Why does the rotator cuff get injured? . To raise your arm, the deltoid muscle needs to engage. It's well-suited for this task, but as it works, it rolls the head of the humerus upward. . In a healthy shoulder, this upward roll is balanced by muscles that insert near the head of the humerus from below, such as the subscapularis and teres minor. . However, if these muscles don't engage properly, the humerus head will roll upward, reducing the space available for the supraspinatus tendon to slide

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