Triceps Brachii Muscle: Basic Anatomy
Why do we need such a powerful extensor of the elbow? Is gravity not sufficient? When in daily life do we need to powerfully extend our elbows? The answer is: not very often. The reason we have such a powerful muscle in the extensor compartment of the elbow is due to our evolutionary ancestors who needed powerful extensor to climb trees and hang from branches.
In this article we will discuss the gross and functional anatomy of the triceps brachii. We will also discuss the clinical relevance of the structure, and provide a summary of key points at the end of the article. We will finally conclude with some review questions to test the reader’s understanding of the article content.
The triceps brachii forms the posterior compartments of our arms. They act 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 & 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 sulcus. 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 sulcus. 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.
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.
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.
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.
Radial Nerve Palsy
The nerve then passes into the radial/spiral groove of the humerus, and runs between the medial and lateral head of the triceps brachii, where it is most commonly affected by fracture of humerus. The medial head is closely associated with the spiral groove, so overuse can impinge the nerve here. Causes of radial nerve palsy include fracture of the humerus, compression of the axilla, or overuse of the triceps e.g. benchpressing is a weight lifting exercise. Compression of the nerve against humerus also occurs known as Saturday night palsy. The patient presents with wrist drop (forearm extensor denervated) and loss of sensation to the posterior arm and forearm.
This reflex is innervated by C7 component of radial nerve and can be elicited by tapping on the triceps tendon as it crosses the olecranon fossa with a tendon hammer, when the arm of the patient is relaxed. A brisk reflex can be a sign of an upper motor neurone lesion. An absent or diminished reflex can be a sign of a lower motor neurone lesion.