SKELETAL-MUSCLE STIFFNESS IN STATIC AND DYNAMIC CONTRACTIONS

Series elastic stiffness of rat gastrocnemius medialis muscle was dete rmined by means of sinusoidal movements (180 Hz, 0.25% of muscle lengt h) for various contraction conditions. The effects of muscle length, a ctivation level, velocity, prestretch, and temperature on the force-st iffness relationship were investigated. All force-stiffness curves wer e transformed to a linear force-alpha curve (Ettema and Huijing, 1993; Morgan, 1977) to distinguish mathematically two series elastic compon ents; a force dependent and force independent compliance. For all isom etric conditions a typical force-stiffness curve was found, where stif fness increased with force, and this increase levelled off at higher f orces. Stiffness in dynamic shortening and lengthening contractions is related to force in a completely different way than in isometric cond ition. An increase in temperature caused a decrease in muscle stiffnes s for a given force, and the effects of muscle length, activation leve l, and prestretch were small. It was concluded that the series elastic component of skeletal-muscle-tendon complex is probably located in mo re than two morphologically identifiable elements. Furthermore, we con cluded that using a single series elastic element in muscle modelling is not appropriate to describe muscle behaviour under all conditions t hat occur during in vivo activation.