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Pronate more but reduce Internal Invertor Moment?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by JasonR, May 3, 2013.

  1. JasonR

    JasonR Member

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    Can someone explain this to me? In the Dorsey Williams paper which Craig often cites, it is evident that there is a ~4 fold decrease in internal invertor moments with the use of inverted devices. Yet some actually pronated more. For a given stance time, I can't see how those particular subjects reduced their IIMs- they would have to pronate faster to get there, with a greater reversal of momentum required to resupinate? Did those particular subjects decrease their IIMs, or was the four fold reduction a group mean that washed out such individual accounts? What am I missing?
    Ta, Jason
  2. Jason:

    Even though I don't have the paper at hand currently to talk specifics, you must be very careful when analyzing papers using inverse dynamics calculations to differentiate between external and internal moments, since they are the exact opposite of each other.

    In addition, in a subject with a four fold decrease in internal inversion moment you would expect an increase in eversion motion unless an increase in external inversion moment occurred and/or a decrease in internal ankle eversion moment occurred and/or an decrease in external ankle eversion moment occurred.

    Confusing? Yes it is at first. But this is the way we must analyze the balance of moments across any joint axis in order to make sense of the summation of moments that will either cause a joint to accelerate, decelerate or be stabilized at any instant in time.
  3. JasonR

    JasonR Member

    Thanks for your reply Kevin,
    I think I broadly understand what you are saying. The net moment (in the frontal plane) is the sum of 4 potential inputs- internal invertor and evertor and external invertor and evertor. The permutations of those cant be picked off from kinematics alone (though if the velocity of motion is increasing in a particular direction, am I correct that you can deduce the net moment- in this case pronatory?).
    The reduction in internal ankle invertor moments is presumably a good thing, because it means those structures such as Tib Post, are working less. The assumption I have is that the inverted device created an external inverting ORF as part of the moments re-balancing equation.
    OK, so for a foot that pronated more in a given time (accelerated?) , it may be that the inverted device did an even better job with that subject if the IIM reduced?
    Last edited: May 4, 2013
  4. Jason:

    It could also be that if the inverted device was "overcorrecting" the runner then the internal rearfoot eversion moment of the runner was increased (e.g. increased peroneal muscle activation).

    I believe, when Irene Davis first spoke to me about the results of her experiments with Blake inverted orthoses, that she thought some of the runners in her experiments were possibly using the peroneals more due to being over-inverted and thus had shown larger internal rearfoot eversion moments. Increased internal rearfoot eversion moment would make the runners pronate faster and/or over larger magnitudes especially if this moment was greater in magnitude than the rearfoot inversion moments (internal or external) that were acting at that same instant in time.
  5. Greg Fyfe

    Greg Fyfe Active Member

    I recently attended Craigs Boot Camp, which included information from this paper.
    I haven't read it myself and it's not an area I work in much.

    However what I came away with, about the paper, was; of the 11 subjects

    1/ one subject pronated more with an inverted orthotic, one pronated more with a standard orthotic orthotic, one showed no difference between a standard and inverted orthotic. and there's a few other variations

    2/ The inverted device decreased the inversion moment by a factor of 4,

    So it would seem that the inverted device reduced the force required to invert/supinate the foot

    However the type of orthotic does not seem to show a consistant effect on what the measure of the movement of pronation was.

    i.e. the force and the movement don't seem to be related


  6. JasonR

    JasonR Member

    Hi Greg
    It would seem that that is an ongoing theme- that forces (kinetics) and movement (kinematics) are not the same. At the heart of my question was that, for a specified stance period, to pronate more on one device (for at least the one subject), you would have had to do it more quickly. Taken from an initial velocity of zero (speculating), then we are dealing with acceleration, which is related to force.
    I am playing around with ways to visualise this, with Kevins comments in mind. I have attached a stylised (FBD?) of the right foot in the frontal plane- trying to represent axis position, internal, external forces and lever arms. The second picture demonstrates a smaller net pronatory moment in the presence of a significantly reduced internal invertor moment due to the input of a new orthotic reaction force ('over-correcting'). The peroneals have kicked in to balance the new ORF. (thats the 1st time I have thought about the moment arm of the peroneals- potentially quite large esp if compared to a med dev STJA!).
    Always after advice as to better ways to analyse whats going on!

