Identifying submaximal muscular effort: Reliability of difference scores calculated from isometric and isokinetic measurements

The present investigation examined the reliability of a derived strength me asurement and also how variability between test sessions affects the utilit y oh this variable for judging an individual's muscular effort. 31 young he althy men (M age = 25 yr.) completed three isometric and isokinetic concent ric contractions of the knee extensors, using maximal and self-selected sub maximal efforts, on each of two test yes sions. Difference scores between i sometric and isokinetic measurements were calculated by subtracting the mea n of the three isokinetic peak torques from the mean of the three isometric peak torques for each individual subject for maximal and submaximal effort s performed on both test sessions. For the group of subjects, difference sc ores were significantly greater during maximal (33 +/- 29 Nm) than submaxim al (13 +/- 30 Nm) efforts, suggesting subjects could not maintain the same relationship between isometric and isokinetic muscular actions across the m aximal and submaximal conditions. However, the test-retest reliability of t he difference scores was only modest even when data were averaged over two test sessions (intraclass correlation coefficients were .82 for maximal, an d .58 for submaximal). As a result, the range of scores within which an ind ividual's true score might be expected to lie was large (+/- 25 Nm for maxi mal, and +/- 37 Nm for submaximal). Although derived strength parameters li ke difference scores may be effective in distinguishing submaximal from max imal efforts completed by groups of subjects, the test-retest reliability o f the present scores suggests that their use in judging an individual's per formance may be limited.