Evaluation of moment-angle curves in isokinetic knee extension

BOBBERT, M. F. and J. HARLAAR. Evaluation of moment-angle curves in isokinetic knee extension, Med. Sci. Sports Exerc., Vol. 25, No. 2, pp. 251–259, 1992. The purpose of this study was to challenge the nation 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°-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° (180° 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° and 145°, no difference 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. Beyond a knee angle of 100°, SREMG levels were found to attain significantly higher values at 120 and 210° ·s−1. Thus, the second hypothesis was rejected. It was concluded that the moment-angular velocity relationship determined in isokinetic knee extension tests does not reflect solely the force-velocity relationship of human knee extensors but also includes an activation element. As long as it is unknown how variations in SREMG should be accounted for, the value of moment-angular velocity relationships for calculation of dynamic properties of human muscle groups remains questionable.