The modulation and strength of the human soleus short latency stretch reflex was investigated by mechanically perturbing the ankle during an unconstrained pedaling task. Eight subjects pedaled at 60 rpm against a preload of 10 Nm. A torque pulse was applied to the crank at various positions during the crank cycle, producing ankle dorsiflexion perturbations of similar trajectory. The stretch reflex was greatest during the power phase of the crank cycle and was decreased to the level of background EMG during recovery. Matched perturbations were induced under static conditions at the same crank angle and background soleus EMG as recorded during the power phase of active pedaling. The magnitude of the stretch reflex during the dynamic condition was not statistically different from that during the static condition throughout the power phase of the movement. The results of this study indicate that the stretch reflex is not depressed during active cycling as has been shown with the H-reflex. This lack of depression may reflect a decreased susceptibility of the stretch reflex to inhibition, possibly originating from presynaptic mechanisms.
M.J. Grey and T. Sinkjaer are with the Center for Sensory-Motor Interaction at Aalborg University, Fredrik Bajers Vej 7D. DK-9220, Aalborg, Denmark. M.J. Grey, C.W. Pierce, and T.E. Milner are with the School of Kinesiology at Simon Fraser University, Burnaby, B.C. Canada V5A 1S6.