Compartimentum posterius brachii

The posterior compartment of the arm is also known as the "extensor compartment", as its main action is extension.

The muscles of this compartment are:

• the triceps brachii and
• the anconeus muscle

and these are innervated by the radial nerve.

Their blood supply is from the profunda brachii.

The triceps brachii is a large muscle containing three heads a lateral, medial, and middle.

The anconeus is a small muscle that stabilizes the elbow joint during movement.

Some embryologists consider it as the fourth head of the triceps brachia as the upper and lower limbs have similar embryological origins, and the lower limb contains the quadriceps femoris muscle which has four heads, and is the lower limb equivalent of the triceps.