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Ankle and forefoot equinus

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Jon UK, Feb 2, 2010.

  1. Jon UK

    Jon UK Welcome New Poster

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    Hi - My first posting
    Can anyone help please?

    If ankle and forefoot equinus present, why is an abductory twist present with minimal arch lowering during pronation

    Thanks very much
    Last edited by a moderator: Sep 22, 2016
  3. Jon UK

    Jon UK Welcome New Poster

    Thank you very much for this information

    How do you think an abductory twist relates to ankle equinus?

  4. Jon:

    Not all subjects with ankle equinus display an abductory twist. However, in an individual that does have the right biomechanical tendencies to develop abductory twist, increased tension in the Achilles tendon during late midstance would likely tend to increase abductory twist...up to a point.
  5. Admin2

    Admin2 Administrator Staff Member

  6. David Smith

    David Smith Well-Known Member

    Kevin and Jon

    Kevin, in your video you mention the torsion in the leg is partly due to the external rotation of the pelvis during late midstance. At first this seemed like a strange way of describing the action since I always think of this as an internal rotation of the hip. Now, pointing that out, it seems that differentiating hip and pelvis rotation is a good thing. In that way we can imagine that while the tibia and femur and therefore hip are internally rotating due to pronation moments from GRF (we'll accept that proposition for the sake of this argument) then the pelvis is externally rotating and so, via the hip external rotator muscles, is driving the leg in the opposite direction than that due to the effects of pronation at the STJ. i.e. internal rotation.

    I thought I would still communicate that since it might clear any confusion and leads on to the next point directed at Jon.

    Jon wrote
    Kevin replied
    Then: my thoughts are--

    Often (by my experience) in the subject with the equinus foot and ankle you will often find restricted internal rotation of the hip. I.E. there is no internal rotation past the position of reference of knee straight ahead. Therefore the midway or neutral hip position = an externally rotated knee and foot position (if there is no malleolar compensation). Some subjects will tend to adopt this toe out foot placement angle that allows normal hip rotation at contralateral swing thru. However some subjects keep a straight ahead foot position, now with reference to Kevin's video and the first comment above, you can see that this can result in increased transverse plane torsion or moments in the longitudinal axis of the leg that will be in and external rotation direction at late midstance phase of the support leg.

    Before heel lift, on the support leg, the frictional forces and moment arm of the fore foot and heel about the longitudinal axis of the leg are both quite large.
    At heel lift the moment arm available to frictional forces becomes much shorter, friction may become increased as the stance goes into propulsion but overall the small moment arm means that the forces from opposite torque in the leg due to external rotation of the pelvis overcomes the opposing frictional forces at the forefoot and so an abductory twist or external rotation occurs.
  7. efuller

    efuller MVP

    I still think there might be some confusion so I'll add a little more. When you describe motion it's helpful to describe the motion as one part to another part or one part relative to the floor. So, external rotation of the hip over the stance leg is the same as internal rotation of the leg relative to the hip. However, those motions may be same as or different from external rotation of the leg relative to the ground.

    On the cause of the abductor twist and the moments that cause the abductory twist. I beleive that it is the momentum of the pelvis that is offset from the leg that is causing the external rotation moment applied to the leg. In the explanation above hip musculature is credited with applying a moment to the leg.

    Right leg stance looking from above: The muscles that would cause the pelvis to rotate clockwise (external rotation of the pelvis relative to the stance leg) will also create an equal and opposite moment on the leg from the pelvis. So this would create an internal rotation moment on the leg at the same time pronation of the foot would be causing an internal rotation moment on the leg. That situation would tend not to cause an abductory twist.

    I would agree the observation is related to restricted internal rotation of the hip. However, you can see restricted rotation of the hip without an equinus so the equinus is not really that related to the out toeing.

    Agreed, at heel lift the frictional forces from the ground have a shorter lever arm to restrict abduction of the foot relative to the ground.



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