What muscle is responsible for abducting the arm?

The muscle responsible for abducting the arm is the deltoid. The deltoid has three distinct parts (anterior, lateral, and posterior), with the lateral head being primarily responsible for the initial phase of arm abduction, particularly from 15 degrees to 90 degrees of movement. This muscle is located on the uppermost part of the arm and shoulder, providing a rounded contour to the shoulder.

While the supraspinatus also plays a crucial role in shoulder abduction by initiating the movement, particularly in the first 15 degrees before the deltoid takes over, its primary function is to stabilize the shoulder joint and assist the deltoid. Understanding this distinction helps clarify the roles of the muscles involved in arm abduction. The teres major, while important for other shoulder movements, does not contribute to abduction. Similarly, the infraspinatus is primarily involved in lateral rotation of the arm rather than abduction. Thus, the clear answer reflecting the primary muscle responsible for abducting the arm is the deltoid.