What is the primary function of the deltoid muscle?

The deltoid muscle primarily functions to abduct the arm at the shoulder, making it essential for raising the arm away from the body. The deltoid is a large, triangular muscle that covers the shoulder joint and is divided into three distinct parts: the anterior, lateral, and posterior fibers. The lateral fibers are particularly important for initiating abduction, starting at approximately 15 degrees, with contributions from the supraspinatus muscle up to about 90 degrees.

This abduction action is crucial in many daily activities, such as reaching overhead or lifting objects to the side. Understanding the role of the deltoid helps clarify its significance in upper extremity movement, particularly in conjunction with other muscles that support shoulder stability and mobility.