Transcript Comparisons to Isokinetic Strength

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
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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
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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
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Muscle Fatigue
Static vs dynamic contractions
Frequency and work / rest ratio
Temperature and Humidity
• inc from 20-27 C - dec 10-20% in capacity 3
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 5
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 11
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
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