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Muscular Function Assessment

• Gallagher - OEH ch 21– Muscle strength is a complex function

that can vary with the methods of assessment

• Garg - – A comparison of isokinetic lifting

strength - speed and box size• Wolf -

– relationships between grip strength work capacity and recovery

• OUTLINE• Definitions and introduction• Assessment methods• Variables impacting performance• Recovery of performance

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Muscle Function• Gallagher• Strength - capacity to produce a force or

torque with a voluntary muscle contraction

• Power - Force * distance * time-1

• Endurance -ability to sustain low force requirements over extended period of time

• Measurement of human strength– Cannot be measured directly– interface between subject and device

influences measurement– Fig 21.1 Biomechanical eg.

• Q = (F * a)/b or c or d• force from muscle is always the same • results are specific to circumstances

• dynamic strength - motion around joint– variable speed - difficult to compare

• static or isometric strength- no motion– easy to quantify and compare – not representative of dynamic activity

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Factors Affecting Strength• Gender• Age• Anthropometry• Psychological factors - motivation

– table 21.1

• Task influence– Posture

• fig 21.2 angle and force production

– Duration• Fig 21.3

– Velocity of Contraction• Fig 21.4

– Muscle Fatigue

– Static vs dynamic contractions

– Frequency and work / rest ratio

– Temperature and Humidity• inc from 20-27 C - dec 10-20% in capacity

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Strength Testing (intro)• Isometric strength testing

– standardized procedures

– 4-6 sec, 30-120 sec rest

– standardized instruction

• postures, body supports, restraint systems, and environmental factors

– worldwide acceptance and adoption

• Dynamic strength

– isoinertial (isotonic)- mass properties of an object are held constant

– Psychophysical - subject estimate of (submax) load - under set conditions

– isokinetic strength

• through ROM at constant velocity

• Uniform position on F / V curve

• Standardized• Isolated muscle groups

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Strength testing• Testing for worker selection and

placement– Used to ensure that worker can tolerate

physical aspects of job– similar rates of overexertion injuries for

stronger and weaker workers

• Key principles– Strength test employed must be directly

related to work requirements• must be tied to biomechanical

analysis

• Isometric analysis fig 21.5– for each task - posture of torso and

extremities is documented (video)• recreate postures using software

– values compared to pop. norms • industrial workers

– estimate % capable of level of exertion– predict forces acting on lumbar spine

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Isometric Considerations• Discomfort and fatigue in isometrics

thought to result from ischemia– Increasing force, increases

intramuscular pressure which approaches then exceeds perfusion pressure - lowering then stopping blood flow

– Partial occlusion at 20-25% MVC– Complete occlusion above 50% MVC

• Fig 15-19 Astrand– Max hold time affected by % of MVC– Recommend less than 15% for long

term requirements

• Fig 15-20 Astrand– With repeated isometric contractions a

combination of Force and Frequency determine endurance

– Optimal work / rest ratio of 1/2– Frequency important as well (Astrand)

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Isoinertial Testing• Consider - biomechanics and grip

– Stabilization requirements

– justification of cut off scores

• Examples from industry• SAT - strength aptitude testing

– air force standard testing

– Pre-selected mass - increase to criterion level - success or failure

– found incremental weight lifted to 1.83m to be best test as well as safe and reliable

• PILE - progressive inertial lifting evaluation

– lumbar and cervical lifts -progressive weight - 4 lifts / 20 seconds

• standards normalized for age, gender and body weight

– variable termination criteria• voluntary, 85 % max HR, 55-60% body

weight

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Psychophysical testing• psychophysical methods

– workers adjust demand to acceptable levels for specified conditions

– provides ‘submax’ endurance estimate• Procedure -

– subject manipulate one variable-weight– Either test : starting heavy or light– add / remove weight to fair workload– Fair defined as : without straining,

becoming over tired, weakened, over heated or out of breath

• Study must use large number’s of subjects– evaluate / design jobs within

determined capacities by workers– 75% of workers should rate as

acceptable• If demand is over this acceptance level; 3

times the injury rate observed to occur

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Psychophysical (cont)• Summary

– Table 21.2 (Snook and Cirello)

• Advantages– realistic simulation of industrial

tasks

– very reproducible - related to incidence of low back injury

• Disadvantages– results can exceed “safe” as

determined through other methodology

– biomechanical, physiological

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Isokinetic Testing• Isokinetic testing

– Evaluates muscular strength throughout a range of motion at a constant velocity

– Consider - velocity, biomechanics– However;

• humans do not move at constant velocity

• isokinetic tests usually isolated joint movements

• may not be reflective of performance ability

• Redesign of isokinetic testing – multi joint simulation tasks for industry

• fig 21.8• Better, as they require core

stabilization• still in development, therefore

limited validity

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Comparing Isokinetic Strength (Garg)

