Isokinetic performance capacity of trunk muscles. Part II: Coefficient of variation in isokinetic measurement in maximal effort and in submaximal effort.

It has been claimed that with the aid of isokinetic trunk strength measuring devices it is possible to distinguish true muscular weakness from submaximal effort in the test. This proposition is based on the presumption that in the isokinetic trunk strength test identical performances can only be reproduced by maximal effort. The purpose of this study was to investigate whether it is possible to distinguish maximal effort from submaximal with the aid of the coefficient of variation (CV) in an isokinetic trunk muscle strength test. The study group included 35 (21 male and 14 female) subjects of whom 12 were healthy, 10 had a mild low-back pain and 13 had a more severe chronic low-back pain. The subjects performed five consecutive bendings both with maximal (100%) and submaximal (50%) efforts at a speed of 90 degrees/second. In maximal effort only healthy subjects reached an average level of CV close to 10% both in extension and in flexion. In the chronic low-back pain group the average CV was close to 20%. The difference in CV was statistically significant (p < 0.05-0.02) between the healthy and the chronic low-back pain subjects. In the submaximal effort all health groups had a CV of approximately 20% or more and no significant differences were found. The group of slightly variable measurements (CV = 11-20%) was remarkably large in both the maximal and submaximal effort. The results suggest that an effort with a CV of 11-20% cannot be classified as definitely submaximal or maximal. When the CV is less than 10% the effort can be fairly certainly classified as maximal.