Background: Sitting posture influences movements of scapulothoracic and glenohumeral joints and changes the shoulder muscle activities. The development and maintenance of correct sitting posture is important for the fundamental treatment of shoulder pain during rehabilitation. Objects: The purpose of this study was to investigate the effects of the sitting postures and the shoulder movements on shoulder muscle activities for both male and female. Methods: Twenty-eight subjects without shoulder-related diseases participated in this experiment. The subjects had randomly adopted three different sitting postures (upright posture, preferred posture, maximum slouched posture) and shoulder flexion angles in scapular plane (30˚, 90˚, 120˚). Surface electrodes were collected from upper trapezius (UT), anterior deltoid (AD), and posterior deltoid (PD) and the active shoulder range of motion was measured in each sitting posture and shoulder flexion angle. Results: The active range of motions of the shoulder external rotation and the flexion in the scapular plane decreased from the upright posture to the maximum slouched posture (p<.05, mixed-effect linear regression with random intercept, Tukey post-hoc analysis). All muscles showed the highest EMG activities at 120˚ shoulder flexion with the maximum slouched posture and did not show the gender differences. Conclusion: Increased shoulder muscle activities may become the potential risk factor for the shoulder impairment and pain if people continuously maintain the maximum slouched posture. Therefore, an upright position is necessary during shoulder exercises, as well as in activities of daily living, including motions involving lifting the arms.