The function of the deltoid muscle is mainly related to shoulder _____.

The deltoid muscle plays a critical role in the movement of the shoulder joint, and its primary functions include abduction, flexion, and extension of the arm. This muscle is divided into three distinct parts: the anterior (front), lateral (middle), and posterior (back) fibers.

The anterior fibers contribute to flexion by raising the arm forward, while the lateral fibers are primarily responsible for abduction, allowing the arm to lift away from the body. The posterior fibers assist in extension, which involves moving the arm backward. Since the deltoid muscle effectively encompasses all these movements through its various parts, it is appropriate to state that its function is mainly related to shoulder abduction, but it also supports the actions of flexion and extension. As a result, the correct answer accurately reflects the comprehensive involvement of the deltoid muscle in these essential shoulder movements.