Muscular Function Assessment

1. 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

2. 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

3. 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

4. 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

5. 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

6. 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)

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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)]

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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