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PP127: Sit-ups and Hip Flexor Paradox

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  • PP127: Sit-ups and Hip Flexor Paradox

    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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