Neuromuscular Adaptations in Degenerative Rotator Cuff Tears: Contributions of Aging and Pathology.

OBJECTIVE: Degenerative rotator cuff tear (RCT) is associated with impaired shoulder function; however, the extent to which altered neuromuscular coordination reflects aging-related changes or pathology-associated alterations remains unclear. This study aimed to characterize shoulder neuromuscular coordination patterns associated with aging and degenerative RCT using a multidimensional electromyographic approach. METHODS: Surface electromyography was recorded from eight shoulder-related muscles during maximal shoulder abduction and flexion tasks in 106 participants, including young controls (YC, n=24), elderly controls (EC, n=24), and patients with degenerative RCT (n=58). Muscle synergy characteristics were quantified using non-negative matrix factorization (NMF). Intermuscular coordination was assessed using beta-band coherence analysis, and muscle interaction patterns were further evaluated using network topology metrics. RESULTS: Compared with YC participants, both EC and RCT groups demonstrated reduced muscle synergy dimensionality, with no significant differences between EC and RCT groups. Synergy structure analyses revealed task-dependent differences among groups, and RCT participants exhibited coordination patterns partially similar to those observed in elderly controls. Intermuscular coherence and network analyses demonstrated group-specific alterations mainly during shoulder abduction, involving deltoid-related and deltoid-scapular muscle interactions. These findings suggest that degenerative RCT is associated with altered muscle coordination patterns superimposed on age-related neuromuscular changes. CONCLUSION: This study provides a comprehensive characterization of age- and pathology-associated alterations in shoulder neuromuscular coordination. The integration of muscle synergy, coherence, and network analyses may provide quantitative insights into functional impairment mechanisms in degenerative RCT and support future development of individualized rehabilitation strategies.