Medial Head of Triceps Brachii Muscle: Anatomy, Function, Imaging

Topic

Medial head of triceps brachii muscle

The medial head of the triceps brachii is one of the three heads of the triceps muscle, located in the posterior compartment of the arm. It lies deep to the long and lateral heads, forming the core muscular mass that extends from the posterior humeral shaft to the olecranon process of the ulna.

The medial head is the most powerful and consistently active portion of the triceps, engaged in all forms of elbow extension, including low-resistance and sustained contractions. Its deep location makes it less visible superficially but crucial for maintaining elbow stability and extension strength.

Synonyms

Deep head of triceps
Medial humeral head of triceps

Origin, Course, and Insertion

Origin: Posterior surface of the humerus below the radial (spiral) groove, extending to the area proximal to the medial epicondyle, and the medial intermuscular septum.
Course: Fibers pass downward and medially, converging into a broad, thick tendon that fuses with tendons of the long and lateral heads.
Insertion: Posterior surface of the olecranon process of the ulna, via the common triceps tendon; also gives expansions into the antebrachial fascia over the elbow.

Tendon Attachments

Merges with the common triceps tendon at the posterior aspect of the elbow.
Inserts into the olecranon process and continues as a fibrous expansion over the anconeus and forearm fascia.
Provides strong posterior reinforcement to the elbow capsule.

Relations

Anteriorly: Humerus and radial nerve (which runs in the spiral groove above its origin).
Posteriorly: Long and lateral heads of triceps.
Medially: Ulnar nerve near the medial intermuscular septum.
Laterally: Lateral head and radial groove region.
Inferiorly: Olecranon process and elbow joint capsule.

Nerve Supply

Radial nerve (C6–C8) — branch to the medial head arises within the radial groove and supplies it before entering the lateral and long heads.
Occasionally, a separate branch to the anconeus arises from the same fascicle.

Arterial Supply

Deep brachial (profunda brachii) artery — via its muscular branches and collateral network around the elbow.

Venous Drainage

Companion veins of profunda brachii artery, draining into the brachial veins.

Function

Primary extensor of the forearm: Provides sustained and precise elbow extension.
Postural role: Active during stabilization of the elbow during forearm movements.
Synergistic activity: Works with anconeus and other triceps heads during pushing, throwing, and weight-bearing tasks.
Clinical role: Predominantly active in slow or low-force extension, while long and lateral heads assist during high power activities.

Clinical Significance

Muscle strain or tear: Rare but may occur with resisted elbow extension or weightlifting.
Radial nerve injury: Affects medial head first due to its proximity to the nerve in the spiral groove.
Posterior arm trauma: Deep hematomas or compartment syndrome may involve medial head fibers.
Triceps tendinopathy: Involves shared tendon near olecranon insertion.
Surgical relevance: Important landmark in posterior arm approaches, and protection of the radial nerve.

MRI Appearance

T1-weighted images:
- Muscle belly: Intermediate signal intensity, clearly demarcated from surrounding fat.
- Tendon: Low signal (dark linear band) merging into common triceps tendon.
- Marrow (humerus): Bright fatty signal adjacent to origin.
- Pathology: Partial tear or denervation changes appear as focal or diffuse increased T1 signal from fatty infiltration.
T2-weighted images:
- Normal muscle: Intermediate-to-low signal, darker than on T1.
- Tendon: Uniformly dark.
- Pathology: Edema, inflammation, or partial tear appear as bright hyperintense regions at musculotendinous junction or tendon insertion.
- Fluid collections or hematoma: Bright hyperintensity with surrounding edema.
STIR:
- Normal muscle: Intermediate-to-dark signal.
- Pathologic muscle: Bright hyperintense signal indicating edema, contusion, or inflammation.
- Ideal for detecting early denervation edema or acute muscle injury.
Proton Density Fat-Saturated (PD FS):
- Normal muscle: Intermediate-to-dark homogeneous signal.
- Partial tears or tendinopathy: Focal bright signal at olecranon attachment or muscle belly.
- Peritendinous fluid or fascial edema: Bright hyperintensity along tendon margins.
T1 Fat-Sat Post-Contrast:
- Normal muscle: Mild uniform enhancement.
- Inflammation, myositis, or tear: Focal or diffuse enhancement corresponding to vascularized granulation tissue.
- Chronic tendinopathy: Peripheral enhancement with central low-signal fibrosis.

CT Appearance

Non-Contrast CT:

Muscle belly: Soft-tissue density posterior to humerus, deep to long and lateral heads.
Tendon: Dense linear structure inserting on olecranon.
Pathology: Detects calcification, partial avulsion, or bony changes at olecranon enthesis.
Useful in: Triceps tendon rupture evaluation, bone avulsion, or fracture involvement.

Post-Contrast CT (standard):

Normal muscle: Homogeneous enhancement pattern.
Inflamed or injured muscle: Focal increased enhancement in acute strain or tear.
Chronic lesions: May show thickened, irregular tendon with peritendinous enhancement.
Valuable for postoperative evaluation and trauma correlation.

MRI image