Relationship between short latency stretch reflex and fascicle behavior in the soleus muscle in vivo

Objectives: Stretch reflex responses were considered to be affected by the velocity of muscle fiber lengthening and angular velocity. However, the results of previous studies in vivo and in vitro are inconsistent in this regard. The purpose of the present study was to investigate the effects of the velocity of fascicle lengthening on the amplitude of the stretch reflex for each trial with a high angular velocity and wide range of motion. Methods: Thirteen healthy men volunteered for this study. While the ankle was passively moved from 100 to 80 deg at five different angular velocities (100, 200, 300, 500, and 600 deg.s(-1)), the velocity of fascicle lengthening in the soleus muscle was measured using ultrasonography. In addition, the amplitude of the short latency stretch reflex in the soleus muscle was also measured. Results: As angular velocity increased, the amplitude of the stretch reflex and velocity of fascicle lengthening significantly increased (both p < 0.001). For each trial in all subjects, the amplitude of the stretch reflex was not correlated with the velocity of fascicle lengthening at any of the angular velocities. Conclusion: In conclusion, the stretch reflex size is not related to the fascicle behavior in each trial.