Strength Testing: Variability and Relevance Matheson et al., Effect of instructions on isokinetic trunk strength testing variability, reliability, absolute value, and predictive validity. pp 249 - 264 Wolf et al., Relationship among grip strength, work capacity and recovery. pp 57 - 70 Garg and Beller, A comparison of isokinetic lifting strength.

OUTLINE Summarize findings and discussions from papers. Complete knowledge of methods and results not required.

Effect of Instructions Matheson et al. 1992 Will high-demand instructions effect isokinetic strength performance variability, reliability, absolute magnitude, and validity. The effect of high-demand instructions on lumbar strength performance was examined in 30 healthy women.

Relevant Research Isokinetic Testing dynamic contraction performed at a constant velocity regardless of the force generated by the muscles. Is this form of testing realistic? Previous discussions emphasized the need for standardized protocol with performance tests. consistent instructions are important

Without uniform instructions are results influenced by motivation are results influenced by cautionary factors results may not be reliable and valid Performance tests that demand maximum strength have a high potential for injury. Results are influenced by the subject’s perception of how much force to generate without getting injured.

Hypotheses High-demand uniform instruction will: decrease variability of performance increase peak torque produce stronger correlation between trunk testing device and lifting capacity Compared to uniform instructions asking subject to put in a consistent effort

Methods 2 instruction sets: put forth high effort put forth consistent effort Isokinetic back test of healthy normal women. tested at 30o, 60o, 90o, 120o, 180oper sec over 80o arc 4 practice trials, 4 test trials at each velocity lumbar flexion and extension collected separately instructions were tape recorded after 30 min rest, test progressive lifting capacity lift and lower a weighted crate,  wt. until max reached

Discussion Hypothesis 1:with high-demand instructions there was a  in performance variability. Table 2:  SD with high demand instructions less variability at slower speeds Hypothesis 2:  in torque with  velocity the fall off did not occur until 120o per sec peak torque - fig 1, 2. tables 3 and 4  torque in high-demand instruction group

Hypothesis 3: there is a strong relationship between isokinetic trunk performance and lifting capacity, as long as high-demand instructions are used. The relationship was significantly greater when the high demand instructions were used compared to the consistent instructions. Tables 6 - 9

When using high-demand instructions: Testing at low velocities (30o per sec) will help limit performance variability and improve reliability. However, testing at lower velocities  the exposure to the load and  the risk of injury. When using high-demand instructions: test on volunteer basis appropriate orientation and familiarization subject must be allowed to quit at any time

Conclusions High-demand instructions were found to have a substantial positive effect on performance variability, reliability, absolute magnitude, and validity. Under these conditions, isokinetic trunk strength was found to be predictive of performance in a frequent lifting-lowering task. A high demand instruction set in an isokinetic trunk strength protocol is the best means of predicting performance in lifting tasks.

Relationships Among Grip Strength, Work Capacity, and Relationships Among Grip Strength, Work Capacity, and Recovery (Wolf et al.) The relationship between strength, fatigue and work capacity is a central issue for occupational rehab. Muskuloskeletal impairments are often expressed as a loss of strength. Questions in paper: how important is strength as a component of work capacity? how do work capacity and strength affect recovery time?

Relevant Research Work (N) = Force x Distance Power (Joules): force/time Capacity to sustain work is inversely related to the power generated. as task demand approaches max capacity, exponential decrease in endurance After injury loss of power will  capacity rest from injury will  muscular and aerobic endurance (de-conditioning)

Rehab strengthen and condition worker to improve capacity various programs (functional restoration, work conditioning) difficult to establish and define dose of intervention The goal is to accelerate the rate of rehab and shorten treatment time. Physical training goals in the workplace are different from those of athletes. Athlete: improve capacity to enhance performance Worker: improve capacity to minimize the risk of injury and reduce the strain of performing tasks

Research indicates that there is a significant  in muscle strength following isometric contraction. Prediction equations for muscular endurance at a given % of max contraction have been derived (Sato). Results: 10-35 % decline in strength following exercise longer bout, lower recovery strength Fatigue theory short duration high intensity exercise: metabolic inhibition longer duration fatigue may be at the level of excitation-contraction coupling Whatever the cause, fatigue following tasks limits work capacity and is often the cause of intervention.

