Which muscle is primarily responsible for abducting the glenohumeral joint?

The deltoid muscle is primarily responsible for abducting the glenohumeral joint due to its anatomical position and muscle fiber orientation. The deltoid consists of three distinct sections: anterior, lateral, and posterior. It is particularly the lateral fibers of the deltoid that perform abduction of the arm, especially when the arm is lifted from the side to about shoulder height.

When the deltoids contract, they pull the humerus away from the body, facilitating movement in the frontal plane. While other muscles may play a role as synergists in arm movement, the deltoid is the main muscle activated for this specific action of abduction.

In comparison, the pectoralis major primarily functions as an adductor and internal rotator of the shoulder, while the trapezius and the triceps brachii serve different roles in shoulder and arm movement. The trapezius is more involved in elevation and stabilization of the shoulder girdle, and the triceps brachii is mainly responsible for elbow extension. Therefore, the deltoid's role as the primary abductor of the glenohumeral joint is well-defined in functional anatomy.