Force Recovery After Activated Stretching in Whole Skeletal Muscle: Transient Aspects of Force Enhancement

The enhancement of isometric force after active stretching is a well-accepted and demonstrated characteristic of skeletal muscle in both whole muscle [1,2] and single-fiber preparations [1,3], but its mechanisms remain unknown. Although traditionally analyzed at steady-state, transient phenomena caused, at least in part, by cross-bridge kinetics may provide novel insight into the mechanisms associated with force enhancement (FE). In order to identify the transient aspects of FE and its relation to stretching speed, stretching amplitude, and muscle mechanical work, a post hoc analysis of in situ experiments in soleus muscle tendon units of anesthetized cats [2] was conducted. The period immediately following stretching, at which the force returns to steady-state, was fit using an exponential decay function. The aims of this study were to analyze and quantify the effects of stretching amplitude, stretching speed, and muscle mechanical work on steady-state force enhancement (FE_ss) and transient force relaxation rate after active stretching. The results of this study were interpreted with respect to prior force depression (FD) experiments [4], to identify if the two phenomena exhibited similar transient and steady-state behaviors, and thus could be described by the same underlying mechanism(s).