The shoulder muscles are a group of three muscles that make up the shoulder joint. The deltoids, or delts, are the most prominent and visible of the shoulder muscles. They are triangular in shape and span the shoulder joint, connecting the upper arm to the shoulder blade. The deltoids are responsible for shoulder abduction, flexion, and rotation. The other two shoulder muscles are the supraspinatus and the infraspinatus, which are located beneath the deltoids and are responsible for shoulder stability and movement.