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Sub talar Rotational Equilibrium and Movement of COP

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  #1  
Old 23rd October 2009, 11:44 AM
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Default Sub talar Rotational Equilibrium and Movement of COP

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I've been mulling the fascinating replies on the earlier thread on forefoot varus extensions. It's a puzzle which has been bouncing around in my head for a little while now and I've really enjoyed everyones view on it.

Of course the reason for its significance lies solidly in the realm of the applied biomechanics and the nitty gritty of where the bulk lies on an orthotic as well as the fascinating atom splitting which we all enjoy.

In that vein, at 3:30 am this morning, whilst attending to a flu ridden toddler (the swine), I mulled thus.

So a varus extention, or indeed a lump, can still reduce the pronation moment even if its lateral to the axis (by moving the COP nearer to the axis and reducing the lever arm).

So the significance of the axis is not that anything lateral to it will increase pronation moment (and vice versa for supination moment). That's certainly how I used to think of it.

BUT, there must be such a line, beyond which a mod stops reducing pronatory moment and starts adding to it, and it must involve the position of the axis.

If its about moving the COP nearer or further from the axis there must be a midpoint.

So we take the STA on one side, and the lateral most significantly weight bearing point (say 5th met). We draw a line in between the two. Thus



Theoretically, is it the case that any forefoot mod placed lateral to THAT line will pronate the foot?


OR


Is it simply the case that wherever the COP is in the forefoot, any mod placed Lateral to THAT line will create a pronatory moment, Thus



or thus



In terms of practical application, I'm thinking firstly in terms of where we put modifications on orthotics (as in forefoot varus extensions) and also in terms of where we find the bulk on a pre fab orthotic. It strikes me that this line, wherever it may be, would be a significant piece of information to know, if indeed it exists!

Would value everybodies thoughts.

Regards
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  #2  
Old 23rd October 2009, 12:21 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Robert, I've only given a quick scan of what you are saying and asking, but my first thoughts are that this depends not only on the position of the axis and modification, but also on the shape and load-deformation characteristics of the modification. And in the case of a forefoot modification, the load-deformation characteristics of the rays.
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Old 23rd October 2009, 01:06 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Also, that it is important to realize that COP defines only the point of application of the ground reaction force vector; it does not define the magnitude or line of action of the GRF vector.
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Old 23rd October 2009, 02:50 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Robertisaacs View Post
I've been mulling the fascinating replies on the earlier thread on forefoot varus extensions. It's a puzzle which has been bouncing around in my head for a little while now and I've really enjoyed everyones view on it.

Of course the reason for its significance lies solidly in the realm of the applied biomechanics and the nitty gritty of where the bulk lies on an orthotic as well as the fascinating atom splitting which we all enjoy.

In that vein, at 3:30 am this morning, whilst attending to a flu ridden toddler (the swine), I mulled thus.

So a varus extention, or indeed a lump, can still reduce the pronation moment even if its lateral to the axis (by moving the COP nearer to the axis and reducing the lever arm).

So the significance of the axis is not that anything lateral to it will increase pronation moment (and vice versa for supination moment). That's certainly how I used to think of it.

BUT, there must be such a line, beyond which a mod stops reducing pronatory moment and starts adding to it, and it must involve the position of the axis.

If its about moving the COP nearer or further from the axis there must be a midpoint.

So we take the STA on one side, and the lateral most significantly weight bearing point (say 5th met). We draw a line in between the two. Thus



Theoretically, is it the case that any forefoot mod placed lateral to THAT line will pronate the foot?


OR


Is it simply the case that wherever the COP is in the forefoot, any mod placed Lateral to THAT line will create a pronatory moment, Thus



or thus



In terms of practical application, I'm thinking firstly in terms of where we put modifications on orthotics (as in forefoot varus extensions) and also in terms of where we find the bulk on a pre fab orthotic. It strikes me that this line, wherever it may be, would be a significant piece of information to know, if indeed it exists!

Would value everybodies thoughts.
To determine the moment from the ground about the STJ axis you have to know where the axis is and where the center of pressure is. So if you put your lump lateral to existing center of pressure then you would increase pronation moment, and if you put the lump medial to the current CoP you would decrease pronation moment. This assumes that there is no change in position of the joints or change in muscle tension. With those restrictions I wouldn't think about it too hard.

