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Published on: August 25, 2020
Shoulder angle effects on neuromuscular activation and rapid force production: sEMG time-frequency and force-time
Salih Çabuk1, Süleyman Ulupınar1, İzzet İnce2
1Department of Coaching Education, Faculty of Sport Sciences, Erzurum Technical University, Erzurum, Türkiye.
Introduction:
Maximal isometric shoulder force production constitutes an angle-dependent neuromuscular task; changes in shoulder elevation can meaningfully modify the relative contribution of glenohumeral and scapulothoracic musculature through concomitant alterations in moment arms, length-tension operating ranges, and the direction of the resultant force vector. Although shoulder testing across different arm elevations is widely used to infer functional capacity and rehabilitation readiness, the underlying neuromuscular strategies are still most often described using amplitude-based surface electromyography (sEMG) metrics or peak mechanical outcomes alone. However, it remains unclear how shoulder elevation angle influences time-frequency features of muscle activation and force-time characteristics.
Methods:
Twenty-four elite male athletes performed maximal isometric efforts. sEMG signals were analyzed wavelet-based mean frequency. Rate of force development (RFD) and mechanical impulse values were calculated from the raw force-time data across predefined intervals from contraction onset to 50, 100, 150, 200, 250 ms, and peak force.
Results:
Mean frequency was higher for the anterior deltoid and pectoralis major at 90° than at 135° and 180°. For the serratus anterior and infraspinatus, mean frequency was greater at 135° and 90° than at 180°. In contrast, upper and lower trapezius showed higher mean frequency at 180° than at 135° and 90°. No significant position-dependent differences were observed for the middle and posterior deltoid. Mechanically, the 180° condition showed higher force, RFD, and impulse values compared to the other angles.
Discussion:
Angle-dependent task sharing was observed within the shoulder complex, whereby lower elevation angles are associated with greater involvement of anterior musculature, while the overhead position is characterized by increased involvement of scapular stabilizers and higher rapid force-production outputs. These angle-specific neuromechanical patterns may contribute to more detailed shoulder profiling in elite athletes.
