Transcript Exercise and Aging Skeletal Muscle

Muscular Function Assessment
Gallagher - OEH ch 21(CCW)
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Outline
• Muscle strength is a complex function that
can vary with the methods of assessment
• Definitions and introduction
• Assessment methods
• Variables impacting 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
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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
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Gender
Age
Anthropometry
Psychological
factors motivation
– table 21.1
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Factors Affecting Strength
• Task influence
– Posture
• fig 21.2 - angle and force production
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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 - decrease of 10-20% in muscle capacity
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Strength Testing (intro)
• Isometric strength testing
– standardized procedures
– 4-6 sec contraction, 30-120 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
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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 stress 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 % MVC
– Recommend less than 15% for long term requirements
• Fig 15-20 Astrand
– With repeated isometric contractions Force and Frequency influence
endurance
– Optimal work / rest ratio of 1/2
– Duration important as well (Astrand - blood flow)
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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 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
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observed to occur
Psychophysical (cont)
• 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
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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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