Abstract

Investigation of the firing behavior of spinal motor neurons (MNs) provides essential neuromuscular control information because MNs form the "final common pathway" in motor control. The MNs activated during human infants leg movements and rodent locomotion, mainly controlled by the spinal central pattern generator (CPG), show highly synchronous firing. In addition to spinal CPGs, the cerebral cortex is involved in neuromuscular control during walking in human adults. Thus, MN firing behavior during adult walking is expected to be similar to that of infants and rodents and has some unique features. Recent technical advances allow non-invasive investigation of MN firing by high-density surface electromyogram (HDsEMG) decomposition. Therefore, we investigated the MN firing behavior of the tibialis anterior muscle during walking by HDsEMG decomposition. We found motor unit recruitment modulation compared with steady isometric contractions, doublet firings, and gait phase-specific firings during walking. We also found high MN synchronization during walking over a wide range of frequencies, probably including cortical and spinal CPG-related components. The amount of MN synchronization was modulated between the gait phases and motor tasks. These results suggest that the central nervous system, including the spinal CPG and cerebral cortex, flexibly controls MN firing to generate appropriate muscle force during human walking. In addition to revealing the neural control mechanisms of walking, our data demonstrate the feasibility of non-invasive investigation of MNs during walking, which will open new frontiers for the study of neuromuscular function in medical and exercise sciences.