The muscles of the shoulder. The muscles of the shoulder can be divided into two distinct groups.2 photos

The muscles of the anterior portion of the shoulder are responsible for bending the arm, while those of the posterior portion facilitate its extension. All the muscles located in the anterior part of the shoulder belong to the flexor group: - **Deltoid muscle**: It consists of two heads that merge together to form the main body of the muscle. This muscle gradually narrows as it descends, giving rise to a strong tendon. When the arm is straight at the elbow, the deltoid muscle bends the forearm. When the arm is bent, it acts as a powerful supinator, turning the forearm so that the palm faces upward. - **Biceps brachii**: It originates from the lower part of the anterior surface of the humerus and runs downward in front of the elbow joint. Its tendon attaches to the olecranon bone and the tubercle of the humerus. Regardless of the position of the forearm, the biceps brachii is the primary flexor muscle at the elbow joint. - **Coracobrachialis muscle**: This muscle originates from the upper edge of the coracoid process of the scapula and extends downward and outward, attaching to the inner surface of the humerus. It helps to bend the upper arm at the shoulder joint and also draws it toward the midline of the body. The muscles of the posterior portion of the shoulder are involved in extending the upper limb at the elbow joint, thereby straightening the forearm in line with the shoulder. Among these muscles is the **triceps brachii**, a large and powerful extensor muscle; there is also the **elbow flexor muscles**, which are much less significant. The triceps brachii has three heads: the long head, the lateral head, and the medial head. All three heads converge in the middle of the upper arm, forming a wide, flat tendon that extends downward, passing over a small bursa, and attaching to the olecranon bone. The primary function of the triceps brachii is to extend the arm at the elbow joint. Additionally, due to its location, the long head of the triceps helps to stabilize the shoulder joint. The **elbow flexor muscles**, which are triangular in shape, are located behind and below the elbow joint. Together with the triceps brachii, they play a role in bending the arm at the elbow joint and also help to stabilize it.