Effect of angular velocity on active muscle stiffness in vivo

We previously reported that active muscle stiffness could be evaluated in vivo. However, we were not able to investigate active muscle stiffness as more than 250 deg·s^-1 due to the limitation of the torque motor of dynamometer. The aim of the present study was to investigate the effect of angular velocities (including higher angular velocities of more than 250 deg·s^-1) on active muscle stiffness. Eighteen males volunteered for this study. Active muscle stiffness of the medial gastrocnemius muscle was calculated according to changes in the estimated muscle force and fascicle length during fast lengthening at five different angular velocities (100, 200, 300, 500, and 600 deg·s^-1). Electromyographic activities of the lateral gastrocnemius muscle (LG) and soleus muscle (SOL) were evaluated over two different phases: before the stretch (mEMGa) and after the stretch (mMEGb). Active muscle stiffness was higher at 300 than at 100 deg·s^-1, but decreased as the angular velocity increased from 300 to 600 deg·s^-1. There were no differences in mEMGa and mEMGb values among the five angular velocities, whereas mEMGb values were higher than mEMGa for all angular velocities. In conclusion, active muscle stiffness was highest at 300 deg·s^-1 and decreased at both slower and faster angular velocities.