Triceps (lateral head) !
The lateral head of the triceps lies on the outer back of the upper arm; it straightens the elbow with force, and the radial nerve runs beneath it.
Official name: Triceps brachii, lateral head
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What it is
The outer head of the triceps, on the back of the humerus.
What it does
Straightens the elbow with force.
Key facts
- Origin
- Posterior humerus above the radial groove; lateral intermuscular septum
- Insertion
- Olecranon
- Action
- Extends the elbow
- Nerve
- Radial nerve (C6–C8)
- Blood supply
- Deep brachial artery
Relations
- Deep
- Radial nerve and deep brachial artery in the radial groove, then the triceps (medial head)
- Medial
- Triceps (long head)
- Above
- Rear deltoid, overlapping its upper part
- Lateral
- Lateral intermuscular septum, which the radial nerve pierces in the lower third of the arm; the brachioradialis arises below
How to find it
On the upper outer back of the arm, below the rear deltoid: pushing up from a chair or straightening the elbow against resistance makes it form the outer side of the horseshoe shape of the triceps.
Examination
- Resisted elbow extension compared with wrist extension: weak wrist extension with strong triceps fits a radial nerve lesion in or below the radial groove; weakness of both suggests a higher lesion or C7
On imaging
Axial MRI of the arm shows it on the posterolateral side, with the radial nerve and deep brachial artery in the radial groove between it and the medial head. In humeral shaft fractures the radiograph shows the bone; if wrist drop appears, ultrasound follows the radial nerve between the triceps heads to look for a trapped or cut nerve.
Clinical notes
- Radial nerve palsy is the classic nerve injury of humeral shaft fractures, especially in the distal third, where the nerve is tethered as it pierces the septum
- In the posterior approach to the humerus the interval between the lateral and long heads is opened and the medial head split; the radial nerve crosses the field in the groove
Related conditions
Test yourself
Think of the answer, then tap the question to see it.
Which structures spiral round the humerus between the lateral and medial heads of triceps?
The radial nerve and the deep brachial artery, in the radial groove.
A humeral shaft fracture causes wrist drop with normal elbow extension. Where is the radial nerve lesion?
At or below the radial groove, since the triceps is still working.
In which third of a humeral shaft fracture is radial nerve palsy especially likely, and why?
The distal third, where the nerve is tethered as it pierces the lateral intermuscular septum.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- caput laterale musculi tricipitis brachii TA2 2473
- Official Spanish (DeCS)
- Cabeza lateral del músculo tríceps braquial
- English (TA2)
- Triceps brachii, lateral head
Related
Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Triceps Muscle.
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Representative model, for educational purposes only.
