OBJECTIVE: To compare quantitative muscle activation between erect and slouched sitting postures in the muscles around the scapula, and to investigate the correlation between the angle of thoracic kyphosis and the alteration of muscle activity depending on two different sitting postures. METHODS: Ten healthy males participated in the study. Unilateral surface electromyography (SEMG) was performed for serratus anterior, middle trapezius (MT), and lower trapezius (LT), which are scapular stabilizer muscles, as well as latissimus dorsi. Participants elevated their shoulders for 3 seconds up to 90° abduction in the scapular plane, tilting 30° anterior in the coronal plane. They were told to hold the position for 10 seconds and voluntary isometric contractions were recorded by SEMG. These movement procedures were conducted for three times each for erect and slouched sitting postures and data were averaged. RESULTS: Activities of MT and LT increased significantly more in the slouched sitting posture than in the erect one. There was no significant correlation between kyphotic angle and the area under curve of each muscle. CONCLUSION: Because MT and LT are known as prime movers of scapular rotation, the findings of this study support the notion that slouched sitting posture affects scapular movement. Such scapular dyskinesis during arm elevation leads to scapular stabilizers becoming overactive, and is relevant to muscle fatigue. Thus, slouched sitting posture could be one of the risk factors involved in musculoskeletal pain around scapulae.