Rotator cuff tendon tear causes alterations to cuff muscle architecture and tendons including muscle fascicle contracture and increased tendon length, fatty infiltration of the muscle fibers, muscle fiber pennation angle changes, asymmetric muscle atrophy, and altered muscle fiber type composition, e.g. [1]. These changes ultimately result in a reduction in muscle force, and frequently lead to a reduction of shoulder strength and a loss of functionality. Recently, division of the cuff tendons in a manner related to cuff tear has been shown to alter tendon excursions and cause muscle moment arms reductions in a cadaver experiment [2] and a computational study [3]. Evaluations of the effects of cuff tear size and location on cuff muscle moment arms have not been conducted. Consequently, the mechanisms by which the muscle force and moment arm changes affect joint strength are not well understood. An improved understanding of these relationships would increase potential for rehabilitation of joint strength following cuff repair. Models for evaluating rotator cuff tear are non-existent since subject-specific models have focused on healthy normal shoulders. Consequently, models have not been used to quantify the effects of clinically observed changes in muscle and tendon architecture on muscle moment arm and force generating capacity.

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