Multifidus

What it is

A deep layer of short muscle bundles filling the groove either side of the spinous processes, lying beneath the erector spinae group. Each bundle spans only two to four vertebral segments, running from a transverse process upward and inward to a spinous process above.

Multifidus means "split into many parts", and it is: dozens of small overlapping bundles from the sacrum to the neck.

It is thickest and best developed in the lumbar region, where it forms a substantial mass, and it is the deepest muscular layer there.

What it does

Extends and rotates the spine weakly — its short lever arms mean it produces little movement.

Its real contribution is segmental control. Where the erector spinae move the whole trunk, these bundles act across a few joints at a time and are positioned to control the relationship between adjacent vertebrae. They also carry a high density of sensory receptors, so their role in reporting spinal position is likely as important as their role in producing force.

What loads it

Anything loading the spine. It works in concert with the erectors and abdominal wall and cannot be loaded selectively by any voluntary movement.

What is commonly got wrong about it

The atrophy story. Multifidus is observed to be smaller and fattier on scans in people with long-standing back pain, and this has been widely presented as a cause requiring specific retraining. The observation is real. Whether the change causes the pain, results from it, or both, is not established — muscles adjacent to a painful area commonly change with reduced use.

That it needs specific activation exercises. This is the same claim as for transversus abdominis, and it has had a similar history: trials comparing specific stabilisation training with general exercise have generally not found the specific approach superior.

That you can feel or isolate it. It lies under the erector spinae group. Nothing you feel in your back is identifiably this muscle.