Reproducibility of maximal versus submaximal efforts in an isometric lumbar extension task

The purpose of this study was to investigate the differences in test-retest reliability between maximal and submaximal efforts in an isometric lumbar extension task and to test the hypothesis that submaximal efforts would be less consistent than maximal efforts. Twenty subjects were asked to produce maximum voluntary contractions at seven different positions in a lumbar extension machine. Each subject was tested twice in a maximal-effort condition and twice in a 50% effort condition. The order of the effort conditions was counterbalanced across subjects so that half of the subjects performed the maximal-effort tests first and half performed the submaximal effort first. Results indicated high test-retest correlations at all angles in both conditions. There were no differences in test-retest reliability between effort conditions. Therapist ratings of consistency did not differ between conditions, and therapists could not discriminate between conditions on the basis of effort consistency. Because subjects could reliably reproduce a submaximal effort, it was concluded that consistency interpretations of isometric strength testing may be misleading.