Trunk muscle endurance measurement. Isometric contrasted to isokinetic testing in normal subjects

This cohort study examined a sample (n = 152) of subjects tested for isokinetic lumbar strength and endurance, using novel endurance measures.

To validate a new lumbar sagittal isokinetic endurance testing protocol comparing reliability in a normal subject cohort with strength test reliability to include presentation of a gender-specific normative database, and then correlating the results to a Sorenson isometric endurance protocol.

The isometric Sorenson test has been virtually the only validated clinical tool for lumbar extensor trunk muscle endurance testing, using an exercise chair and permitting the subject to maintain the trunk horizontal against gravity for a single timed contraction. Alternative isokinetic sagittal lumbar performance measurement methodology has been developed recently to measure trunk muscle endurance by determining declining work performance on repeated, reciprocal dynamic contractions.

We compared protocols, performance measures, normative data, and reliability for the static isometric Sorenson test to three different isokinetic endurance measurements: the endurance ratio, final fatigue ratio, and recovery ratio. Subjects were tested on a sagittal Cybex TEF (Lumex, Inc., Ronkonkoma, NY) unit for a strength trial followed by an endurance trial. Subgroups also performed reliability and isometric endurance protocols.

We found an extremely low test-retest correlation for the isometric Sorenson trunk extension test. All test-retest correlations for the corresponding isokinetic endurance measures were significant and markedly higher. Extensor muscle performance declined more substantially on all endurance measures than flexors and showed more variability. In contrast, trunk strength measures were more reliable and less variable than corresponding endurance measures. Moreover, correlations between the isometric Sorenson test and the isokinetic endurance measures were all negative; i.e., increases in Sorenson time are moderately correlated to greater decline in work performance.

Reliability of the Sorenson isometric endurance test is unacceptably low, showing negative correlations to all isokinetic endurance tests for lumbar extensors. Isokinetic strength and endurance tests are far more reliable than isometric tests, with normative data showing more consistent results from men than women. Both genders display more substantial fatigue (and greater variability) in extensor endurance compared with flexor testing. Endurance measures are identified as human performance cognitive constructs, and reasons for greater variability than usually shown by trunk strength measurements are discussed.