What Humeral Position is Better to Prevent Shoulder Impingement during Arm Elevation?

What Humeral Position is Better to Prevent Shoulder Impingement during Arm Elevation?

Background: Secondary impingement can be caused by an imbalance in the force couple or co-activation of the deltoid and rotator cuff, as well as by an imbalance in the scapular upward rotators. Purpose: The purpose of this study was to evaluate the effects of humeral position and shoulder flexion angle on the activity of the scapular upward rotators, infraspinatus, and deltoid during arm elevation. Study design: Cross-sectional comparative study. Methods: Twenty-three healthy participants were included in this study. Electromyography (EMG) electrodes were attached to the upper trapezius, lower trapezius, serratus anterior, infraspinatus, anterior deltoid, and middle deltoid. The participants were seated on a chair with no arm rest and instructed to maintain the trunk in an upright position. They were then asked to elevate the arm until it reached a bar set at 90 degrees and 120 degrees elevation. The movement was performed in the scapular plane to ensure shoulder joint stability. Participants were then asked to grasp a 2-kg dumbbell and maintain arm elevation for 5 seconds in the palm-down, neutral, and palm-up position. Muscle activity was measured during maximal voluntary isometric contraction (MVIC), and %MVIC values were obtained. Two-way repeated-measured analysis of variance was used for the analysis; when significant differences were found in the %MVIC values of each muscle according to the arm elevation position, post hoc paired-sample t-test with Bonferroni correction were performed. Results: The EMG activity of the upper trapezius, lower trapezius, serratus anterior, infraspinatus, anterior deltoid, and middle deltoid varied significantly according to the shoulder flexion angle (all p<0.05) and according to the humeral position (all p<0.05) in all muscles except the lower trapezius. Conclusions: We recommend that consideration be given to the humeral position and shoulder flexion angle during the performance of strengthening exercises for the scapular upward rotators and shoulder external rotators.