View Full Version : PP127: Sit-ups and Hip Flexor Paradox

Mel Siff
04-23-2002, 08:07 PM
Here is another Puzzle & Paradox (PP127) in my ongoing series.


For newcomers to this forum, these P&Ps are Propositions, not facts or
dogmatic proclamations. They are intended to stimulate interaction among
users working in different fields, to re-examine traditional concepts, foster
distance education, question our beliefs and suggest new lines of research or
approaches to training. We look forward to responses from anyone who has
views or relevant information on the topics.

PP127 The risks of straight-legged sit-ups may be grossly exaggerated in the
healthy individual, especially regarding the action of the hip flexors on the
lumbar spine.

Theoretically it might appear to some folk that the small angle subtended in
the straight-legged sit-up by psoas in its proximal attachment to the spine
will markedly accentuate the lumbar concavity (as seen from behind), this
does not appear to have been shown via clinical measurement. If the psoas
angle were more perpendicular to the length of the spine, then it would be
far more likely for the psoas to exert sufficient force to increase the
lumbar concavity, but the major component of force exerted by psoas is along
the length of spine, not vertical to the spine.

Paradoxically, the bent-knee (actually flexed-hip) sit-up would seem to place
psoas in a position which enables it to pull more vertically on the lumbar
spine, so that that flexed hip sit-ups would appear to be less safe than
straight-legged sit-ups! As a matter of interest, has anyone come across
any MRIs which have examined exactly how much an activated psoas in a healthy
person during sit-ups increases the lumbar concavity (not 'lordosis', since
this term accurately refers only to pathology)?

What is far too commonly assumed is that psoas definitely DOES increase the
lumbar concavity, and in order to do so, it has to produce extremely large
force because of its low angle in the supine or straight body. This
certainly is correct IF one assumes that psoas always definitely increases
the lumbar concavity. This assumption seems to be totally incorrect, because
the poor mechanical advantage of psoas in the supine position does not permit
it to produce high levels of torque about the hips. One probably could
insert microelectrodes directly into psoas to electrically activate it in a
supine person and we still would not be able to generate sufficient vertical
force to produce enough torque to deform the lumbar spine, at least to an
extent which causes spinal pathology.

All too often, it is presumed that large psoas activation necessarily can and
will produce enough force to deform the lumbar spine to such an extent that
it will lead to vertebral displacements which will cause nerve impingement in
the non-pathological spine. Surely, the likelihood that this level of
displacement will occur is vastly exaggerated because the healthy spine and
its discs are surrounded by very strong connective tissue that minimise the
degree of this displacement.

Then, even if the lumbar spine is subjected to large psoas forces, adjacent
vertebrae are not going to experience local dislocations which will produce
nerve impingement - instead the lumbar spine, according to basic mechanics,
is simply going to exhibit a slightly smaller radius of curvature over a
length of many vertebrae. It certainly will not experience the type of
deformation and discal stress that is imposed when the lumbar spine is
subjected to loaded flexion and rotation.

Moreover, it is impossible or virtually impossible for the average person to
sit up in such a way as to allow psoas to strongly become activated before
the rest of the abdominal musculature has become activated and minimised the
possibility of this happening. The very act of raising the head and shoulders
during the early stages of sitting up activates the abdominal musculature so
strongly and increases the psoas angle to such an extent that the ability of
psoas to increase the lumbar concavity is minimal. If this point is doubted,
try sitting up by keeping the back of your head and shoulders in touch with
the floor while starting a sit-up and attempt to raise your trunk by relying
largely on psoas action. First of all, this will not be possible unless your
feet are restrained and you are extremely strong, which hardly is the case
with the average fitness fan.

Does all of this not suggest that most or all of the well-meaning advice
about executing sit-ups or crunches is exaggerated or redundant, especially
regarding the risks of psoas activation necessarily imposing pathological
levels of stress on the lumbar spine or its components? Does the body not
automatically act to minimise the risks of any high levels of stress as soon
as the head and shoulders are raised during the early stages of any supine
sit-ups or 'crunches'? Comments from anyone?

Dr Mel C Siff
Denver, USA

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