Evaluation of moment-angle curves in isokinetic knee extension

The purpose of this study was to challenge the notion that the relationship of moment of force to angular velocity, determined in isokinetic knee extension tests, reflects the force-velocity relationship of human knee extensors. For this purpose, maximum-effort isometric knee extension moments were collected at seven different knee joint angles, and maximum-effort isokinetic knee extension moments were measured as a function of knee joint angle at angular velocities of 30, 60, 120, and 210 degrees.s-1. For the isokinetic contractions two protocols were used with a different threshold moment, i.e., the minimum knee extension moment that had to be reached isometrically before knee extension was initiated. In one protocol (LTM, for low threshold moment) the threshold was set at 5% of the maximum isometric knee extension moment produced at the starting angle of 85 degrees (180 degrees being full knee extension), in the second protocol (HTM, for high threshold moment) it was set at 95% of this isometric moment. During all contractions, electromyograms were recorded from m. vastus medialis, m. vastus lateralis, m. rectus femoris, and m. biceps femoris. The EMG signals were rectified and smoothed to yield SREMG. Two hypotheses were specifically tested. The first was that the threshold moment does not affect knee extension moments in the middle of the range of motion. Between knee angles of 100 degrees and 145 degrees, no differences were found between moments produced in the HTM protocol and those produced in the LTM protocol. Thus, the first hypothesis was supported. The second hypothesis was that subjects achieve the same level of muscle excitation at different speeds.(ABSTRACT TRUNCATED AT 250 WORDS)