Relationship between lower limb muscle coordination and knee flexion angle during the swing phase of gait in...

Stroke patients with hemiplegia often show inefficient gait patterns, including reduced knee flexion during the swing phase, which may increase fall risk. Post-stroke gait frequently involves merged muscle synergies that affect lower limb kinematics. However, it remains unclear how muscle synergy merging and fractionation relate to knee flexion during the swing phase. Therefore, this study aimed to examine the association between knee flexion during the swing phase and muscle synergy merging and fractionation patterns in patients with stroke.

The study comprised 21 stroke patients with hemiplegia. Surface electromyography was recorded from eight lower-limb muscles on the paretic side during comfortable gait. Maximum knee flexion angle (MKFA) during the swing phase was measured using a markerless motion capture system. Using non-negative matrix factorization, the number of muscle synergies, their spatiotemporal structure were calculated. Participants were classified into a low-synergy group (LS; n = 5; one or two synergies) or a high-synergy group (HS; n = 16; three synergies). Group comparisons of MKFA during the swing phase were performed. Furthermore, we investigated whether muscle synergies of the HS group could be fractionations of those of the LS group.

The HS group showed significantly greater MKFA compared with the LS group (p = 0.032). In the HS group, the ankle plantar flexors constituted an independent muscle synergy, whereas in the LS group, these muscles had high weightings within a muscle synergy associated with load response. Furthermore, the independent muscle synergies observed in the HS group were shown to be fractionated from the merged muscle synergies present in the LS group.

Our results showed that merged muscle synergies were associated with reduced MKFA during the swing phase, whereas an independent synergy involving the plantar flexors was associated with greater knee flexion. These findings suggest that fractionation of the plantar flexor synergy may be important for improving knee kinematics after stroke and could inform targeted rehabilitation strategies. Given the relatively small and imbalanced sample size, cautious interpretation of the findings is warranted. Further studies with larger, balanced samples are needed to further strengthen the evidence for these findings.