Named for its three heads, the long, medial, and lateral, triceps brachii is one of the larger muscles on the back of the upper limb. Arising from the scapula’s infraglenoid tubercle, the long head travels in front of the teres minor and behind the teres major. Visible only distally on the humerus, the medial head arises from four different areas; from behind the humerus, from within the radial nerve, from the medial intermuscular septum; and also from the lateral intermuscular septum. The final, lateral head arises from the rear of the humerus, and spans down to the region of the lateral intermuscular septum. All three converge into a single tendon which itself inserts into the ulna and to the posterior wall of the elbow joint capsule.
Mainly responsible for elbow joint extension or in laymen’s terms, straightening the arm, the triceps is an extremely important muscle with many functions. When painting or writing (or performing any action requiring fine dexterity), the triceps locks the elbow in place. When there is a need for dual control of elbow and shoulder, or when sustained force is required, the long head comes into play. When high-intensity force is called for, the lateral head is engaged, and for precise, low force actions, the medial. While it has historically been believed that innervation comes from the radial nerve, it has been proven that in fact the axillary nerve is responsible.