Exercise and Aging Skeletal Muscle

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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
• 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

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Factors Affecting Strength

• 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 - 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 numbers 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

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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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