Final answer:
The deltoid muscle is a large, triangular-shaped muscle that covers the shoulder and is responsible for various movements of the arm. Its posterior fibers are specifically located in the back of the shoulder.
Step-by-step explanation:
The deltoid muscle is a large, triangular-shaped muscle that covers the shoulder. It is responsible for abducting the arm (moving it away from the midline), as well as flexing and medially rotating it. The deltoid is named after the Greek letter delta, which resembles a triangle. Its posterior fibers are specifically the part of the deltoid muscle that are located in the back of the shoulder.