Phase-dependent modulation of percutaneously elicited multisegmental muscle responses after spinal cord injury

Phase-dependent modulation of monosynaptic reflexes has been reported for several muscles of the lower limb of uninjured rats and humans. To assess whether this step-phase-dependent modulation can be mediated at the level of the human spinal cord, we compared the modulation of responses evoked simultaneously in multiple motor pools in clinically complete spinal cord injury (SCI) compared with noninjured (NI) individuals. We induced multisegmental responses of the soleus, medial gastrocnemius, tibialis anterior, medial hamstring, and vastus lateralis muscles in response to percutaneous spinal cord stimulation over the Th11-Th12 vertebrae during standing and stepping on a treadmill. Individuals with SCI stepped on a treadmill with partial body-weight support and manual assistance of leg movements. The NI group demonstrated phase-dependent modulation of evoked potentials in all recorded muscles with the modulation of the response amplitude corresponding with changes in EMG amplitude in the same muscle. The SCI group demonstrated more variation in the pattern of modulation across the step cycle and same individuals in the SCI group could display responses with a magnitude as great as that of modulation observed in the NI group. The relationship between modulation and EMG activity during the step cycle varied from noncorrelated to highly correlated patterns. These findings demonstrate that the human lumbosacral spinal cord can phase-dependently modulate motor neuron excitability in the absence of functional supraspinal influence, although with much less consistency than that in NI individuals.