Short latency, non-reciprocal group I inhibition is reduced during the stance phase of walking in humans.

Activation of group Ib afferents from extensor muscles produces an inhibition in the parent muscle and its synergists in a resting cat. This reflex switches to excitation of the parent muscle and its synergists when the cat walks. This study determined if a similar reflex undergoes the same type of reversal in the intact human. A putative Ib reflex was elicited by conditioning the Hoffmann (H) reflex in the soleus muscle with stimuli to the nerve innervating the medial gastrocnemius muscle. The reflex was observed while subjects: (1) sat quietly; (2) sat and activated the triceps surae muscle isometrically at a low level; (3) stood and activated the triceps surae to the same level as (2); and (4) walked on a treadmill. Condition-test intervals of 1 to 16 ms were used. Ten out of the 15 subjects studied in quiet sitting showed an early, presumably disynaptic inhibition. Walking resulted in a significant reduction in the size of this inhibition at condition-test intervals of 4, 5, 6, and 8 ms for these subjects. No significant differences were observed at longer condition-test intervals. As a group, the inhibition of the conditioned H-reflex was diminished during walking, but not significantly excited. Four out of the 10 subjects, however, showed a significant excitation of the conditioned H-reflex during walking. The inhibition was significantly reduced at a condition-test interval of 7 ms when the triceps surae group was activated isometrically. No differences were seen in the reflex between matched levels of contraction in sitting and standing. It is concluded that the short latency group I inhibition seen at rest is reduced during walking, in a manner similar to that seen in spinal and decerebrate cats. The reduction may be accounted for, at least partially, by activation of the triceps surae.