Background: In massive rotator cuff tears a teres major (TMj) tendon transfer to the insertion of the supraspinatus (SSp) reverses its adduction moment arm into abduction which is supposed to be an adequate salvage procedure. Analysis of muscle function to find biomechanical ground of such success is scarce. Methods: We compared pre- and postoperative clinical outcome of TMj transfer, i.e. Range of Motion, pain, Constant Shoulder scores and arm force. TMj activation was evaluated in 14 patients suffering massive cuff tears using activation ratios to describe the desired 'in-phase' and undesired 'out-of-phase' contribution to the external arm moment. Additionally, we analyzed activation of the latissimus dorsi (LD) and the medial part of the deltoids (DE). The activation ratios were compared to controls and TMj activation ratios were related to clinical outcome. Findings: TMj tendon transfer improved arm function. Pre-operatively, we observed 'out-of-phase' abduction activation of TMj and LD. After transfer patients activated TMj according to its new anatomical position. 'Out-of-phase' LD abduction activation persisted. The clinical improvements coincided with changes in activation ratio of TMj. Interpretation: 'Out-of-phase' TMj adductor activation is associated with compromised arm function in patients with irreparable cuff tears. After transfer, TMj is activated in correspondence with its new anatomical function, which was supportive for the improved arm function.