The deltoid is a large, three-headed, thick, triangular muscle which originates from the clavicle and the scapula at the rear of the shoulder and extends down to its insertion in the upper arm.
BASIC FUNCTION: To rotate and lift the arm. The anterior deltoid lifts the arm to the front; the medial deltoid lifts the arm to the side; the posterior deltoid lifts the arm to the rear.
The trapezius, is the flat, triangular muscle that extends out and down from the neck and then down between the shoulder blades.
BASIC FUNCTION: To lift the entire shoulder girdle, draw the scapula up, down, and to either side, and help turn the head.