Muscle · Shoulders

Deltoid

Musculus deltoideus

The thick triangular cap that gives the shoulder its round outline. It is really three muscles wearing one name: front fibres from the collarbone that lift the arm forward, side fibres from the point of the shoulder that lift it out, and back fibres from the ridge of the shoulder blade that draw it behind you. All three converge on a single rough patch halfway down the outer arm bone, so which direction the deltoid pulls depends entirely on which part fires. It is the muscle people cannot use when the shoulder hurts overhead — and it is almost never the culprit. On its own its pull drags the head of the arm bone straight up into the bony roof above it; it only moves the arm cleanly when the rotator cuff holds that head down and the shoulder blade turns upward underneath it.