Triceps brachii muscle

Synonyms: three-headed upper arm muscle, arm extensor, triceps
German: Musculus triceps brachii

1 Definition

The triceps brachii muscle is a three-headed muscle that belongs to the muscles of the upper arm. The three heads of the muscle are called "Caput longum", "Caput laterale" and "Caput mediale".

2 Devolution

2.1 Origin

Two heads of the triceps brachii muscle have their origin at the humerus (upper arm bone), the third head at the scapula (shoulder blade):

• The fleshy origin of the medial head of the triceps (Caput mediale) lies on the back (Facies posterior) and at the medial margin (Margo medialis) of the humerus, medial and distal to the radial sulcus. In its upper part, this muscle head is becoming stout and tapering.
• The fleshy origin of the lateral head of the triceps (Caput laterale) lies on the back surface (Facies posterior) and at the lateral margin (Margo lateralis) of the humerus, lateral and proximal to the radial sulcus.
• The origin tendon of the long head of the triceps (Caput longum) passes through the joint capsule of the shoulder joint to its origin on the infraglenoid tubercle of the scapula. The fibers of this part of the muscle passes by between the teres major muscle and the teres minor muscle in distal direction. Then, they radiate between the other two triceps heads and unite with them to form the insertion tendon of the muscle.

Most parts of the medial triceps head are covered by the lateral and long heads. You can just see its distal part from the outside.

2.2 Insertion

The tendineous insertion of the triceps brachii muscle is formed by the olecranon of the ulna. The insertion tendon already starts in the middle of the muscle. It consists of two combined aponeuroses, of which one covers the lower part of the muscle to the outside, while the other one is passing deep into the muscle mass.

Thus, some parts of the triceps brachii muscle pass over 2 joints, the shoulder and the elbow joint.

3 Innervation

The innervation of the triceps brachii muscle is provided by the radial nerve from the brachial plexus (segments: C6 to C8). More recent findings show that the long triceps head frequently is also innervated by a branch of the axillary nerve. ^[1]

4 Function

Together with the anconeus muscle, the triceps brachii muscle is responsible for the extension of the forearm in the elbow joint. Thus, it works as direct antagonist of the biceps brachii muscle and of the brachialis muscle. Another task is the fixation of the elbow joint during the precision movements of the hand, eg. when writing.

The Caput longum drawing to the scapula effects a retroversion and a slight adduction of the upper arm.

5 Sources

1. ↑ de Se`ze MP et al.: "Does the motor branch of the long head of the triceps brachii arise from the radial nerve?". Surg Radiol Anat 26 (6): 459-461; 2004 PMID 15365769