with a
diameter of 30 mm was used, which was used without supporting tools. Shown are
averages of the maximum achievable static action forces, that are valid for specific
collectives, e.g. men aged 20 to 25 years, and therefore are not representative for the
total population. The maximum forces are represented in the form of “isodynes”. For
different working conditions (e.g. concerning posture or required force direction), the
transferability of the data has to be checked, that is contours of equal maximum
exerted action force. For example, in DIN 33411-3 and DIN 33411-5 maximum static
action forces for other working conditions were presented.
The illustrated isodynes (contours of equal maximum exerted action force) apply to
males with an average age of 22.8 ± 2.2 years, an average body height of 176.8 ± 5.9
cm, and an average body weight of 72.73 ± 12.47 kg.
The illustrated isodynes also apply to males with an average age of 22.8 ± 2.2 years,
an average body height of 176.8 ± 5.9 cm, and an average body weight of 72.73 ±
12.47 kg.
The figures show the behavior of the heart rate during and after work with shorter and
longer breaks and with steady proportion between work phase and break. The work
process is almost purely energetical and consists of cycling on a bicycle ergometer.
The average power generated through cycling is 200 [W]. Because of the exponential
character of the exhaustion and recreation phases it is not functional to work until the
occurrence of exhaustion. There is a need for disproportionately long recreation
phases. It is physiologically more favorable to arrange short cyclical work and
recreation phases. Human performance is limited in time due to limited energy
sources. The efficient execution of the work, while maintaining full performance
therefore depends critically on an appropriate distribution of the physical workload. In
selecting rest breaks, it should be taken into account that the onset of fatigue requires
disproportionately long recovery periods. The required recovery phase increases
disproportionately with increasing work phase durations, where the recovery from
static work requires a longer period than from dynamic work. A quick-alternating work
and rest regime is therefore more favorable for physiological and economic reasons.
An example for the choice of the working process with the highest efficiency is the
loading of an industrial furnace. During the loading of an industrial furnace, the human
energy demand can be lowered by reducing the lifting height. Regarding human work,
a positively directed work has to be performed while lifting and a negatively directed
work while lowering the workpiece. A reduction of the lifting height comes along with a
decrement of the metabolic rate and therefore with a lower strain of the cardiac
circulatory system. Simultaneously the shorter movement lengths lead to a
considerable increase in efficiency as well as to a higher effectiveness when
activating leg- and trunk musculature.
External loads are not the only ones considered when evaluating job performance,
but also the entire movement loads and the mass of the involved body parts. This
especially applies to movements that include large body parts and small loads.
The figure shows the dependency of the average bending force on the angle of the
cubital joint. The maximum of the bending force is reached when the cubital joint is at
an angle of about 100 degrees.
The diagram above shows the relationship between contraction force and velocity of
contraction with the thereout calculated power. The maximum power of about 135
Watt is achieved with a generated force of only 45 Newton. The velocity of contraction
then amounts to approx. 2.9 m/s.
When considering the dynamic work forms of the muscle, not only does the
unavoidable moment of inertia play a role, but also the gliding of the muscle’s actin-
and myosin-filaments. As a velocity dependent part of the total force occurs
(analogous to an internal friction), the maximum force decreases with increasing
velocity of contraction of the muscle length (so-called Hill’s force-velocity relation).
Avoiding activities that impose repeated static load on the internal structure leads to a
considerable relief for the organism. One possibility is to replace static work with
dynamic work (e.g. moving a lever instead of pushing a device for fixing a work
object), another way is to install appropriate retaining devices (e.g. weight reducing
suspension of tools).
The diagram shows the stress related to the three design alterations in the static
case. Dynamic forces (e.g. acceleration forces, as they arise, when pulling tools)
should be considered, when analyzing workplaces and for the installation of
suspension.
The human body has over 600 muscles available. The muscle mass constitutes
approx. 40% of men’s body mass, while for women it is 26%. The face alone has
more than 43 muscles. A health hazard for energetic-effectoric work forms emerges
especially when handling loads. Here, especially the spine is mechanically
vulnerable, which is shown schematically on the right hand side in the figure. At this,
the lumbar spine, the thoracic spine and the cervical spine can be differentiated. As
for the consequences of possible impairment, in 1993 the list of occupational disease
was extended with spinal disc related ailments of the lumbar spine through carrying
and lifting heavy loads (BK 2018) and also with longstanding carrying of loads on the
shoulders.
