TricepsTriceps Brachii

arms3 musclesMusculus triceps brachii

The triceps brachii is the three-headed muscle on the back of your upper arm, and it straightens your elbow. It is about two thirds of the arm's total muscle — considerably more than the biceps.

Triceps: its 3 muscles highlighted on an anatomical figure, from behind — Long Head, Lateral Head and Medial Head.
Where the triceps sit, from behind.

Three heads converge onto one tendon at the point of the elbow. The long head starts on the shoulder blade just below the socket; the lateral and medial heads both start on the back of the upper arm bone itself, above and below the shallow groove where the radial nerve crosses. All three end together on the olecranon, the bony point you lean on.

Their shared job is straightening the elbow, and this is most of what the back of the arm does. It is worth stating plainly that the triceps is the larger of the two arm muscles by a considerable margin — roughly two thirds of the upper arm's muscle mass — which makes it the more productive target for anyone training arms for size.

The long head is the odd one out and the reason the three are worth separating. It crosses the shoulder as well as the elbow, so it also pulls the arm backward and inward toward the body. That gives it a second job the other two heads do not have, and it makes the long head's length depend on where the elbow is.

The medial head sits underneath the other two and is largely hidden. It is the workhorse: active in elbow extension at every load and through the whole range, where the lateral head contributes more as resistance increases.

Training notes

The single most useful thing to know is that the long head needs the elbow overhead. Because it crosses the shoulder, it is only at full length when the arm is raised — so overhead extensions train it in a range that pushdowns, dips and close-grip pressing never reach. A triceps routine of pushdowns alone leaves the largest head short.

For the other two heads, pressing does most of the work already. Close-grip bench press, dips and any heavy pressing loads them hard, and pushdowns add volume to the same range.

Arm size responds better to triceps work than to biceps work, purely because there is more muscle there. Anyone splitting arm volume evenly between the two is giving the smaller muscle equal billing.

Elbow position is the variable that matters, and it maps cleanly: elbow overhead lengthens the long head, elbow at the side works all three in the middle of their range, elbow behind the body shortens the long head so the other two take over.

Anatomy

Origin
  • Long head: infraglenoid tubercle of the scapula
  • Lateral head: posterior surface of the humerus, above and lateral to the radial groove
  • Medial head: posterior surface of the humerus, below and medial to the radial groove
Insertion
  • Olecranon of the ulna
Actions
  • Extends the elbow, straightening the arm
  • Extends and adducts the arm at the shoulder, through the long head only
Innervation
Radial nerve (C6–C8)

Parts of this muscle

Exercises

Antagonists

Synergists