Jun 2015 06

The muscle at the front of your neck - sternocleidomastoid

By: Dave Wheeler

This week we look at one of the main muscles of the neck - the Sternocleidomastoid, or SCM.

The anatomy of the Sternocleidomastoid



The 2 different heads of SCM originate in 2 different places:

  • The Sternal Head starts at the top edge of the sternum (the bony bit in the centre top of your chest)
  • The Clavicular Head starts at the top edge of the clavicle, just a couple of centimetres out from the sternum

‚ÄčThese origins are the fixed points of the SCM muscle.

Both heads the merge as they go up diagonally out to insert at the mastoid process which is the bony bit just behind the bottom of the ear.


What does the sternocleidomastoid do?

The actions of sternocleidomastoid

You can work out what happens when the SCM contracts by looking at the diagram above.

Remember that the origins at the sternum & clavicle don't move - so if the muscle contracts, it pulls on the head and causes it to nod forward.

So the action of the SCM is to flex the neck.

Remembering, though, that there's an SCM each side of the neck, they don't both have to work at the same time. If the muscle on only 1 side contracts then SCM causes 2 actions:

  • It flex the head & neck to that side, and
  • It rotates the head & neck to the opposite side

The secondary action of sternocleidomastoid

To be complete, it's not strictly true that the origin stays fixed in one place all the time. There's one occasion when the head stays still (so the insertion at the mastoid process remains fixed) - then when the SCM contracts, it pulls up on the sternum and clavicle.

This happens when we breathe in.

So SCM helps to elevate the entire ribcage during inhalation

The function of sternocleidomastoid

The average head weighs 4½ - 5½ Kg. That weight balances on the top of your spine.

That's the equivalent of 5 bags of sugar balanced on top of a broom-stick handle.

What keeps it in place (what stops the 5 bags of sugar falling off the broom-stick handle) are 3 sets of muscles under equal tension, either side of the neck.

At the front is the Sternocleidomastoid

At the back are the Levator Scapulae

At the side are the Scalenes

So apart from the actions of the muscle, the function of the SCM is to help balance and support the weight of the head.

In a previous blog I've talked about the fact that we spend most of our day with our head forward, which means that those muscles at the back (Lev Scap) are extended and over worked.

By contrast, the  SCM - is underused, and therefore weak.

This means that even when we try to stand with our head back in the "correct" anatomical position, the SCM is too weak to support the head for more than a couple of minutes. So our head comes foward again.

This weakness can be corrected, but it takes patience. Slow, gentle forward neck raises - only 3 at a time - and only raising by about an inch will do the trick.

Do 3 neck raises, 2 or 3 times a week. Eventually, this will strengthen the SCM.

If you need more help, give me a call.