Anatomy of Motion on Instagram: "Why does the rotator cuff get injured?
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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.
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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.
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However, if these muscles don't engage properly, the humerus head will roll upward, reducing the space available…