Brachialis

Origin, Insertion, Action & Nerve

Attachments explained

The Brachialis is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.

Action & function

When the Brachialis contracts, it produces the following movement: Flexion of forearm at the elbow (in all positions).

As part of the Arm group of the Upper Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.

Nerve supply & clinical relevance

The Brachialis receives its nerve supply from the Musculocutaneous nerve (C5, C6); small contribution from radial nerve (C7). This nerve carries fibres from spinal segment(s) C5, C6, C7.

Because a muscle can only contract when its nerve is intact, injury to the Musculocutaneous nerve or to its spinal roots (C5, C6, C7) can weaken or paralyse the Brachialis, impairing the movements it normally produces (flexion of forearm at the elbow (in all positions)). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Brachialis (the OIANS method)

The key insight for the brachialis is its insertion on the ulna, a bone that does not rotate. Because the ulna cannot pronate or supinate, the brachialis flexes the elbow equally well in any forearm position, which is why it, rather than the biceps, is called the true workhorse of elbow flexion. Reasoning from that single attachment gives you the whole action.

Group its nerve with the biceps under the musculocutaneous nerve, then note the small radial nerve twig as a memorable exception. Comparing it to the biceps side by side (biceps supinates and flexes, brachialis only flexes) keeps the two anterior flexors distinct.

Frequently asked questions

Related muscles