The Changes in Synergistic Patterns of Peri-Shoulder Muscles During Shoulder Abduction and Flexion in Patients With Rotator Cuff Tears

Understanding muscle synergy variability and its clinical relevance in rotator cuff tear (RCT) patients is crucial for elucidating motor control mechanisms and informing rehabilitation. This study uses non-negative matrix factorization (NMF) to assess the influence of age and pathological factors on synergy patterns during abduction (ABD) and flexion (FL) tasks. Fifteen young controls (YC), fifteen elderly controls (EC), and twenty elderly RCT patients were recruited. Surface electromyography (sEMG) signals from eight shoulder muscles were recorded for NMF analysis, and the correlation with the Constant-Murley Score (CMS) was evaluated via regression. Results revealed that the number of synergies in the FL task was significantly lower in the RCT group compared to the EC ( $4.5~\pm ~1.3$ vs. $5.6~\pm ~1.1$ , P=0.01) and YC groups ( $4.5~\pm ~1.3$ vs. $5.6~\pm ~1.0$ ,P=0.03), with no significant difference in the ABD task (P=0.10). In ABD, synergy pattern 1 was dominated by the upper trapezius and middle trapezius, while in the EC group, pattern 4 was driven by the supraspinatus, and in the YC group, both supraspinatus and posterior deltoid were involved. In FL, the middle deltoid weight was significantly increased in pattern 3 in the RCT group, suggesting compensatory activation or a pathological marker. The root mean square value of middle deltoid strongly correlated with CMS (ABD: R ${}^{2} =0.53$ ,P< 0.001; FL: R ${}^{{2}} =0.46$ ,P< 0.001). These findings reveal task-specific compensation mechanisms in RCT patients and provide insights for targeted rehabilitation strategies.