Effect of Instructions on Measurement of Isometric Whole Body Strength: Reliability and Validity

Matheson LN, Danner R, Grant J, Mooney V. Effect of computerized instructions on measurement of lift capacity: Safety, reliability, and validity. Journal of Occupational Rehabilitation, 1993;3(2), 65–81.


The evaluation of lifting capacity is an important part of most functional capacity evaluations. Several different methods have been developed to evaluate lifting capacity in a safe, reliable, and valid manner. Isometric strength testing is one approach which has been demonstrated to be highly reliable. However, questions have been raised about the safety and validity of isometric strength testing as a predictor of “real world” lifting capacity. One method to improve safety is to provide real time performance feedback so that the evaluee is able to increase psychophysical input to appropriately gauge his or her effort level. One method to improve reliability is to provide standardized instructions that are well-understood by the evaluee. Both of these approaches have been utilized in the ERGOS Work Simulator, a multiple-task evaluation instrument that presents instructions to the evaluee visually and auditorially. Primary instructions are presented auditorially through the use of a synthesized “voice” in combination with text presented on a color video display terminal. Secondary instructions are presented pictorially with text presentation, using synthesized photographs that depict the posture to be used with the evaluation task. Subsequently, during the task, the evaluee is provided “real time” feedback concerning force generation through the use of a force curve that presents performance on a 24-cycles-per-second basis along a logarithmic scale. The safety, reliability, and validity of this automated approach in comparison to an experienced human evaluator was evaluated in this research project.


Twenty-seven healthy adult males were randomly assigned to one of two groups, the first receiving standardized machine-driven instructions and the second receiving standardized human-provided instructions. For these latter subjects, the auditory and visual feedback mechanisms provided by the ERGOS were disabled. Isometric testing at knuckle height and elbow height was performed on a repeated-measures basis with a 90-minute interval between trials. In addition, lifting capacity from the floor to bench height was measured through the use of the EPIC Lifting Capacity test on a one-lift-per-cycle and four-lifts-per-cycle basis.


No statistically significant differences exist between the two randomly selected groups on any of the demographic variables. There were no injuries reported in either group. There were no differences between the groups in terms of next-day discomfort. A two-factor repeated measures analysis of variance on average isometric force values demonstrates that there was no effect due to type of instructions. However, there is an effect on the repeated-measures factor for the isometric lift at knuckle height. The lift at elbow height has no significant effect attributable to either the instructions or to the repeated measure. Correlation coefficients for each of the test-retest comparisons are excellent and exceed r =.90. It appears that there is a practice effect for the knuckle lift such that subjects uniformly improve from the first trial to the second trial. There was no difference in improvement comparing the machine-instructed subjects and the human-instructed subjects.

In terms of validity, each instruction method was able to dependably predict lifting capacity, with correlations in the range of r = .49 to r = .90 which is consistent with earlier research on isometric measures. As can be expected, the relationship between knuckle height isometric performance and lifting capacity was generally better than the relationship between elbow height performance and lift capacity. Additionally, the human instructions led to slightly higher correlations between each isometric task and either lifting task.

The results of this study demonstrate the efficacy of machine-driven instructions in a isometric strength testing system to achieve safe, reliable, and valid results. A comparison of the machine-driven isometric test with the machine-driven progressive lifting capacity test on the ERGOS is warranted in order to better understand the effect of this mode of instruction on human performance.