The deltoid muscle, located on the outside of the shoulder, helps rotate the upper arm and extend the arm at the shoulder. The deltoid has three different sets of fibers that help perform specific tasks.
The front (or anterior) fibers of the deltoid allow the arm to flex at the shoulder, moving the upper arm upward and to the front. The anterior fibers also rotate the upper arm medially, or inward around the axis of the bone.
The side (or lateral) fibers of the deltoid move the arm at the shoulder, moving the upper arm upward and to the side. The rear (or posterior) fibers extend the arm at the shoulder, moving the upper arm downward and to the rear. The posterior fibers also rotate the upper arm laterally, or outward around the bone axis.
By allowing these movements, the deltoid muscle helps lift the arms, do jumping jacks, lift objects, wave or hug. It also allows a person to carry objects at a safe distance from the body and helps prevent injury when carrying heavy objects.