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

Subscapularis muscle

10 Best Exercises To Strengthen Your Rotator Cuff - BuiltLean