Deltoid Muscles

The deltoid is the triangular muscle of the shoulder that forms the rounded flesh of the outer part of the upper arm. It passes up and over the shoulder joint. The wide end of the triangle is attached to the shoulder blade (scapula) and the collarbone (clavicle). The apex of the triangle is attached to the upper arm bone (humerus), about halfway down its length. Its strongest point is the central section, which raises the arm sideways. The front and back parts of the muscle twist the arm. The ligaments which connect the muscles to the bone endings are called "deltoid ligaments." Deltoid comes from the Greek word "deltoeides," meaning shaped like a (river) delta.