Differences between measured and resultant joint moments during isometric contractions at the ankle joint.

The purpose of this study was to examine two hypotheses: (a) during voluntary and electrically induced isometric contractions the moments measured at the dynamometer are different from the resultant moments in the same plane around the ankle joint and (b) at a given resultant moment during electrically induced isometric contractions the ankle angle while loading is different from the ankle angle while unloading. Twenty-seven long distance runners participated in the study. All subjects performed isometric maximal voluntary contractions (MVC) and contractions induced by electrostimulation at four different ankle-knee angle combinations on a Biodex-dynamometer. The kinematics of the leg were recorded using the vicon 624 system with eight cameras operating at 120 Hz. The main findings were: (a) the resultant moment at the ankle joint and the moment measured by the Biodex-dynamometer during isometric contractions are different, (b) during a plantar flexion effort the ankle angle changes significantly, whereas the knee angle shows only small and in most cases not significant changes, and (c) at identical resultant ankle joint moments the ankle angles are different between the loading and the unloading phases. The observed differences may lead to erroneous conclusions concerning the following: (a) diagnostic of muscle architecture, (b) estimation of the moment-ankle angle relationship and (c) estimation of the strain and hysteresis of tendons and aponeuroses.