The lateral head of triceps is the most superficial and lateral portion of the triceps, forming the visible posterior arm contour from the deltoid to the elbow. It originates on the posterior humerus above the spiral groove and inserts into the olecranon via the common triceps tendon. The lateral head provides the greatest cross-sectional area of the three heads and is the primary contributor to rapid elbow extension against resistance.
| Origin | Posterior surface of the humerus above and lateral to the spiral groove, from the lateral intermuscular septum |
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| Insertion | Olecranon process of the ulna via the common triceps tendon |
| Nerve Supply | Radial nerve (C7, C8) |
| Blood Supply | Profunda brachii artery (posterior muscular branches) |
| Actions | Extends the elbow joint — primary contributor during fast or heavy extension; Contributes to shoulder adduction and extension when the arm is elevated |
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The lateral head of triceps overlies the radial nerve in the spiral groove, making it the key muscle in radial nerve palsy localisation. Damage to the spiral groove affects the lateral head's innervation below the posterior cutaneous nerve of the arm branch but above the lateral head motor branch, so lateral head involvement helps localise the lesion level. Lateral head hypertrophy is prominent in powerlifters and produces the characteristic horseshoe appearance of the posterior arm. Partial tears of the lateral head at its humeral origin occur in heavy eccentric loading.
Palpated on the posterolateral upper arm with the elbow in slight flexion. It forms the lateral half of the posterior arm muscle mass between the deltoid insertion and the olecranon, becoming firm with resisted elbow extension.
Partial tear at the humeral origin from heavy eccentric elbow extension loading, producing posterolateral upper arm pain and elbow extension weakness, confirmed by MRI showing origin oedema.
Compression of the radial nerve beneath the lateral head of triceps in the spiral groove producing wrist drop and finger extensor weakness, typically from prolonged arm compression during sleep or anaesthesia.
