Test-retest variation in lifting capacity and indices of subject effort

Tests of lifting capacity ideally produce consistent outcomes from one test to the next. Physiological indices of effort, such as heart rates, coefficients of force variation, and isokinetic force-distance curve variability, should also remain constant. This study examined the test-retest reliability of isometric, isokinetic and isoinertial maximal lifting tests in pain-free, well-motivated subjects. First, isokinetic and isoinertial maximum lifting forces and weights were quite consistent between test sessions, while isometric forces increased slightly in the second session. Second, heart rates did not vary significantly between test sessions of isokinetic and isoinertial modes. However, other indices of effort, including force variances, force-weight differences and ratios, and force-weight curve variability, varied significantly between test sessions. Test-retest variations in these non-cardiac effort indices imply that these indices may not reliably distinguish maximal from submaximal lifting efforts even in normal subjects.

Efforts to reduce occupational low-back injuries and disability have focused recently on measurements of lifting capacity. Accurate and reliable estimation of subject effort is critical to the validity of such measurements. This study demonstrates a need for reconsideration of previously reported indices of lifting effort.