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Introduction: The roles of the pectoralis major (PM) and latissimus dorsi (LD) were studied in a new biomechanical model of massive rotator cuff tear. Methods: Five normal cadaveric shoulders with intact muscular insertions were examined using a custom shoulder testing device. Specimens were tested in three different humeral rotation positions at 0°abduction within the scapular plane. Testing was performed for the intact, with the supraspinatus removed (SS), and with the supraspinatus/infraspinatus/teres minor removed (SS/IS). The rotator cuff muscles, the PM, and the LD were each loaded with 40 N while the deltoid was loaded with 80 N. The PM and LD muscles were loaded or unloaded to determine their effect. Glenohumeral joint forces, humeral head kinematics, and acromiohumeral contact area and pressure were measured. ANOVA with a p value of 0.05 was used for statistical analyses. Results: At 0°abduction, 0°rotation there was a significant in-