Association between muscle synergy during gait and knee flexion angle during the swing phase in stroke patients

This study aimed to assess the association between impaired spatiotemporal muscle coordination and knee flexion angle during the swing phase in stroke patients using muscle synergy analysis.

11 stroke patients who were able to walk with supervision or independence　participated in this study. Eight muscle activity of the paretic lower limb during gait with comfortable speed were recorded using surface electromyography. They were filmed from the sagittal plane using a high-speed camera. The maximum knee flexion angle during the swing phase was calculated using markerless motion capture from the recorded videos. The number of muscle synergies and their spatiotemporal patterns, as well as Variability Accounted For (VAF) 1 were extracted through non-negative matrix factorization (NMF). Furthermore, the muscle synergies extracted from each participant were classified using k-means clustering. Spearman's rank correlation analysis between maximum knee flexion angle during the swing phase and VAF1 was performed to determine whether motor control complexity is related to reduction in knee flexion angle during swing phase. Mann-Whitney U-test was also performed to compare group differences in maximum knee flexion angle during the swing phase for each muscle synergy number.

A significant negative correlation was found between VAF1 and the maximum knee flexion angle during the swing phase (ρ=-0.706, p=0.015). The participants were classified into 2 (three participants) or 3-synergy groups (eight participants) based on the number of extracted muscle synergies. The maximum knee flexion angle during the swing phase tended to be higher in the 3-synergy group (49.8±13.6°) than in the 2-synergy group (31.7±16.5°), but the difference was not significant (p=0.125). Only in the 3-synergy group had muscle synergy, mainly composed of the ankle plantarflexor muscles.A significant negative correlation was found between VAF1 and the maximum knee flexion angle during the swing phase (ρ=-0.706, p=0.015). The participants were classified into 2 (three participants) or 3-synergy groups (eight participants) based on the number of extracted muscle synergies. The maximum knee flexion angle during the swing phase tended to be higher in the 3-synergy group (49.8±13.6°) than in the 2-synergy group (31.7±16.5°), but the difference was not significant (p=0.125). Only in the 3-synergy group had muscle synergy, mainly composed of the ankle plantarflexor muscles.

VAF1, an indicator of the complexity of motor control, showed a significant correlation with the maximum knee flexion angle during the swing phase. Additionally, only the 3-synergy group had the muscle synergy mainly of the ankle plantarflexors and tended to have a higher maximum knee flexion angle during the swing phase.

This study may be useful in developing interventions based on the characteristics of lower limb muscle coordination to address insufficient knee flexion during the swing phase in individuals with stroke.