Effects of muscle fatigue on 3-dimensional scapular kinematics

Objective: To determine the effects of fatigue during an external rotation task on 3-dimensional scapular kinematics. Design: A single-group, pretest-posttest measurement design. Setting: Research laboratory. Participants: Thirty healthy subjects. Interventions: Not applicable. Main Outcome Measures: Three-dimensional scapular kinematics were recorded with a Polhemus magnetic tracking device during arm elevation in the scapular plane. Results: There was a significant fatigue effect for all scapular rotations in the early to middle phases of humeral elevation. Significantly less posterior tilting (up to 90degrees of elevation), external rotation (up to 120degrees of elevation), and upward rotation (up to 60degrees of elevation) were observed. Additionally, there were fair to good correlations (r range, .39-.60) between the changes in scapular posterior tilting and the amount of muscle fatigue. Conclusions: Fatigue in shoulder external rotation altered the scapular resting position and the movement of posterior tilting in the early range during arm elevation in the scapular plane. Observed changes in scapular kinematics may affect the amount of area in the subacromial space and facilitate impingement. Data regarding changes produced by fatigue of the external rotators may also help with the development of a model of diminished rotator cuff function.