Corpus: Triceps brachii muscle

Origin

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

• The fleshy origin of the medial head of the triceps lies on the posterior side and the medial margin of the humerus, medial and distal to the radial groove. The upper part of this muscle head is pointed and compact.
• The fleshy origin of the lateral head of the triceps lies on the posterior surface and the lateral margin of the humerus, lateral and proximal to the radial groove.
• The tendon of origin of the long head of the triceps runs through the joint capsule of the shoulder joint to its origin at the infraglenoid tubercle of the scapula. The fibers of this part of the muscle run distally between the teres major and teres minor muscles, then radiate between the two other triceps heads and unite with them in the muscle's insertion tendon.

The medial head of the triceps is largely covered by the lateral and long head. Only its distal part is visible from the outside.

Insertion

The tendinous insertion of the triceps brachii muscle is the olecranon of the ulna. The tendon begins in the middle of the muscle. It consists of two composite aponeuroses, one of which covers the lower part of the muscle on the outside, while the other extends deep into the muscle mass. Some parts of the triceps brachii muscle thus run over 2 joints, the shoulder and elbow joints.

The triceps brachii muscle is innervated by the radial nerve from the brachial plexus (segments: C6 to C8). According to more recent findings, the long head of the triceps is often also supplied by a branch of the axillary nerve.

Together with the anconeus muscle, the triceps brachii muscle is responsible for the extension of the forearm in the elbow joint. It therefore acts as a direct antagonist of the biceps and brachialis muscles. Another task is to fix the elbow joint during fine movements of the hand, e.g., when writing. The long head, which pulls towards the scapula, also causes retroversion and weak adduction of the upper arm.

Injuries to the triceps muscle can occur because of severe trauma. More frequently, however, minor injuries are sufficient to cause severe damage in cases of underlying tendinopathy, e.g., diabetes mellitus, chronic renal insufficiency, gout, or rheumatoid arthritis.