Effects of spinal transection and locomotor speed on muscle synergies of the cat hindlimb

It has been suggested that during locomotion, the nervous system controls movement by activating groups of muscles, or muscle synergies. Analysis of muscle synergies can reveal the organization of spinal locomotor networks and how it depends on the state of the nervous system, such as before and after spinal cord injury, and on different locomotor conditions, including a change in speed. The goal of this study was to investigate the effects of spinal transection and locomotor speed on hindlimb muscle synergies and their time-dependent activity patterns in adult cats. EMG activities of 15 hindlimb muscles were recorded in nine adult cats of either sex during tied-belt treadmill locomotion at speeds of 0.4, 0.7 and 1.0 m/s before and after recovery from a low thoracic spinal transection. We determined EMG burst groups using cluster analysis of EMG burst onset and offset times and muscle synergies using non-negative matrix factorization (NNMF). We found five major EMG burst groups and five muscle synergies in each of six experimental conditions (2 states × 3 speeds). In each case, the synergies accounted for at least 90% of muscle EMG variance. Both spinal transection and locomotion speed modified subgroups of EMG burst groups and the composition and activation patterns of selected synergies. However, these changes did not modify the general organization of muscle synergies. Based on the obtained results, we propose an organization for a pattern formation network of a two-level central pattern generator that can be tested in neuromechanical simulations of spinal circuits controlling cat locomotion. KEY POINTS: Analysis of muscle synergies during locomotion can reveal the organization of spinal locomotor networks. We recorded EMG activity of 15 hindlimb muscles in cats locomoting on a treadmill at speeds 0.4, 0.7 and 1.0 m/s before and after recovery from spinal cord transection at low thoracic level. We found five muscle synergies in all six experimental conditions (2 spinal states x 3 speeds) that include two flexor synergies operating in the swing phase and three extensor synergies operating in the stance phase. Major features of found synergies (the number, muscle composition and activation patterns) were not substantially affected by spinal transection and locomotion speed, suggesting that spinal control mechanism operates muscle synergies. Based on the obtained results, we proposed an organization of a pattern formation network of a two-level central pattern generator controlling locomotor activity of hindlimb muscles.