Backward and forward walking use different patterns of phase-dependent modulation of cutaneous reflexes in humans.

1. The phase-dependent modulation of medium-latency (P2) (70-80 ms) responses in semitendinosus (ST), biceps femoris (BF), rectus femoris (RF), and tibialis anterior (TA) was studied with the use of low-intensity stimulation (2 times perception threshold) of the sural nerve. The shocks were given in a random order at 16 phases of the step cycle in 10 normal subjects during forward walking (FW) or backward walking (BW) on a treadmill. 2. All subjects exhibited P2 responses in all muscles studied both during BW and FW. The amplitude of the facilitatory P2 responses showed phase-dependent changes that could not have been predicted on the basis of the variations in background activity throughout the step cycle. 3. During FW, the P2 facilitatory responses in BF were large (with respect to the background activity) throughout the whole step cycle except for a short period near the end of the swing phase. In ST the responses were smaller and appeared primarily at the end of the stance phase and during the first part of the swing phase. During the second half of swing the P2 responses were basically suppressive. A modulation pattern similar to the one in ST was found in RF and TA, except that there was no reversal to suppressive responses in the swing phase in RF. Instead, a reduction in the amplitude of the facilitatory P2 responses occurred. 4. During BW, the modulation pattern recorded in the same subjects was different from the one seen during FW. Large facilitatory P2 responses were present in all muscles in middle and late swing. In the first half of stance the responses were most prominently seen in BF and RF. At the end of stance and/or at the onset of swing the facilitatory responses decreased in amplitude (BF and RF) or reversed to P2 suppressions (ST and TA). 5. We conclude that there are both facilitatory and suppressive pathways from the sural nerve to the leg muscles studied and that the balance of activity in these paths is phase dependent during both FW and BW. It is suggested that the phase-dependent modulation of P2 responses could largely rely on a central motor program. During BW the same motor program is used as during FW, but possibly running in reverse, thereby causing a shift both in the timing of the reflex reversal and in the periods of reflex suppression.