Scalenus anterior

What it is

A deep muscle of the side of the neck, running from the cervical vertebrae down to the first rib. It is one of three scalenes — anterior, medius and posterior — named for their ladder-like arrangement, from scalenus, uneven.

Its importance is largely as a landmark, because of what passes on either side of it. The brachial plexus — the nerve bundle supplying the arm — and the subclavian artery pass behind it, between it and scalenus medius. The subclavian vein passes in front. The phrenic nerve descends across its front surface on its way to the diaphragm.

That gap between the anterior and middle scalenes is the interscalene triangle, one of the places the nerves and vessels to the arm can be compressed.

What it does

Side-bends the neck towards its own side, and assists rotation. With the neck fixed, it lifts the first rib, making it an accessory muscle of breathing in.

What loads it

Resisted neck side-bending, and laboured breathing, where the scalenes work with the diaphragm to expand the ribcage.

What is commonly got wrong about it

Thoracic outlet syndrome as a common, self-treatable condition. Compression of the brachial plexus or subclavian vessels in this region is real, uncommon, and notoriously difficult to diagnose — there is no single agreed test and the diagnosis is contested even between specialists. It is widely diagnosed online on the basis of neck and arm aching. Arm symptoms with numbness, weakness, or colour change need proper assessment and are not a stretching project.

That the scalenes should be stretched to relieve neck tension. They lie deep to sternocleidomastoid and are difficult to reach specifically.

That accessory breathing muscles are a sign of dysfunction. They are recruited in heavy breathing by everyone. Their use during exertion is normal.