Relevance of isometric evaluation not prevalent in daily activity probably not the most valid test for task ability greater relevance for the hand The purpose of this Wolf et al. study was to address the issue of expected work duration, work capacity, and recovery rates for sustained high output work tasks. Research goals: develop technology necessary to support a treatment strategy dose of exercise is able to be closely tied to expected levels of recovery

Methods 40 healthy subjects (20 male, 20 female; 18 - 55yrs) repetitive gripping task at a set pace standard body position and instructions establish pre trial isometric max resistance 75, 50 and 25 % of max isometric strength 1 sec reps until fatigue (could not hold pace) 2 min break between each resistance level measure isometric grip strength 1, 5, 10 and 20 min of recovery average of 3 trials dynamometer handle set at position 2 for females and 3 for males

Results Males had  initial isometric and isotonic strength. Correlation between isometric / isotonic strength. No correlation between isometric or isotonic strength and duration of work at either 75 or 50 %. Significant relationships between isometric and isotonic strength and work capacity at 75 and 50% level. Isotonic strength was the best predictor of work capacity at 75 % level (compared with duration). Work duration and isotonic strength had a similar predictive ability at the 50% resistance level.

Results Recovery time for males and females were similar. The independent variables were not significantly predictive (age, height, weight, and pre-work isotonic strength). Recovery rate and time to recovery: subjects categorized by time to 100% recovery significant differences in degree of recovery (Fig 2) no differences in rate similar slope, different starting points degree of loss of strength predictor of length of recovery

Discussion Significant gender differences in initial isometric strength, isotonic strength, work performed, and postwork isometric strength. No gender differences for work duration. Using an isotonic exercise protocol you can expect an average 20% decrement in isometric strength immediately after exercise and full recovery after 20 minutes. If decrement is < 20% full recovery in 10 min. If decrement is > 20% full recovery after 20 min.

A Comparison of Isokinetic Lifting Strength Garg and Beller Goal of research determine the effects of speed of lifting and box size on isokinetic strength compare isokinetic lifting strength with static lifting strength psychophysically determined maximum acceptable weights

Relevant Research Measurement of human strength is important for job design. It is important to match physical strength requirements with capabilities of the worker to prevent overuse injuries. The measurement of dynamic strength is very complex. Isokinetic strength (constant speed) is commonly used to measure dynamic strength. Boxes should be used instead of a bar to simulate an actual lifting task.

Methods 9 male college students - range in age 22-36 (table 1) Measure: MAW, static strength, isokinetic strength MAW, lift box for 1 hr @12 lifts/hr, adjust weight static strength measured at origin of lift isokinetic - 3 speeds (.42, .51, .6 m/s) subjects told to lift as hard as possible without jerking load cell on tension line to the box 3 box sizes 25 - 50 cm wide lift floor to bench (.8m) open technique - subjects choice ** rate .2 /min (every 5 minutes) - 1 hr RPE for all lifts - low back

Results Progressive  in mean and peak isokinetic strength with  speed and  box width. Speed had greater impact than width. This confirms the ergonomic recommendation that heavy loads should be lifted slowly. However, high speed lifting perceived to be less stressful. RPE 10.7 (fast) vs 12.7 (slow)

At a slow speed (.41/ms) mean isokinetic strength = mean static strength. Thus static strength can represent dynamic capabilities of workers that lift heavy objects slowly. Maximum acceptable weights had a higher correlation with mean isokinetic strength than with static strength or peak isokinetic strength. Which strength test should be used for job design and physical ability testing. static strength and MAW appear to be more appropriate than isokinetic

Conclusions Both speed of lifting and box width should be controlled carefully. At slow speed mean isokinetic strength = mean static strength. At higher speed mean isokinetic strength = maximum acceptable weight.

The results of this study indicate that maximum acceptable weight and static strength testing should be used for job design. It should be remembered that static strength results in higher allowable limits. The effectiveness of maximum acceptable weight has not been well documented. The complexities of isokinetic strength testing, and its relationship to safe lifting capability, are not fully understood.

Conclusions on Types of Strength Testing High-demand instructions(isokin)  variability, reliability and validity good predictive ability for lifting capacity Recovery 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

Isokinetic tests depends on lifting speed and box width speed has more influence than weight difficult to establish safe lifting capacity difficult to design, many variables better to use static strength and maximum acceptable weights

General Conclusions The goals of physical training programs in the workplace are: improve a worker’s physical capacity minimize the 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.

General Conclusions 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 subject’s motivation and perception of how much force to generate without getting injured.