Triceps brachii muscle: Basic anatomy

Location

The triceps brachii forms the posterior compartment of our arm. It acts to extend the elbow and also extend the shoulder, in part. The muscle has a close relationship with the humerus and with the radial nerve. It is differentiated from the triceps surae of the leg, which consists of the two heads of gastrocnemius, and the soleus muscle. The other muscle that is present in the extensor compartment of the arm is anconeus (abducts the ulna during pronation).

Origins and insertions

The triceps brachii muscle is so named, as it is a muscle with three heads. The triceps brachii forms the bulk of the posterior or extensor compartment of the arm. The triceps is an extensor of the elbow, as well the long head acting as an extensor of the shoulder. Histological studies of the triceps muscle heads have shown that the three heads of triceps brachii consist of different types of muscle fibers.

The long head arises from the infraglenoid tubercle of the scapula, a small bony projection from the lower part of the rim of the glenoid fossa. It consists of type IIa fibers, and is therefore at maximal function when sustained contraction is required.

The lateral head arises from the posterior lateral surface of the humeral shaft, above the radial groove. It consists of largely type IIb fibers, and is therefore at maximal function when short intensity high power contraction is required.

The medial head arises from the posterior medial surface of the humeral shaft, below the radial groove. It consists of type I fibers, and is therefore required for continuous long-term contraction. It lies close to the radial sulcus and if it is overused, may compress the radial nerve as it descends. The radial sulcus is where the radial nerve and profunda brachii artery run in the arm. The three heads of triceps brachii converge into a single tendon that inserts onto the olecranon process.

The quadrangular space is bordered superiorly by teres minor, inferiorly by teres major, medially by the long head of triceps and laterally by humerus. The axillary nerve and posterior circumflex humeral artery pass through this space.

The triangular space is inferior to the quadrangular space, and has the same medial and lateral borders. It is however bordered superiorly by the teres minor muscle, inferiorly by teres major and laterally by long head of triceps. It allows the radial nerve to enter the arm, as well as the profunda brachii artery.

Innervation

The radial nerve (ventral rami of C5 - T1) is a branch of the posterior cord of the brachial plexus. Triceps is innervated by C6,7 and 8 component of radial nerve, a separate branch for each head. It passes through the triangular space accompanied by the profunda brachii branch of the brachial artery, and the circumflex scapular artery. Here it supplies the long head of triceps.

The nerve then passes into the radial/spiral groove of the humerus, and runs between the medial and lateral head of the triceps brachii and supplies them. It emerges by passing between brachialis and brachioradialis, and emerges anterior to the lateral epicondyle. The nerve continues to descend into the extensor compartment of the forearm, which it innervates via its branch, the posterior interosseus nerve. The radial sulcus is also known as the spiral groove or radial groove.

Blood supply

The blood supply to the triceps comes from the: 

• profunda brachii artery
• superior ulnar collateral artery
• posterior circumflex humeral artery

Anatomically the upper limb is divided into the arm and the forearm. The profunda brachii artery supplies all of the muscles of the flexor and extensor compartment of the arm.

The venous drainage is via the brachial vein that runs with the profunda brachii artery.

Action

The triceps acts as an elbow extensor and the long head acts as a shoulder extensor, as it arises above the shoulder joint i.e. from the infraglenoid tubercle of the scapula. It also supports the lower part of the capsule.