As soon as we hear arm muscles, we think biceps. But there are also other muscles in the arm and together they completely wrap the humerus, the arm bone. In anatomy, ‘arm’ only refers to the part of the upper limb from the shoulder joint to the elbow joint.
Find out where the muscles of the arm are located, to what structures they are attached and what their functions are. Learning arm muscle anatomy will also help you better understand how arm muscle exercises tone these muscles.
The muscles of the arm are:
- biceps brachii
- triceps brachii
- coracobrachialis
- brachialis
Arm Muscles – Biceps and Coracobrachialis
The biceps muscle, or the biceps brachii, arises from two places on the shoulder blade bone or the scapula. They are called its two heads – a short head and a long head. The short head starts from the coracoid process, a projection on the scapula. The long head starts from above the glenoid cavity, the shallow depression on the scapula that forms part of the shoulder joint. It first runs above the head of the humerus, the arm bone, within the shoulder joint. The fleshy part of the muscle occupies most of the front of the arm.
Both parts of this muscle join towards the lower third of the humerus and form a short, thick tendon which attaches itself to the upper end of the radius, one of the two forearm bones. The biceps tendon also forms an extension called the bicipital aponeurosis, a sort of thickened fascia that runs round the forearm to attach itself to the back of the ulna, the other forearm bone.
The coracobrachialis is a smaller muscle that occupies the front of the arm along with the biceps. It starts from the coracoid process of the scapula, same as the short head of the biceps. It travels along with the short head, but leaves it at the level of the middle of the humerus to attach itself to the inner surface of the arm bone.
Both the muscles are controlled by the musculocutaneous nerve, a branch of the lateral cord of the brachial plexus. While the biceps short head and the coracobrachialis flex the shoulder joint, the long head of the biceps helps stabilize the humerus at the shoulder joint. Also, since the aponeurosis gives the biceps an attachment to the back of the forearm, the muscle also helps rotate the forearm along its long axis so that the palm of the hand comes to face above. This action is called supination.
Brachialis Muscle
This muscle begins with an attachment to the front of the lower half of the arm bone humerus. It then crosses the elbow joint from the front and attaches itself to the ulna in the forearm. This muscle is also controlled by the musculocutaneous nerve.
The brachialis muscle causes flexion of the elbow when it contracts, that is, it helps fold the forearm to touch the arm.
Triceps Muscle
The triceps arises from three places and has three heads:
- long head – arises from just below the glenoid cavity of the scapula
- lateral head – starts from the back surface of the upper part of the humerus
- medial head – arises from the back surface of the humerus, below the groove for the radial nerve
The three heads unite to form the fleshy part of the triceps and run along the back of the humerus, forming the bulk of the back of the arm. The muscle then forms a strong tendon that attaches itself to the olecranon process of the ulna. This process is the prominent bone you can feel at the tip of your elbow.
The triceps is controlled by the radial nerve. When it contracts, the triceps causes extension at the elbow joint, that is, it straightens out or unfolds the upper limb. The long head also helps stabilize the shoulder joint similar to the long head of the biceps.
The arm muscles, biceps, triceps, brachialis and coracobrachialis, all arise from either the scapula or the humerus and get attached to the arm bone or the forearm bones. They form the fleshy part of the arm, almost completely covering the humerus.
Sources
Romanes, G.J. “Cunningham’s Manual of Practical Anatomy” (Oxord University Press; 15th Edition, 2003)
Lakshmi Ananth - Lakshmi Ananth is a practising ENT, head and neck surgeon and has faculty position at a medical college. She is specially interested in ...
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