Effects of swimming fatigue on neuromuscular parameters in young swimmers with unilateral shoulder pain

Background: A cross-sectional study was conducted to investigate the effects of fatigue on isometric strength and surface electromyography (sEMG) in swimmers with unilateral shoulder pain compared to healthy controls. Methods: Twenty front-crawl swimmers participated in the present study and were grouped according to the presence (PAIN, N.=10, 3 females, 23±3 years) or absence (controls, N.=10, 4 females, 22±5 years) of unilateral shoulder pain, based on subjective reporting and physiotherapy evaluation. Before and after swimming fatigue training, the sEMG signals were recorded during a functional upper-limb task and a strike task and were normalized according to their maximal voluntary isometric contraction (%MVIC), and an isometric strength assessment was performed for the shoulder. Results: A significant interaction effect time × group was found during the functional task in the sternocleidomastoid (m.SCM) muscle (P=0.014, pη2=0.294) and pectoralis major (m.PM) muscle (P=0.027, pη2=0.243), whereas during the strike task, only the serratus anterior (m.SA) muscle was characterized by a significant interaction (P=0.006, pη2=0.346). In particular, in the PAIN group, fatigue reduced m.PM activation by 0.8% MVIC (Cohen's d=0.440) and increased m.SCM activation by 0.7% MVIC (Cohen's d=0.633), as well as increased m.SA activation by 24.2% MVIC (Cohen's d=1.356), whereas in controls m.SCM decreased by 0.6% MVIC (Cohen's d=0.485). Finally, the PAIN group was characterized by lower IR strength than controls after the training by 0.127 N/kg (Cohen's d=0.988). Conclusions: Pain and fatigue can affect muscle strength and sEMG activity in swimmers with unilateral shoulder pain. These findings could provide novel insights into the physiological and pathophysiological mechanisms underlying unilateral shoulder pain in swimmers, supporting the development of new training and treatment strategies.