BACKGROUND: The upper and lower portions of the subscapularis muscle are independently innervated and activated. HYPOTHESIS: Upper and lower portions of the subscapularis muscle demonstrate different activation levels and require different exercises for rehabilitation. STUDY DESIGN: Controlled laboratory study. METHODS: Fifteen healthy subjects performed seven shoulder-strengthening exercises. Electromyographic data were collected from the latissimus dorsi, teres major, pectoralis major, infraspinatus, supraspinatus, and upper and lower subscapularis muscles. RESULTS: Upper subscapularis muscle activity was greater than lower subscapularis muscle activity for all exercises except for internal rotation with 0 degrees of humeral abduction. The push-up plus and diagonal exercises consistently stressed the upper and lower subscapularis muscles to the greatest extent. CONCLUSIONS: Humeral abduction was found to have a strong influence on the selective activation of the upper versus the lower subscapularis muscle and thus supported the design of different exercise continuums. In addition, the push-up plus and diagonal exercises were found to be superior to traditional internal rotation exercises for activating both functional portions of the subscapularis muscle. CLINICAL RELEVANCE: Our results showing that the upper and lower portions of the subscapularis muscle are functionally independent may affect training or rehabilitation protocols for the rotator cuff muscles. Copyright 2003 American Orthopaedic Society for Sports Medicine