Abstract: BACKGROUND: Limitation of knee motion is a relatively common post-injury feature, but it remains to be understood how muscle synergy under knee motion limitation is affected.  
OBJECTIVE: To study the similarities and differences between unilateral knee flexion limitation and normal exercise lower extremity muscle synergy.
METHODS: Twenty healthy adults (12 males and 8 females) were recruited as subjects, and the surface electromyogram signals of the lower limbs in natural walking with or without flexion limitation were collected. Muscle synergy was extracted from the surface electromyogram signals of 10 muscles on the restricted side (right leg) by a non-negative matrix factorization algorithm, and one-way analysis of variance was used to study the significant differences in gait parameters of walking in different conditions.  
RESULTS AND CONCLUSION: (1) Under flexion limitation, the number of muscle synergies decreased during the walking task after restraint, showing different muscle coordination mode control and recruiting new muscle synergy units. With the decrease of the synergistic module, the activation time of the medial gastrocnemius, lateral gastrocnemius and soleus muscle shifted more obviously from the support phase to the swing phase. After the restriction, it mainly relied on the rectus femoris, vastus medialis, vastus lateralis and tensor fascia lata for compensation, making it abnormally active at the activation level, and at the same time, the restricted side showed movements such as circles and crotches, so after the restriction, the hip joint on the restricted side was mainly used as compensation. (2) Under flexion limitation, there were significant differences in gait parameters (frequency, pace, gait cycle, and support ratio) between the restricted side and the non-restricted side compared with those before restriction (P < 0.05). The single-leg support time was significantly increased (P < 0.01) compared to before the restriction. The proportion of support and the time of single-leg support on the restricted side were significantly smaller than those on the non-restricted side (P < 0.01). (3) Therefore, the weakening of knee flexion movement can be considered as a key factor leading to the impaired coordination of lower extremities. The results of the study have implications for the quantitative analysis of motor function and the guidance of human lower extremity rehabilitation, and provide important insights for understanding the neuromuscular control behind motor function impairment mechanism.

Key words: knee joint, flexion limitation, muscle synergy, surface electromyography, non-negative matrix factorization