Phase-dependent modulation of short latency cutaneous reflexes during walking in man.

In reduced animal preparation (cat fictive locomotion) most of our knowledge on the phase-dependent modulation of cutaneous reflexes concerns early- (P1 responses) rather than medium-latency (P2) responses. In contrast, in humans, virtually only P2 responses have been studied because P1 responses are relatively rare in adults. In this work, human P1 and P2 responses following sural nerve stimulation were compared in BF (biceps femoris). Some 14% of all subjects showed P1 responses, which could be either suppressive or facilitatory. The suppressive responses were most common, and they occurred primarily at end swing. When a subject showed both suppressive P1 and P2 responses, the P2 suppressions were stronger than the P1 suppressions (as measured in the same phase at end swing). However, the P2 suppressions were less consistent across subjects than the P1 suppressions. In some subjects, there were also facilitatory P1 responses, mostly throughout stance (when facilitatory P2 responses occur as well). These facilitatory P1 responses were consistent across experimental conditions in these subjects. Similar to the cat, the facilitatory P1 responses were much smaller than the facilitatory P2 responses. It is concluded that P1 and P2 reflexes can appear independently from each other, but that their modulation in the step cycle is mostly quite similar in some phases of the step cycle (end swing).