Differences between measured and resultant joint moments during voluntary and artificially elicited isometric knee extension contractions

Objective. Examine two hypotheses: (a) during isometric knee extension contractions the moment measured at the dynamometer is different from the resultant moment in the same plane around the knee joint and (b) during isometric contractions, at the same given resultant moment the knee angle while loading is different from the knee angle while unloading. Design. Comparative study in which the geometrical and the kinetic differences between joint and dynamometer were determined. Background. It is usually assumed that the moment measured by the dynamometer is equivalent to the resultant joint moment. The non-rigidity of the dynamometer-leg system can influence the equivalence of these two moments. Method. Twenty seven subjects performed isometric maximal knee extension contractions and contractions induced by electro-stimulation on a dynamometer. The kinematics of the leg were recorded using 8 cameras (120 Hz). Results. The resultant moment at the knee joint and the moment measured by the dynamometer are different. During a knee extension effort the knee angle changes significantly. At identical resultant knee joint moments the knee angles are different when comparing the loading and the unloading phases. Conclusions. The differences between the measured and the resultant joint moments might influence the estimation of parameters as: muscle forces, moment-angle relationship and strain and hysteresis of tendons and aponeuroses.