In Plain English
The shoulders are the rounded muscles that cap the top of each upper arm, extending from the collarbone and shoulder blade to the upper arm. Made up of three distinct heads, they lift your arm in different directions, rotate it, and help keep the shoulder stable through an enormous range of motion while giving the upper body much of its width.
Technical Definition
The shoulders refer primarily to the deltoid, a three-headed muscle consisting of the anterior, lateral, and posterior deltoid. It originates from the lateral third of the clavicle, the acromion, and the spine of the scapula before converging onto the deltoid tuberosity of the humerus. The anterior fibers contribute primarily to shoulder flexion, horizontal adduction, and internal rotation, the lateral fibers to shoulder abduction, and the posterior fibers to shoulder extension, horizontal abduction, and external rotation, with all three heads working together to stabilize the glenohumeral joint throughout arm movement.























