The aim of this study was to clarify the influence of changes in the angle of horizontal shoulder flexion on the muscle activities of the clavicular and sternal parts of the pectoralis major. Surface electromyography (EMG) was performed on the clavicular and sternal parts of the pectoralis major of 10 healthy male subjects (mean age: 24.2 ± 2.5 years). Recordings were taken with the shoulder maintained in horizontal flexion on the side being measured. The angles of horizontal shoulder flexion were 0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80°, and 90°, and each position was maintained for 5 s. The EMG activity of the clavicular portion of the pectoralis major significantly increased during horizontal shoulder flexion of 70°, 80°, and 90° (p < 0.05). However, the EMG activity of the sternal part of the pectoralis major demonstrated no significant change at any angle of horizontal shoulder flexion. These results suggest that the clavicular part of the pectoralis major is involved in pulling the humerus to the clavicle between 70° and 90° of horizontal shoulder flexion, and the sternal part of the pectoralis major does not participate in maintaining the angle of horizontal shoulder flexion.
