A submaximal test to assess back muscle capacity: evaluation of construct validity.

A functional endurance test more specific to common occupational tasks is proposed for assessing back muscle capacity. The test involves static intermittent contractions (8-s work-rest cycles) using a predefined absolute load (90 Nm) across subjects. Since the test involved an absolute endurance task, it was hypothesized that performance would be influenced by both the strength and relative endurance of the subjects, thus demonstrating the construct validity of this new test. Fifteen males and 17 females were assessed on three different days to allow familiarization and to measure their Strength as well as their absolute and relative endurance. Absolute and relative endurance were defined as the time to reach exhaustion (Tend(abs) and Tend(rel), respectively) during a fatigue protocol including both an absolute (90 Nm) and a relative (40% of individual strength) load (extension moment at the L5/S1 joint). Strength and Tend(rel) each explained an almost equivalent portion of Tend(abs) (total variance explained: 61.5%), thus confirming the construct validity of the functional endurance test. This new test should better identify the back muscle impairments (weakness, fatigability) often observed in chronic low-back-pain patients.