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Sub talar Rotational Equilibrium and Movement of COP

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Robertisaacs, Oct 23, 2009.

  1. This one is interesting too. http://jp.physoc.org/content/581/1/99.full.pdf html
    look at the emg activity of soleus when the treadmill was inclined versus declined. What does the forefoot valgus wedge do to the dorsiflexory moment at the ankle joint?
     
  2. Bruce:

    I would have said "touche" if you had instead of using the example of "conflict avoidance", you used the example of "conflict engagement".:drinks

    Life would be much too simple without conflict.
     
  3. Bruce Williams

    Bruce Williams Well-Known Member

    Kevin;

    Touche.:santa:

    Bruce
     
  4. Bruce Williams

    Bruce Williams Well-Known Member

    Simon;

    I'll try to give those sights a look see tomorrow before traveling.

    Thank you for your reply and for mentioning the phases of gait and their interaction within the process.

    Bruce
     
  5. efuller

    efuller MVP

    To recount: We are talking about an observation where we have seen increased STJ supination after the addition of a forefoot valgus wedge. Do we all agree that we have seen it?

    Bruce questioned why do we get supination of the STJ with a modification that looks like it should increase pronation moment from the ground.

    Kevin replied that it was CNS mediated response that created more supination moment from the muscles.

    Bruce asked: how do we know this?

    Is everybody in agreement so far?

    Here is my explanation:
    We see supination of the STJ, therefore there must be a net supination moment acting on the STJ. If we added a modification that increased pronation moment from the ground and we see supination of the joint, then there must be some other source of supination moment that is larger than the pronation moment from the ground.

    This is simple physics. Newton's second law for angular motion is that the net moment = the moment of inertia x angular acceleration. The direction of the acceleration is in the direction of the moment.

    So, now we have to look at the foot and leg for potential sources of supination moment. The muscles are the most likely candidate to respond to an external pronation moment from a forefoot valgus wedge. I can't think of any other source of supination moment to explain the extra supination that is seen with a forefoot valgus wedge. If you don't accept that it is the muscle you either have to say that resupination does not occur or come up with some other source of the supination moment. If it supinates there is a net supination moment.

    I hope this helps.

    Eric
     
  6. efuller

    efuller MVP

    Jeff,
    I still don't think you need to think about forefoot to rearfoot relationships to explain the phenomenon that we are talking about. Forefoot to rearfoot relationship measured in neutral position is not necessarily relevant to explaining how wedging will change the forces from the ground under the foot. In your example, you describe how the foot "compensates" from neutral. However, we need to look at the foot in the position that it stands in. For example, some feet with a rigid forefoot valgus may have an extremely laterally deviated STJ axis and these feet will tend to supinate in stance and have a high amount of force under their lateral column in resting stance position. Conversely, a foot with a measured forefoot valgus could have a medially positioned STJ axis and these feet may have higher forces under the medial forefoot. I don't recall if your father talked about a rigid and flexible forefoot valgus, but others have used that concept when trying to explain the above phenomenon that I feel can be better explained using STJ axis location.


    You can also measure a perpendicular forefoot to rearfoot relationship with both feet with medially and laterally deviated STJ axes. These will behave differently.

    I agree with Simon's comments about apply forces across multiple joints. A force that could apply moments across multiple joints will simultaneously create a moment about all of those joints. The joint that moves the most will be the one with the greatest net moment. A real life example of this can be seen with a patient seated in a chair with the plantar surface of their foot pointed toward you. Push upward on the fifth metatarsal head. In feet with more medially positioned axes you will see more pronation of the STJ. With feet with a more laterally positioned axis you see less pronation and more ankle dorsiflexion. Now, invert the STJ (which will move the fifth met head closer to the STJ axis.) Now push in the same direction (relative to the body, but not relative to the foot.) and you can find a position of the STJ ( if enough range of motion available) where you will not get any STJ motion and get ankle dorsiflexion. Even though you are pushing distal to the MTJ you are getting motion at more proximal joints in relation to the amount of moment at those joints.

    Cheers,

    Eric
     
  7. efuller

    efuller MVP

    The moment at the ankle joint is dependent upon the distance from the line of action of the force at the center of pressure to the STJ axis. In standing we can assume the line of action of force is vertical. Two situations, A cluffy wedge with the FHL muscle active and another situation where the EHL muscle is active. With the FHL muscle active there will be a tendency to move the center of pressure more distal. With the EHL more active it will tend to move the center of pressure more proximal. So the effect of the cluffy wedge is dependent on how it changes the location of center of pressure. If the long tendons and the attachments of the sesamoids to the proximal phalanx were cut then the cluffy wedge would probably do very little to change the location of the center of pressure.


    The lateral forefoot wedge has two different effects. It will tend to increase the pronation moment at the STJ which will tend to increase tension in the plantar fascia. It will also tend to shift weight from the medial forefoot to the lateral forefoot which will tend to decrease tension in the plantar fascia. Sometimes one is more important than the other.

    Does the fascia insert medial to the axis. It crosses the STJ with attachments on both sides. The better question is what kind of moment at the STJ does tension in the fascia produce. Most of the time the sagittal view is the best analysis. With pronation there is a relative anterior shift of the talar head relative to the anterior aspect of the calcaneus. Tension in the fascia will push the met head backwards which pushes the cuneiform, navicular and then the talus backward. This push will tend to supinate the STJ. However, in the transverse plane it is possible to have the forefoot so abducted on the rearfoot that the proximal push on the metatarsal head will cause the talus to adduct (and the leg to internally rotate.) This is pronation of the STJ. This phenomenon was originally described by Jack (the push up test of Jack for flat foot) See my paper on the windlass for diagrams of what I'm saying.

    Yes we can leave the MTJ and forefoot to rearfoot relationship out of the discussion.

    Respectfully,
    Eric
     
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