Effects of repetitive passive movement on ankle joint on spinal reciprocal inhibition.

Repetitive passive movement (RPM) activates afferent Ia fibers. The input of afferent Ia fibers from antagonist muscle may modulate the extent of spinal reciprocal inhibition (RI). However, effects of RPM on RI remain unknown. We aimed to clarify these effects in 20 healthy adults. Four RPM tasks (40°/s, 80°/s, 120°/s, and 160°/s), with the range of ankle joint movement set to 40°, ranging from 10° in dorsiflexion to 30° in plantar flexion, were performed for 10 min. For measuring RI, a deep peroneal nerve as a conditioning stimulus, tibial nerve as a test stimulus, and three condition-test stimulus intervals (CTIs; single, 2 ms, and 20 ms) were used. The stimulation frequency was 0.3 Hz for 36 times (3 stimulation conditions × 12 sets). RI was measured before, immediately after, and 5, 10, 15, and 20 min (Pre, Post 5, 10, 15, and 20, respectively) after the task. The extent of reciprocal Ia inhibition (CTI 2 ms) significantly increased in Post 5 and 10 at RPM speed of ≥ 120°/s. The extent of D1 inhibition (CTI 20 ms) significantly increased in Post 5 and 10 at RPM speed of ≥ 80°/s, and continued to increase until Post 15 at RPM speed of 160°/s. The extent of RI was the highest at RPM speed of 160°/s for both Ia and D1. Therefore, high RPM may increase the extent of reciprocal Ia inhibition and D1 inhibition, suggesting that rapid movements affect RI by increasing the firing frequency from the muscle spindle to afferent Ia fibers.