• Goal of research– determine effects of speed of lifting and

box size on isokinetic strength– compare isokinetic with

• static lifting strength• psychophysically determined maximal

acceptable weight (MAW)

• Relevance of Research– Measurement of human strength is

important for job design– Important to match physical strength

requirements with worker capabilities to prevent injury

– Measurement of dynamic strength is complex

– Isokinetic strength is commonly used to measure dynamic strength

– The use of boxes instead of a bar is a better simulation of actual lifting tasks

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Methods• 9 male college students - range in

age 22-36 (table 1)• 12 lifts per hour (every 5 minutes)• lift floor to bench (.8 m)• 3 box sizes 25 - 50 cm wide• open technique - subjects choice **• Measure MAW, static strength,

isokinetic strength– MAW - adjust weight till comfortable– Static measured at origin of lift– Isokinetic evaluated at 3 speeds

• RPE on low back evaluated for all lifts

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Results• Progressive decline in mean and peak

isokinetic strength – with inc speed and inc box width– Fig 1 and 2– speed had greater impact than width

• Recommend lifting slowly

• However, high speed lifting perceived to be less stressful– RPE 10.7 (fast) vs 12.7 (slow)– Fig 3

• static strength and MAW higher correlation with mean than peak isokinetic strength– high speed - mean isokinetic - within 6%

of MAW– low speed - mean - equal to mean static

strength– Fig 4

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Recommendations• recommend

– both speed of lifting and box width should be controlled carefully

– using MAW and Static strength testing

• Static testing results in higher allowable limits for workers

• MAW - effectiveness not yet as well documented

• the complexities of isokinetic strength testing and its relationship to safe lifting capability are not fully understood

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Grip Strength, Work Capacity and Recovery

• Wolf• Investigates relationships between

strength, fatigue and work capacity that are central to occupational rehabilitation

• Musculoskeletal impairments are often expressed as loss of strength– % disability

• correlation between strength and endurance is greater than .90– endurance tests

• often assess repetitions to failure using a % of body weight

– strength test often use one rep max (isotonic) ; not always appropriate

• 1 RM= (weight) / [1- (RM * .02)]

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Grip Strength, Work Capacity and Recovery

• questions in paper– how important is strength as a

component of work capacity?– how do work capacity and strength

affect recovery time?

• Relevant research• Capacity to sustain work activity is

inversely related to power required– exponential decrease in endurance, as

demand approaches max– Walsh (Fig 1 and 2)

• after injury - loss of power leads to loss of capacity– rest from injury - often increases impact

due to muscular de-conditioning

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Background• Rehabilitation

– strengthen and condition worker to improve capacity

– Various programs (functional restoration, work conditioning, work hardening)

– Often difficult to establish and define dose of intervention precisely

• The goal is to accelerate the rate of rehab and shorten treatment time

• Physical training goals in the workplace are different from those ot athletes– Athlete: improve capacity to enhance

performance– Worker: improve capacity to minimize

the risk of injury and reduce the strain of performing tasks

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Background• Prediction equations for muscular

endurance at a given % of max contraction - constants for each muscle group (Sato)– results 10-35 % decline in strength– longer bout, lower recovery strength

• Fatigue - theory– short - high intensity exercise -

metabolic inhibition– longer duration - fatigue may be at

level of E-C coupling - ? K+ ?

• Relevance of isometric evaluation– low - due to low prevalence of

isometric activity– Greater relevance for hand

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Relationships• Research goals of Wolf study

– develop technology necessary to support a treatment strategy

– dose of exercise is able to be closely tied to expected levels of recovery

• Address issues of ;– expected work duration and capacity

– and recovery rates

• Methods- 40 healthy subjects-1/2 male

– Standard body position and instructions

– Measure isometric and isotonic max’s

– Repetitive isotonic gripping task at 25, 50 and 75 % of pre-trial max to failure

• measure isometric grip strength after 1, 5 10 and 20 min of recovery

• Take average of three trials

• Plot recovery rates of return to max strength

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Results• correlation between isometric and

isotonic strength maximums (.63)• poor correlation between isometric

or isotonic strength and duration (time) of work at either 75 or 50 %

• strong relationships between isotonic strength and work capacity (strength * time) at 75 and 50% levels (>.8)

• Isotonic strength best predictor of work capacity at 75 % level - – When compared with duration

• Work duration and isotonic strength had a similar predictive ability fro work capacity at the 50% resistance

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Recovery Results • No significant gender differences

– either for recovery time or % at any time points

– table III and fig 1

• Recovery rate and time to recovery– subjects categorized based on their time

to reach 100%– significant differences in initial degree

of recovery Fig 2 after fatigue– no differences in rate– similar slope, different starting points - – Rate of recovery, therefore related to

degree of initial strength loss (%)• This is therefore a good predictor of length

of recovery (time)

• Healthy standards - avg 20% decline in strength with protocol - 20 min recovery– variation - abnormal - intervention– standards - tables 4 and 5