Serratus posterior inferior - a neglected muscle

[Jim Pickles]

The serratus inferior posterior – a muscle that is commonly neglected. For instance, it doesnt get a mention in Stretching and Flexibility.

The muscle runs diagonally downwards from the lower ribs to join the T11, T12, L1 and L2 vertebrae (see fig). Its function is described as “drawing the lower ribs backward and downward to assist in rotation and extension of the trunk”. Drawing the lower ribs downwards is the function most often mentioned, and this is commonly used to suggest a role in forced expiration.

I’ve had a bit of an interest in this muscle for a long time. It started in the days when I stood up from backbending. I found it I was part way up, if I contracted a muscle in this area, it made standing up from a backbend much easier – it was as though someone had placed a hand in the area to help me up. It was the only muscle that I could identify as a candidate in this area, but is was a bit of a puzzle, because we would expect it to ENCOURAGE back bending (by pulling the ribs back and down) rather than helping to get out of one. However every muscle has two ends, and it we view the upper (rib) ends as fixed (by the tensions in the rib cage) then the action would be to lift the T11-L2 region, maybe transferring the bend from lower in the lumbar region up towards the T-L junction, where different musculature could then help lift the torso further. Not sure if this explanation holds water but maybe someone has better ideas.

I was more recently reminded of this muscle when doing a sphinx. It came in strongly and powerfully assisted in bringing the torso more vertical. I am maybe feeling it more now because it is also strongly activated when going into the arch body hold (arms up), in the way demonstrated in Olivia’s Vimeo video by going through intermediate stages first, with arms down, then out to sides.

So I’ll emphasise use of this muscle routinely now when doing sphinxes with my students (which we do every week), as its so powerful in its effect. As I would encourage everyone else to do, if you're not aware of using already.

Not sure if anyone will read of all this, but if anyone else has any use for this muscle, I’d appreciate hearing it.

Jim.