Properly performed deadlifts primarily strengthen and tone your upper and lower back muscles; however, deadlifting a barbell also requires some work from the shoulder muscles. The rotator plays a small yet important stabilizing role in the deadlift motion and thus experiences some stress from performing the exercise.
When you hold the bar in front of your body while performing a deadlift, the force of the bar pulling down on your shoulder causes the humerus, or upper arm bone, to pull out away from the shoulder joint. The rotator cuff engages in opposition to the downward force of the hanging bar, which helps keep the head of the humerus bone secured in the shoulder joint.
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Although the rotator cuff serves a relatively minor stabilizing function in the deadlift, stress from supporting the shoulder joint while lifting may aggravate a nagging rotator injury. Avoid using excessive weight, especially when rehabbing from an injury, and consult your physician before starting a deadlifting program or if you feel any pain.