For the assessment of force proportions of the spine, mainly specific biomechanical
models (e.g. “Der Dortmunder”) are used. The inspection of the validity of such
models (especially regarding non-linearities, idealizations, coefficients standards and
so on) as well as the establishment of the load-carrying capacity limit of the spine is
only possible empirically. Because of the difference in load, the consideration of every
single vortex shows a complex picture.
The state of the spine can be evaluated very accurately by a computer tomographic
analysis, yet by this means only ex-post insights about the effect of past strains can
be determined. A spine overload only becomes noticeable after damage of the spine.
It is therefore necessary to estimate the risks of spine impairments preventively.
The spine consists of 24 osseous vortices, between which cartilaginous intervertebral
discs are situated. The intervertebral discs impart their agility and elasticity to the
spine. The nutrition of the rubbery invertebral discs is entirely dependent on diffusion
because there is no blood flow. Sustained compressive loading reduces the pressure-
dependent fluid shift and can lead to a metabolic impairment in the intervertebral
discs. When handling loads, the spine is heavily stressed because of the leverage
effect of the external load and the resulting internal forces. Dependent on the position
of the held load and the diffraction of the back the elastic intervertebral discs are
exposed to enormous compressive stress and are strained by internal transverse
forces. Health risks include damage of intervertebral discs, deforming of vortices or
ruptures of muscular fibers can result. Damages to intervertebral fabrics are
irreversible.
For the sample calculation a 50th percentile man is used and the assumption is
made, that the force related to the body mass, acting on the intervertebral disc only
through the upper body parts, is reduced by approximately 50% of the real value.
The determination of the action forces on the spine in industry is usually based on
empirically tested biomechanical models. When considering a load, usually the spinal
area L5-S1 is of main interest because it is a common injury point (95% of all disc
damage accounts for the three lowest lumbar intervertebral discs). The body parts (i)
above the lumbar sacrum transition L5-S1 themselves each generate a moment of
force around the point of reference for the calculation. The lever arms (ai) depend on
body position and therefore represent variables as a function of time during the
execution of movements. If there is not only a body movement executed, but also a
manipulation of a “load“, then there additionally occurs an action force (FA). That in
turn applies a moment of force on L5-S1 over the lever arm aA and as a consequence
the strain increasesHowever, the abdominal pressure also constitutes a certain help:
Through holding one’s breath an abdominal pressure can be set up so that the
solidified abdomen builds a supporting force for chest and spine (pABD). Alongside the
torque, the forces that take effect on the spine also constitute a measure for the stress
of the spine. On the one hand the weight of the body parts above the lumbar sacrum
transition leads to a compression of the intervertebral discs and transverse loads
occur because of the ascent even in an upright body position. On the other hand
additional forces are set up by muscles, e.g. by the back muscles. These forces build
up a counter torque against the moments mentioned above.
A weight of 10 kg held close to the body is equal to a load more than 10 times as high
while standing upright on the intervertebral discs in the lumbar vertebra region,
according to the law of levers.
Even a load 6-7 times as heavy and carried on the head would not result in greater
internal load. A weight of only 10 kg already results in an intervertebral disc strain that
corresponds to a mass of 300 kg directly compressing the disc. Such a strain
corresponds to a 230 kg heavy load directly on the head.
The mechanical effects inside the abdomen also have to be regarded. On lifting
heavy loads the air is held in the lungs by pressure breathing and is highly
compressed inside the body. A pressure like this is required for stabilization of the
trunk, but not without danger. It is therefore necessary to keep the body in an upright
position when lifting heavy loads. Only with an upright position there is an evenly
distributed pressure of the intervertebral disc. The spine should only be strained
axially, in no case eccentrically. For this case of strain, high surface pressures occur
at the margin of the intervertebral disc. With inflected spine and a lifted load of ~50kg
the surface pressure affects the intervertebral discs inconsistently. Additionally, the
contact stress at the right margin reaches values at around 300 N/cm2 (R), while with
straight stance the stress is only half as much (Rohmert 1983).
This illustration shows, how strew exerted on the intervertebral disc L5-S1 increases
or declines with variation of the masses of the lifted loads under otherwise equal
conditions. The compressive forces acting on L5-S1 for the dynamic, two-handed
lifting of loads with a mass of 0 kg up to 50 kg are therefore illustrated in the figure.
