What are the primary functions of the deltoid muscles?

The primary functions of the deltoid muscles center around the movement and stabilization of the shoulder joint. These muscles are crucial for various actions involving the humerus, which is the bone of the upper arm.

The deltoids consist of three distinct heads: the anterior (front), lateral (middle), and posterior (back) fibers. Each head contributes to a range of movements. The anterior fibers facilitate shoulder flexion, the lateral fibers are primarily responsible for abduction of the arm, while the posterior fibers assist in extension and external rotation of the humerus.

Therefore, when considering the functions of the deltoid muscles, they indeed play a key role in horizontal abduction (like moving the arm away from the body in a horizontal plane), flexion (lifting the arm forward), extension (lifting the arm backward), and rotation (turning the arm inward or outward). This comprehensive range of movements supports versatility in upper-body activities.