Shoulder abduction occurs when you move your arm away from the midline of your body, as when doing jumping jacks. A common abduction exercise is the lateral dumbbell raise.
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Shoulder Joint Structure
Your shoulder joint is a ball and socket structure formed by the upper head of the humerus, or upper arm bone, and the scapula. This unique structure allows for a broad spectrum of movement, making the shoulder joint the most mobile joint in the body. Many muscles and bony structures work together to produce movement of the arm from the shoulder joint. During abduction, movement is initially generated by the rotator cuff, then carried out by the deltoid muscle.
Rotator Cuff Muscles
The rotator cuff is a set of four muscles that serve to stabilize and rotate the shoulder, attached by strong tendons to the upper head of the humerus. During abduction, the supraspinatous and infraspinaoous muscles abduct and rotate the head of the humerus. The subscapularis abducts and depresses the head of the humerus to prevent impingement on the overhanging acromion that protrudes from the scapula. The long head of the biceps, which attaches to the scapula, also presses down on the humeral head as it abducts.
Once movement has been initiated by the rotator cuff, the powerful deltoid muscle, originating from the acromium and attaching at the mid-shaft of the humerus, takes over to lift the arm further to the side. According to the American Medical Association, functional shoulder abduction can take place up to 120 degrees when the humerus is externally rotated, placing the hands in a palms forward, thumbs up position.