The overall duration for the leverage operation was supposed to be 1.5 sec. It is
apparent that for the 0-kg-graph even in an upright body position the forces are not
zero, which can be attributed to the weight of the body parts above L5-S1. With
increasing load, the curves get more skewed. This results from the increasing
influence of movement-related shares because of the moment of inertia of the load.
The figure on the right illustrates how the force on L5-S1 varies if a load is lifted in
different body positions.
The limiting value of 3400 N given by NIOSH insufficiently accounts for the difference
in physiological attributes such as age and gender. Therefore the limit applies only to
healthy individuals below the age of 50. The limits suggested by JÄGER however do
differentiate between various age and gender groups. For older people, the NIOSH-
limits are already considered to be too high. In case of ascent, torsions, sudden jerky
movements or asymmetric loads the strain on vortices and intervertebral discs
increases. According to this, the listed percentages are to be set against the denoted
load limits. The limit suggested by Jäger are the consequence of biomechanical
modeling, not a pure risk consideration.
If loads are too heavy and/or operated with a high frequency over a long period of
time and/or are handled in unfortunate body positions, there is a high risk of damage
to the musculoskeletal system for the manual handling of loads. Ailments of the
musculoskeletal system are prevalent in whole Europe. Risks emerge when the work
place does not correspond to the ergonomic principles of design. The European
standard DIN EN 1005-2 is a method for safety-related occupational medical care on
the basis of the European guideline 90/269/EWG „about the minimum regulations
regarding the safety and health protection for the manual handling of loads which
carries a risk to damage the lumbar column” and the load handling-regulation.
The model for risk assessment, which was presented in the European standard DIN
EN 1005-2, encompasses three processes. These processes repose on the same
foundation, however they differ in the complexity of application. The first process is a
quick rough analysis. The second process is easier to handle and has to be used
whenever the rough analysis indicate risks. Process 2 additionally considers a few
risk factors. Process 3 is a comprehensive thorough evaluation process, which
considers additional risk factors that are not included in process 1 and 2. These 3
processes possess different complexities. For logical reasons the risk assessment
starts with process 1 (the easiest process). Process 2 and 3 are only used when the
requirements or the loading cases from process 1 do not apply. The European
standard DIN EN 1005-2 apply for the manual handling regarding lifting or else
lowering of loads with a mass of a minimum of 3 kg and the carrying over a distance
of less than 2 m. The European standard DIN EN 1005-2 does not apply for holding of
property without walking, for pushing or pulling of property, for manually operated
machines or for handling load manually while sitting.
The actual mass of the work object that needs to be handled also comprises its
packaging or batteries, as well as the (for the manual handling needed) technical
auxiliaries. The position of the center of mass is determined by the property’s mass
distribution. The center of mass should ideally be in the middle between both hands
and as close as possible to the body. A shift of the center of mass within the work
object during the handling should best be avoided. The work object should be
constructed as compact as possible. Based on the risk index-calculation the design
recommendation can be derived, so that a low level of risk for the manual handling of
loads can be achieved; these recommendations include reducing the actual mass and
to hold it low from the very beginning (e.g. for the configuration of new workstations).
The weight limit RML, for a particular set of working conditions, is defined as the
weight of a load that virtually all healthy employees can manage for a particular time
(e.g. 8 hours) without an inacceptable risk of back injuries. The weight limit RML
determines by means of a multiplicative connection of reference load Mref and the
nine shown multipliers a maximum load with a low level of risk. The vertical distance
V between the center of both hands and the floor can be varied between 60 cm and
90 cm, the ideal value accounts for 75 cm. The horizontal distance H between the
center of both ankles and both hands as well as the vertical lifting distance D should
not exceed 25 cm. The asymmetry angle A should be 0 degrees and the grasping
quality should be judged as good. The lifting frequency F should be less than or equal
to 0.2 strokes per minute. Single handed work, working in pairs and spare-time work
are to be avoided.
Model basis:
• Dynamics
• Calculation of movements on the basis of the force progression in individual
muscles, resp. calculation of the force progression in muscles by means of
movements Model of the muscular-skeletal system (including muscles, tendons
and insertion points at the bone)
Functionality
• Positioning of marker points (for movement recording)
• Optimization functions for muscle recruitment and motion sequences (e.g.,
balance)
• Programming occurs with the AnyScript scripting language
• Analysis possibilities (graphically, numerically) e.g., direct calculation of
muscular strain