Synergistic Muscle Activation Patterns in Shoulder Strength Testing Using sEMG

Muscle strength is a key indicator of shoulder function, but standard force-based assessments offer limited insight into underlying neuromuscular strategies. Muscle recruitment patterns can vary significantly depending on taskspecific demands, joint positioning, and individual neuromuscular strategies. Such variability may affect the interpretation of strength measurements and underscores the need to investigate muscle activation to gain a more comprehensive understanding of shoulder biomechanics. This feasibility study aims to characterize the muscle activation profiles of key shoulder muscles involved in clinically relevant strength assessment tasks, focusing on quantifying the relative contribution of individual muscles and investigating coactivation patterns through cosine similarity analysis in healthy subjects. Two participants performed maximal voluntary isometric contractions during two tasks: scaption and elbow flexion. sEMG data were simultaneously recorded from five shoulder muscles. Muscle activation ratios were calculated to assess the relative contribution of each muscle to each task. Cosine similarity analysis was applied to assess the patterns of co-activation across tasks. Scaption elicited predominant activation of the middle and posterior deltoid, whereas elbow flexion produced a more distributed pattern involving significant recruitment of the biceps brachii. Cosine similarity results revealed consistent co-activation profiles, suggesting the presence of synergistic recruitment rather than isolated, taskspecific activation. Results demonstrate that combining sEMG with force measurements enables a more detailed interpretation of shoulder muscle function. This integrated approach may enhance the clinical assessment of strength deficits by identifying altered neuromuscular coordination, thereby informing more individualized and effective rehabilitation protocols for patients with shoulder disorders.