1 Muscular Function Assessment Gallagher - OEH ch 21(CCW)

2 Muscle strength is a complex function that can vary with the methods of assessment Definitions and introduction Assessment methods Variables impacting performance Outline

3 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 Muscle Function

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5 Gender Age Anthropometry Psychological factors - motivation –table 21.1 Factors Affecting Strength

6 Task influence –Posture fig 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 C - decrease of 10-20% in muscle capacity Factors Affecting Strength

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9 Isometric strength testing –standardized procedures –4-6 sec contraction, sec rest –standardized instruction posture, 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 Strength Testing (intro)

10 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 stress on lumbar spine Strength Testing

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12 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 Astrand –Max hold time affected by % MVC –Recommend less than 15% for long term requirements Fig Astrand –With repeated isometric contractions Force and Frequency influence endurance –Optimal work / rest ratio of 1/2 –Duration important as well (Astrand - blood flow) Isometric Considerations

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15 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 Isoinertial Testing

16 psychophysical methods –workers adjust demand to acceptable levels for 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 capacities of workers –75% of workers should rate as acceptable If demand is over this acceptance level; 3 times the injury rate observed to occur Psychophysical testing

17 Summary –Table 21.2 (Snook and Cirello) (see Job analysis lecture) 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 Psychophysical (cont)

18 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 Isokinetic Testing

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