PP127: Sit-up and Psoas Paradox

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More feedback on my PP127:

According to the following study, the psoas muscle has only a minor effect on

lumbar lordosis. Flexion, extension of the hip or lumbar spine do not change

the effect that this muscle exerts on the lumbar spine.

J Biomech 1995 Mar; 28(3):339-45

The psoas major muscle: a three-dimensional geometric study.

Santaguida PL & McGill SM

The purpose of this study was to use anatomical data obtained from cadavers,

and geometrical scaling data obtained from MRI scans of living subjects, to

assess the line of action and mechanical function of the psoas major muscle

in three dimensions about each lumbar spine level. In addition, the line of

action of the psoas major was documented as a function of lordosis. A total

of seven cadavers were dissected from which fibre/tendon architecture was

measured, while MRI scans were performed on 15 males to obtain centroid paths

and area scales of the muscle over its length. In this way, the curving path

of muscle line of action was accommodated, together with force and moment

predictions that recognized the presence of a tendon at lower lumbar levels

(up to L3 in some subjects) significantly increasing the stress.

Results confirm that the mechanics of the psoas cannot be adequately

represented with a series of straight line vectors from vertebral origins to

insertion. Moreover, the mechanical action of the psoas major does not change

as a function of lumbar spine lordosis, because the muscle path of action

changes in accordance with changes in spine posture.

Functionally, contrary to claims, the psoas cannot act as a 'derotator' of

the spine, does not impose large shear forces on the spine in any posture

except at L5-S1, and cannot have major affects to 'control lordosis'. It has

the potential to stabilize the lumbar spine with compressive loading and with

bilateral activation, to laterally flex it, and can create large anterior

shear forces, but only at L5-S1.

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Dr Mel C Siff

Denver, USA

Supertraining/