Triceps brachii

The three-headed muscle on the back of the arm and the main straightener of the elbow. It makes up about two-thirds of the upper arm's mass — most of your 'arm size' is triceps, not biceps.

This muscle in 3D

Drag to rotate · scroll to zoom — see the shape, origin and insertion of the triceps brachii. 3D model via Sketchfab (CC-BY).

Anatomy

Muscles: Triceps brachii (long, lateral, medial heads)

Origin: Long head: scapula (infraglenoid tubercle). Lateral & medial heads: back of the humerus.

Insertion: Olecranon of the ulna (point of the elbow).

Actions:

Elbow extension (all three heads)
Long head assists shoulder extension and adduction
Stabilise the elbow in pressing

How the muscle works

All three heads converge on the elbow to straighten it, so every press and push-up finishes with the triceps. The long head crosses the shoulder too, so it also helps pull the arm down and back and is stretched most with the arm overhead.

Fibre-type bias: Mixed, with good fast-twitch capacity for pressing power.

Functional role: Pushing and pressing; locking out any straight-arm movement; throwing.

Common problems

Triceps tendinopathy at the elbow
Weakness limiting press lockout

Training & stretching

Close-grip bench, dips, push-ups, overhead extensions (long head), pushdowns.

Overhead triceps stretch (bend the elbow, gently pull with the other hand).