Various aspects of rotator cuff tears have been studied in depth, including the primary pathogenesis and mechanical properties. There is, however, a lack of information available on the injury mechanics associated with sudden torque loading of the shoulder. To investigate the mechanical response of the Supraspinatus tendon and the joint capsule when exposed to repeated near-instantaneous torques, an in vitro test apparatus was developed. Supraspinatus and capsular reaction forces, capsular surface strains, and shoulder accelerations were measured over 30 cycles of applied torque. A decreasing logarithmic decay was found in the difference between the initial and final loads within each cycle. A generalized force-displacement fiber recruitment model was used to determine the protective role of the capsule by evaluating changes in stiffness. The results suggest that while relatively more damage happens in the first few high-torque load cycles, sudden adduction loading injuries are likely fatigue-based in nature
