Shoulder
The deltoid muscle is located on the outer aspect of the shoulder and is recognized by its triangular shape. The deltoid muscle was named after the Greek letter Delta due to the similar shape they both share. The deltoid muscle is constructed with three main sets of fibers: anterior, middle, and posterior. These fibers are connected by a very thick tendon and are anchored into a V-shaped channel. This channel housed in the shaft of the humerus bone in the arm. The deltoid muscle is responsible for the brunt of all arm rotation and allows a person to keep carried objects at a safer distance from the body. It is also tasked with stopping dislocation and injury to the humerus when carrying heavy loads. One of the most common injuries to the deltoid muscle is a deltoid strain. Deltoid strain is characterized by sudden and sharp pain where injured, intense soreness and pain when lifting the arm out from the side of the body, and tenderness and swelling caused by (and located at) the deltoid muscle.
Credit: https://www.healthline.com/human-body-maps/deltoid-muscle#1
