TRUNK MUSCLE ENDURANCE MEASUREMENT - ISOMETRIC CONTRASTED TO ISOKINETIC TESTING IN NORMAL SUBJECTS

Study Design. This cohort study examined a sample (n = 152) of subject s tested for isokinetic lumbar strength and endurance, using novel end urance measures.Objectives. To validate a new lumbar sagittal isokinet ic endurance testing protocol comparing reliability in a normal subjec t cohort with strength test reliability to include presentation of a g ender-specific normative database, and then correlating the results to a Sorenson isometric endurance protocol. Summary of Backround Data. T he isometric Sorenson test has been virtually the only validated clini cal tool for lumbar extensor trunk muscle endurance testing, using an exercise chair and permitting the subject to maintain the trunk horizo ntal against gravity for a single timed contraction. Alternative isoki netic sagittal lumbar performance measurement methodology has been dev eloped recently to measure trunk muscle endurance by determining decli ning work performance on repeated, reciprocal dynamic contractions. Me thod. 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 T EF (Lumex, Inc., Ronkonkoma, NY) unit for a strength trial followed by an endurance trial. Subgroups also performed reliability and isometri c endurance protocols. Results. We found an extremely low test-retest correlation for the isometric Sorenson trunk extension test. All test- retest correlations for the corresponding isokinetic endurance measure s were significant and markedly higher. Extensor muscle performance de clined more substantially on all endurance measures than flexors and s howed more variability. In contrast, trunk strength measures were more reliable and less variable than corresponding endurance measures. Mor eover, correlations between the isometric Sorenson test and the isokin etic endurance measures were all negative; i.e., increases in Sorenson time are moderately correlated to greater decline in work performance . Conclusions. Reliability of the Sorenson isometric endurance test is unacceptably low, showing negative correlations to all isokinetic end urance tests for lumbar extensors. Isokinetic strength and endurance t ests are far more reliable than isometric tests, with normative data s howing more consistent results from men than women. Both genders displ ay more substantial fatigue (and greater variability) in extensor endu rance compared with flexor testing. Endurance measures are identified as human performance cognitive constructs, and reasons for greater var iability than usually shown by trunk strength measurements are discuss ed.