Muscle coactivation changes following a fatiguing overhead drilling task: Implications for subacromial impingement syndrome

Individuals with subacromial impingement syndrome (SAIS) demonstrate altered upper limb muscle coactivation during humeral elevation and overhead tasks. This study investigated the effect of an occupationally-relevant overhead task designed to induce global shoulder muscle fatigue on scapular stabilizer and deltoid-rotator cuff muscle coactivation, and upper limb kinematics. Upper limb muscle activity and kinematics were analyzed from 33 right-handed male (n = 17) and female (n = 16) participants before and immediately following a simulated overhead drilling task. Co-activation was generally not affected by fatigue, however scapulothoracic, thoracohumeral, and elbow angles all displayed significant responses to fatigue. Muscle activity and kinematics were highly variable, with a considerable number of participants displaying muscular and kinematic adaptations that would increase SAIS risk. Disadvantageous scapular kinematics adaptations to fatigue correlated with disadvantageous scapular stabilizer coactivity when working in postures requiring more scapular internal rotation.