Effects of static stretching on active muscle stiffness with and without the stretch reflex

The purpose of the present study was to examine acute changes in active muscle stiffness with and without the stretch reflex following static stretching. Before and after static stretching for 10 min, active muscle stiffness was measured according to changes in exerted torque and fascicle length during short-range stretch of faster (peak angular velocity of 250 deg·s-1; without the stretch reflex) and slower (peak angular velocity of 100 deg·s-1; with the stretch reflex) angular velocities. During the measurement of active muscle stiffness, the electromyographic activities of plantar flexor muscles were recorded and averaged over two different phases: just before (mEMGa) and after (mEMGb) stretch. In addition, the mEMGb/mEMGa ratio was used to evaluate the effects of stretch reflex. After 10 min of stretching, the mEMGb/mEMGa ratio tended to decrease under the 100 deg·s-1 condition, but not 250 deg·s-1. Under both conditions, active muscle stiffness did not change after 10 min of static stretching. In conclusion, the prolonged static stretching did not affect active muscle stiffness with or without the stretch reflex, but tended to decrease the stretch reflex. In addition, these results imply that active muscle stiffness measured during contractions was not influenced by the stretch reflex.