Relief increases select shoulder muscle activity, following subacromial injection in subacromial pain syndrome

Subacromial pain syndrome (SAPS) is commonly linked to disrupted neuromuscular function, yet the effects of pain reduction on shoulder muscle activity remains unclear. This study examined how immediate and long-term (six weeks) decreases in shoulder pain influence muscle activation during humeral elevation in individuals with SAPS. Twenty-one participants with clinically diagnosed SAPS completed three sessions of humeral elevation at: baseline (T1), immediately after a subacromial injection (T2), and after six weeks of physical therapy (T3). Electromyography (EMG) was recorded from eight shoulder muscles. Due to EMG signal variability and recording errors, muscle-specific sample sizes varied. Statistical parametric mapping (SPM) ttest's assessed differences in EMG amplitude across timepoints. Significant changes (0.33 ≤ ηp² ≤ 0.58) were observed in the anterior deltoid (n = 21), middle deltoid (n = 20), and upper trapezius (n = 17); with greater activation at T2 compared to T1 in the early stages of movement (p < 0.05), indicating a short-term neuromuscular response to pain relief. No significant differences were observed between T1 and T3, nor in the posterior deltoid or supraspinatus muscles. Results suggest a selective, short-term adaptation in the anterior and middle deltoid and upper trapezius following immediate pain relief, with no sustained changes after physical therapy.