    Attached Files:

  7. Bruce Williams

    Bruce Williams Well-Known Member

    Nice diagram, I think tht is a good representation except for the AJ moments that Kevin mentioned. If you have a copy of the paper, could you share it please?
    I think the individual issue of increased pronation likely needs to be analyzed on a single patient basis and there are many factors that likely are not taken into account in studies like this. But, I can't know that for sure until looking over the paper.
    That said, patients will often attempt to pronat through a device if it is posted in varus too much or inhibits medial column function at the level of the 1st mpj and if AJ equinus issues were not properly addressed.
    This is where art meets science and where those of us who try to better our understanding of the whys's and how's greatly appreciate the input of Drs Payne and Kirby in trying to look at these issues in the way that engineers and PhD's will. It is difficult to shift gears for me since I tend to visualize kinemetically, but it helps me to broaden my understanding to think it through, usually.
    Bruce Williams
  8. Greg Fyfe

    Greg Fyfe Active Member

    Hi Jason

    I'm still digesting your diagram, as Bruce says there's a lot going on and maybe looking at the specific subject would help, I think I'm going to have to track down the paper.

    from your latest reply

    When I looked over the notes I had, the Eversion Excursion between no orthoses , standard orthoses and the inverted device ranged between 15 and 15.8 degrees. Not much variation .

    It looks like that while there was for some subjects, more pronation on the orthotic, that the net range of movement did'nt change. I guess they started midstance at a less inverted position and then wouldn't need the extra acceleration to get to the more pronated position.

    I do think I'll have to read the paper , as I think your point about a "washed out mean " effect is worth considering.


    Last edited: May 6, 2013
  9. Craig Payne

    Craig Payne Moderator

    OK, Here is a video of what i talked about at the Boot Camps for those who want to know what is being talked about:

    Last edited by a moderator: Sep 22, 2016
  10. Jason and Colleagues:

    After looking at the diagram provided, I see that I may be able to help.

    Let's first take a step back and just talk about the movement of subtalar joint (STJ) pronation and try to answer this very basic question:

    What causes the STJ to begin pronating?

    (This is actually an easy question to answer once you understand the difference between kinetics and kinematics and understand the concept of rotational equilibrium.)

    Answer to the question: To accelerate the STJ into pronation motion, the net pronation moment acting across the STJ will need to increase.

    What are some of the possible ways that the net STJ pronation moment can be increased? Let's list them:

    1. Increase in external STJ pronation moment (e.g. ground reaction force (GRF) is increased on the plantar foot lateral to the STJ axis)

    2. Increase in internal STJ pronation moment (e.g. peroneus brevis contractile activity is increased)

    3. Decrease in external STJ supination moment (e.g. GRF is decreased on the plantar foot medial to the STJ axis)

    4. Decrease in internal STJ supination moment (e.g. posterior tibial muscle contractile activity is decreased)

    Therefore, until all possible external and internal pronation and supination moments acting across the STJ axis at any instant in time are considered, so that the net moments acting across the STJ can be calculated, then it is impossible to know if the STJ will begin accelerating into pronation or not.

    However, if the STJ is already in rotational equilibrium and is static (i.e. not rotating), [also called static equilibrium] then we can easily predict that, from this state of static equilibrium either an increase in magnitude of STJ pronation moments (internal or external) or a decrease in magnitude of STJ supination moments (internal or external) will accelerate the STJ into pronation motion.

    With these facts in mind, let's now proceed to the original point of this thread. If you now apply an inverted orthosis to a subject's foot and that foot undergoes increased pronation acceleration and motion, we know that this likely occurred not because of the direct mechanical effect of the orthosis, since an inverted orthosis will increase the external STJ supination moment. Rather, the increase in pronation must be due to some neuromotor effect of the orthosis, mediated by the central nervous system, which either increases the internal STJ pronation moment and/or decreases the internal STJ supination moment. In other words, there are only a finite number of sources that will cause pronation motion to occur.

    You may want to read this thread on direct mechanical vs neuromotor effects of foot orthoses for further background on this concept.

    Hope this helps.:drinks

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