Skeletal muscle stiffness in static and dynamic contractions.

Series elastic stiffness of rat gastrocnemius medialis muscle was determined by means of sinusoidal movements (180 Hz, 0.25% of muscle length) for various contraction conditions. The effects of muscle length, activation level, velocity, prestretch, and temperature on the force-stiffness relationship were investigated. All force-stiffness curves were transformed to a linear force-alpha curve (Ettema and Huijing, 1993; Morgan, 1977) to distinguish mathematically two series elastic components; a force dependent and force independent compliance. For all isometric conditions a typical force-stiffness curve was found, where stiffness increased with force, and this increase levelled off at higher forces. Stiffness in dynamic shortening and lengthening contractions is related to force in a completely different way than in isometric condition. An increase in temperature caused a decrease in muscle stiffness for a given force, and the effects of muscle length, activation level, and prestretch were small. It was concluded that the series elastic component of skeletal-muscle-tendon complex is probably located in more than two morphologically identifiable elements. Furthermore, we concluded that using a single series elastic element in muscle modelling is not appropriate to describe muscle behaviour under all conditions that occur during in vivo activation.