Serratus Anterior Fatigue Reduces Scapular Posterior Tilt and External Rotation During Arm Elevation

Abstract

Context: Fatigue of the scapular stabilizing muscles resulting from repeated arm motion has been reported to alter scapular kinematics, which could result in shoulder pathology, especially impingement. Objective: This study aimed to examine the effects of fatigue of the serratus anterior muscle on scapular kinematics, specifically, decrease scapular posterior tilt and upward scapular rotation during arm elevation. Design: Repeated measures. Setting: Laboratory. Participants: Thirty participants were included in the investigation. Interventions: Scapular kinematics and shoulder strength were measured before and immediately following a serratus anterior fatigue protocol. Main Outcome: Scapular 3-dimensional position during arm elevation. Results: No difference in upward rotation of the scapula between prefatigue and postfatigue conditions (ascending: P = .188; descending: P = .798). Scapular posterior tilt decreased during arm elevation following the fatigue protocol between 60° and 90° and 90° and 120° of arm elevation during the ascent (P = .004) and the descent (P = .013). Fatigue by arm elevation angle interaction was found for clavicular elevation during the ascent (P = .050) between 90° and 120° of arm elevation. Scapular internal rotation increased during the ascent (P = .027). There was no difference in clavicular protraction between the prefatigue and postfatigue conditions (ascending: P ≤ .001; descending: P ≤ .001). Conclusion: Fatigue of the serratus anterior decreases posterior scapular tilt and greater clavicular elevation and scapular internal rotation at higher arm elevation angles. These findings are consistent with the scapular kinematic patterns associated with shoulder pain. Improving serratus anterior endurance might delay the changes in scapular kinematics associated with repeated arm motion and shoulder injury mechanisms.