The triangular muscle that gives the shoulder its rounded shape is the deltoid. It caps the shoulder joint and creates the visible contour commonly associated with broad, rounded shoulders.
The deltoid has three named portions: anterior fibers arising near the clavicle, middle fibers arising from the acromion of the scapula, and posterior fibers arising from the spine of the scapula. Together, these sections attach to the deltoid tuberosity on the humerus. Their different positions allow the muscle to perform several actions rather than one simple movement.
When its fibers work together, the deltoid is especially important for lifting the arm out to the side, or shoulder abduction. The front portion assists with shoulder flexion and inward rotation, while the rear portion helps extend the shoulder and move the arm backward. The rotator-cuff muscles help stabilize the joint during these actions.
The name comes from the Greek capital letter delta, Δ, because the muscle’s outline is roughly triangular. A common mix-up is the trapezius: it spans the neck and upper back, whereas the deltoid forms the rounded cap of each shoulder.