Examining Muscle Synergy Deficits in Rotator Cuff Tendinopathy

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Neuromuscular deficits observed through electromyography (EMG) of individual muscles are common in rotator cuff (RC) tendinopathy. However, given that muscles act in coordination, muscle synergy analyses can identify altered neuromuscular patterns that can inform specific interventions to address these deficits. PURPOSE: Characterize shoulder muscle synergies in RC tendinopathy. METHODS: 54 individuals with RC tendinopathy (32M, 29.4 ± 8.9 yr) and 54 healthy controls (matched for age, sex, height, weight, and hand dominance) completed four velocity-controlled shoulder elevation tasks with seven repetitions of scaption, flexion, forward reaching, and contralateral reaching. During each task, muscle activity was collected from the supraspinatus, infraspinatus, deltoid, upper trapezius, lower trapezius, and serratus anterior via surface EMG (2000 Hz). Normalized EMG data was used to calculate muscle synergies of the ascending and descending phases of each task using non-negative matrix factorization (NMF). Relative muscle contribution was calculated as a percentage of total EMG activation for the two synergies of each task. Paired t-tests compared relative muscle contribution between RC tendinopathy and healthy control groups; alpha=0.05. RESULTS: The RC tendinopathy group had lower infraspinatus contribution than healthy controls in all synergies of shoulder scaption and flexion (mean difference= -0.40%, p CONCLUSION: RC tendinopathy is characterized by reduced relative contribution of the infraspinatus during shoulder flexion and scaption tasks. Increased relative contribution of the upper trapezius may compensate for this reduction, especially during the descending phase of scaption. Further research is needed to understand if changes in muscle synergy patterns are related to changes in shoulder disability and improvements with resistance exercise aimed at correcting the infraspinatus and upper trapezius deficits in patients with RC tendinopathy.

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