Muscles in the arms
The arm is the part of the upper extremity between the shoulder joint and the elbow joint. It has a bone called the humerus, which is attached to the trunk with the help of the shoulder blade (scapula). The joint between the scapula (shoulder blade) and humerus is called the glenohumeral joint. The scapula connects the arm with the collar bone (clavicle). Various movements occur at the arm, such as raising the arm above the head, rotating the arm, extending the arm, and movement of the arm toward (adduction) and away (abduction) from the midline. These movements are enabled by muscles present in the shoulder, arm, and forearm.
The muscles of the arm may be divided into two groups:
The muscles in front of the arm (the anterior compartment): These muscles are mainly involved in the flexion of the arm at the elbow and shoulder joints, which helps in raising the arm. These muscles are supplied by a nerve called the musculocutaneous nerve. Their blood supply is through the muscular branches of the artery called the brachial artery. The muscles in the anterior compartment of the arm are:
- Biceps: The biceps or biceps brachii is the main muscle of the arm. It originates from the shoulder blade and is attached to the forearm bone (called the radius). Thus, it can cause movements at the shoulder and elbow joints. The term bicep comes from the fact that this muscle has two heads at the origin, the short and the long heads. The two heads fuse to form a single muscle. The long head of the biceps is prone to rupture because of injuries. The rupture produces a characteristic bulge in the arm called the “Popeye sign.” The functions of this muscle include flexion of the arm and supination of the lower arm or forearm. Supination is the movement of the forearm that makes the palm and forearm face upward.
- Brachialis: This muscle lies deeper than the biceps brachii. The muscle originates at the lower half of the front of the humerus and is attached to the inner bone of the forearm called the ulna. This muscle forms the floor of the area present under the elbow, called the cubital fossa. Due to its position, it is mainly involved in the flexion of the forearm at the elbow joint.
- Coracobrachialis: It is a long and slender muscle that extends from the shoulder blade (the coracoid process of the scapula) to the shaft of the humerus. The main function of this muscle is the flexion of the arm. It is also involved in the adduction of the arm (movement of the arm toward the trunk).
Muscle in the back of the arm (the posterior compartment): The posterior compartment is mainly supplied by the artery called the profunda brachii artery. The nerve supply is mainly through the radial nerve. The muscle present in this compartment is called the triceps brachii muscle. This muscle is called the triceps because it has three heads: the lateral, long, and medial heads. The long head originates from the shoulder blade whereas the lateral and medial heads arise from the back of the humerus. The heads unite to insert into the ulna (bone of the forearm). The muscle helps in the extension of the arm (movement of the arm toward the back) and the forearm (straightening of the forearm at the elbow joint). The muscle also helps to stabilize the shoulder joint.
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Alshammari SM, Bordoni B. Anatomy, Shoulder and Upper Limb, Arm Muscles. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020. https://www.ncbi.nlm.nih.gov/books/NBK554420/