Strength Testing: variability and relevance

1. Strength Testing: variability and relevance • Readings - • Matheson - Effects of instructions on isokinetic trunk strength variability, reliability absolute value and predictive value • Garg - A comparison of isokinetic lifting strength • Wolf - relationships among grip strength work capacity and recovery • OUTLINE • Summaries of above findings and discussions • Complete knowledge of methods and results not required

2. Psychological Factors • Classic experiments - arm strength • normal conditions • immediately after loud noise • while subject was screaming loudly • influence of alcohol and amphetamines • hypnosis • observed increased strength - most with hypnosis • theorized temporary modification in CNS - argued most people - neural inhibition (protective reflex) • reduced inhibition • highly trained athletes • arousal - unexplainable feats • early phase of strength training

3. Effect of Instructions: (Matheson) • Previous discussions emphasized the need for standardized protocol • consistent instructions important • Matheson paper supports this tenet • asks - what type of instructions produce the best results? • Optimize reliability, validity • Hypotheses • high demand instruction in isokinetic strength testing will show; • decreased variability • increased peak torque • produce stronger correlation with performance capacity • When compared to uniform instructions asking subject to put in a consistent effort

4. Instructions - Discussion • With high demand instructions • Hypothesis one • performance variability • Tables 2 and 3 - % coefficient of variation - four trials each speed • Lower variability with high demand • less variability at slower speeds with both types of instruction • Hypothesis two • peak torque - fig 1,2 tables 4 and 5 • higher in high instruction group • groups affected similarly by changes in velocity • Hypothesis three • Predictive ability - table 6 through 9 • significant correlation between isokinetic strength and lifting capacity in progressive test • Only with high demand instruction

5. 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

6. 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 for all lifts

7. 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

8. Recommendations • recommend • both speed of lifting and box width should be controlled carefully • using MAW and Static strength testing • SST 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

9. Grip Strength, Work Capacity and Recovery • Wolf • Relationships between strength, fatigue and work capacity are central to occupational rehabilitation • Musculoskeletal impairments are often expressed as loss of strength • % disability • strength and endurance correlation exceeds .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)]

10. 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

11. 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

12. 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

13. 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 • work capacity • and recovery rates for sustained high output work tasks • Methods - 40 healthy subjects - 1/2 male • Standard body position and instructions • repetitive gripping task at set pace • Measure pre trial isometric max • Utilize 25, 50 and 75 % of max • 1 sec reps until fatigue - 2 min recovery • measure isometric grip strength 1, 5 10 and 20 min of recovery • average of three trials

14. Methods (continued) • evaluate isotonic strength • LIDO - isotonic - progressive protocol • complete ROM in less than 1 sec • inc resistance - repeat to fatigue • Test - based on isotonic results • repetitive gripping to fatigue • 75% and 50% of isotonic max • then 25% for twice as long as 50%

15. Results • correlation isometric / isotonic strength • no correlation between isometric or isotonic strength and duration of work at either 75 or 50 % • significant relationships between isotonic strength and work capacity at 75 and 50% levels • Isotonic strength best predictor of work capacity at 75 % level - • When compared with duration • Work duration and isotonic strength had a similar predictive ability at the 50% resistance

16. Recovery Results • No significant gender differences • either recovery time or % at time points • table III and fig 1 • Recovery rate and time to recovery • subjects categorized - time to 100% • significant differences in degree of recovery Fig 2 • no differences in rate • similar slope, different starting points - • Related to degree of loss of strength • Good predictor of length of recovery • Standards - avg 20% decline in strength with protocol - 20 min recovery • variation - abnormal - intervention • standards - tables 4 and 5

17. Conclusions on Types of Strength testing • High demand instructions(isokin) • Reduce variability with high demand instructions • higher correlation to isotonic • good predictive ability for lifting capacity • Isokinetic testing • correlations depend on lifting speed and box width • difficult to establish safe lifting capacity • Difficult to design - many variables • Better to use static strength and MAW

18. Conclusions on Types of Strength testing • Recovery from fatigue - FCA • duration of work difficult to correlate to either isotonic or isometric strength • work capacity correlates better to isotonic strength than isometric • recovery rates similar between genders and degree of strength loss • useful in diagnosis - hand grip only from this study

19. General Conclusions • The goals of physical training program in the workplace are • Improve a workers physical capacity • Minimize risk of injury • Reduce the strain of performing daily tasks • The lack of physical fitness is a contributing factor for musculoskeletal injuries at work • The logistics of implementing changes in a workplace to reduce strain must be considered • Using a bar is inappropriate for manual materials handling assessments • The type of strength assessed will have a significant impact on the design of weight limits for job design • Results from strength tests are influenced by the subjects motivation and perception of how much force to generate without getting injured