Effect of short latency stretch reflex on passive and active muscle stiffness in the soleus muscle in vivo.

Purpose: The purpose of the present study was to determine the effect of the short latency stretch reflex on passive and active muscle stiffness in the soleus muscle.

Methods: Fourteen males participated in this study. Passive muscle stiffness was calculated from passive muscle force and fascicle length during passive lengthening at four different angular velocities (5, 50, 100, and 200 deg s). Active muscle stiffness was also calculated according to changes in muscle force and fascicle length during fast lengthening at five different angular velocities (100, 200, 300, 500, and 600 deg s) after submaximal isometric contraction (30% of maximal voluntary contraction). During the measurements of passive and active muscle stiffness, the amplitude of the short latency stretch reflex in the soleus muscle was measured.

Results: Change in passive torque and passive muscle stiffness significantly increased, whereas change in the fascicle length decreased, as angular velocity increased. At 100 and 200 deg s (stretch reflex responses were not observed at 5 and 50 deg s), the amplitude of the short latency stretch reflex was highly correlated with passive muscle stiffness. Change in torque and active muscle stiffness were highest at 100 deg s and decreased as angular velocity increased. At all angular velocities under active conditions, the amplitude of the short latency stretch reflex was not correlated with active muscle stiffness.

Conclusion: These results suggest that the short latency stretch reflex affects passive muscle stiffness in the soleus muscle, but not active muscle stiffness.