Psoas Major - quick facts and theories

Due to the interest in the Psoas release post on this blog, I thought a few facts and theories might be of interest to readers.

Fact - Psoas Major (PM) is not an effective hip flexor.  The structure of the muscle and fibre orientation, this being unipennate, with a medial to lateral orientation of 45-75 degrees, allows a total shortening of only  1.5 (posterior fascicles) and 2.25cm (anterior fascicles).  This does not create sufficient shortening ability to contribute to the full range of hip flexion, but does create sufficient shortening to coincide with average range for pelvic tilt.  Acknowledgement - Comeford & Gibbons.

Fact - The PM tendon is not proximal to the Iliopectineal Bursa, this lies lateral to it under the body of the Iliacus.  This, again, draws into doubt the function of the PM as a flexor.

Fact - The PM has fascial attachment onto the anterior surface of the innominate and Sacrum.

Fact - The anterior and posterior fascicles of PM have different innervation, with the former being Femoral nerve and the latter directly and segmentally from the Ventral Ramii.

Theories of PM function,

- PM can be classified as both a Local Stabiliser (posterior fascicles) and a Global Stabiliser (anterior fascicles).
- PM local stabiliser function, segmental stabilisation of lumber spine, acting with other "inner unit" spinal stabilisers and demonstrating the same mechanisms of dysfunction, i.e. segmental atrophy/weakness and activation delay, post trauma/pain.
- PM global stabiliser function, posterior tilt of the pelvis at the SIJ (force closure of the SIJ!), centring of the femoral head in the acetabulum via tensioning of the PM tendon - note no significant movement is required of the tendon over the pelvic brim to accomplish this, hence, no Bursa.

Specifically regarding the final point above, readers should note the huge multi-planar hip ROM improvements possible when PM is activated, this being due to, in the absence of other restriction, joint congruency facilitated by PM.

Readers may be aware, and wish to challenge some of the above, with note to the available evidence that PM is maximumly active at 90 degrees of hip flexion (and therefore a hip flexor?).  The proposal here, as presented by Comerford,  is simply that PM co-contracts to control pelvic tilt as the hip flexes, a force couple.

Further posts will discuss activation and training strategies............