Triceps brachii

What it is

The muscle occupying the whole back of the upper arm, with three heads converging on a common tendon at the point of the elbow.

• Long head — from the scapula, just below the shoulder socket. Crosses both joints.
• Lateral head — from the back of the humerus, above the radial groove
• Medial head — from the back of the humerus, below the radial groove, lying deepest

Only the long head crosses the shoulder, and that is the distinction that matters. The medial head lies underneath the other two and is the one doing quiet, constant work.

Despite the attention biceps receives, triceps makes up the majority of the upper arm's muscle mass.

What it does

Extends the elbow — straightens the arm — and it is the only significant muscle that does, with anconeus contributing a little.

Through its long head it also extends and adducts the shoulder, pulling the arm backward and inward.

Because the long head crosses two joints, its length depends on both. Raise your arm overhead and it is lengthened at the shoulder, leaving less range at the elbow — which is why overhead extension movements feel so different from pressdowns, and why the long head is loaded more in the overhead position.

What loads it

Any pressing movement — push-ups, dips, overhead pressing — and any resisted elbow extension. Overhead positions preferentially load the long head, and this is a well-founded application of two-joint mechanics rather than gym lore.

What is commonly got wrong about it

"Toning the back of the arms." The most persistent aesthetic request in the whole of fitness, and the answer is the same as everywhere else: working a muscle does not preferentially remove the fat sitting over it. The clearest direct test used abdominal exercise and found no effect on abdominal fat.¹

That triceps is a small muscle relative to biceps. It is substantially larger.

"Three heads, three exercises". The long head is genuinely differently positioned and responds to shoulder angle. The lateral and medial heads cross one joint and are loaded together by everything.

That the medial head is unimportant because it is hidden. It is the head most consistently active across elbow extension at all loads.

Sources

1. Vispute SS, Smith JD, LeCheminant JD, Hurley KS. The effect of abdominal exercise on abdominal fat. Journal of Strength and Conditioning Research 2011; 25(9):2559–2564. doi:10.1519/JSC.0b013e3181fb4a46 PMID 21804427 24 sedentary adults, six weeks of abdominal exercise five days a week on an isocaloric diet. No significant effect on body fat percentage, abdominal fat, abdominal circumference or skinfolds — while abdominal muscular endurance improved substantially. A direct test of the premise.

Every source here was checked against PubMed rather than recalled. Where this site says something is disputed or unsupported and no source appears, that is deliberate: it means the claim is being conceded rather than cited.