Regards,
Eric
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Old 23rd October 2009, 03:05 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Simon Spooner View Post
Robert, I've only given a quick scan of what you are saying and asking, but my first thoughts are that this depends not only on the position of the axis and modification, but also on the shape and load-deformation characteristics of the modification. And in the case of a forefoot modification, the load-deformation characteristics of the rays.
Simon makes a very valid point here when talking about FF modifications.

I regularly use a digital wedge 1-5 and sometimes just sub hallux, ala cluffy wedge. I think Howard talked about this first though.

Basically it extends the toes so PF's the metatarsals depending on DFion stiffnessof the metatarsals. This effectively pronates the medial column and vice versa the lateral column if you do it correctly when casting your CFO.

I'll also use a lateral wedge sub 3-5 from the sulcus to the base of the mets valgus posted 5 degrees or more. This will pronate the foot, not the STJ in late midstance as the heel starts to unweight.

Both of these mods, and specific casting positions, are used to stabilize the midfoot/forefoot as quickly as possible to allow for undeterred or to eliminate the delay of progression in midstance.

i would suppose they re-supinate the STJ earlier than if they were not used at all, and I see that in both the CoP positioning in-shoe as well as the F/T cuve timing.

According to the theory above, this should not be possible.

BRuce
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Old 23rd October 2009, 05:22 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Bruce Williams View Post
According to the theory above, this should not be possible.

BRuce
Bruce:

For at least the last two decades, I have noted the paradoxical effect that sometimes forefoot valgus wedges cause more STJ supination during the late midstance and propulsive phases of gait. It not only is possible, but it also happens often and is quite predicatable in many feet. In addition, this paradoxical effect is also easily explained using the basic mechanical principals embodied within the STJ axis location/rotational equilibrium theory of foot function.

My theory is that the increase in external STJ pronation moment during late midstance and propulsion which occurs due to the forefoot valgus wedge will often cause an increase in duration and force of central nervous system mediated gastrocnemius-soleus contractile activity which, in turn, causes an increase in internal STJ supination moment, during late midstance and propulsion. This increase in internal STJ supination moment from increased force and duration of gastrocnemius-soleus contractile activity causes increased STJ supination motion during late midstance and propulsion even though the valgus forefoot wedge is increasing the external STJ pronation moment during this same phase of gait.

This is an excellent example of the neuromotor effect of foot orthoses which is mediated by the central nervous system, which should importantly be distinguised from the direct mechanical effect of foot orthoses which is caused by the valgus forefoot wedge mechanically pushing harder on the lateral forefoot which, in turn, causes an increase in external STJ pronation moment.

Simon.............any ideas.........??
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Old 23rd October 2009, 07:30 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Kevin Kirby View Post
Bruce:

For at least the last two decades, I have noted the paradoxical effect that sometimes forefoot valgus wedges cause more STJ supination during the late midstance and propulsive phases of gait. It not only is possible, but it also happens often and is quite predicatable in many feet. In addition, this paradoxical effect is also easily explained using the basic mechanical principals embodied within the STJ axis location/rotational equilibrium theory of foot function.

My theory is that the increase in external STJ pronation moment during late midstance and propulsion which occurs due to the forefoot valgus wedge will often cause an increase in duration and force of central nervous system mediated gastrocnemius-soleus contractile activity which, in turn, causes an increase in internal STJ supination moment, during late midstance and propulsion. This increase in internal STJ supination moment from increased force and duration of gastrocnemius-soleus contractile activity causes increased STJ supination motion during late midstance and propulsion even though the valgus forefoot wedge is increasing the external STJ pronation moment during this same phase of gait.

This is an excellent example of the neuromotor effect of foot orthoses which is mediated by the central nervous system, which should importantly be distinguised from the direct mechanical effect of foot orthoses which is caused by the valgus forefoot wedge mechanically pushing harder on the lateral forefoot which, in turn, causes an increase in external STJ pronation moment.

Simon.............any ideas.........??
Wow Kevin, you have a very interesting way of saying that the stabilization of the midfoot, from the digital wedging and FF valgus posting, activates the peroneals and thereby decreases the prolonged activation of the gastro-soleal complex.

Craig Payne summed it up nicely by stating that the triad of the achilles, peroneals, and tibialis muscles need to function as a unit in a stable midfoot structure. Failure of one leads to prolonged activation of the others.

This decreased need of the achilles complex to fire may likely lead to an increase in the range of motion at the AJ, and is likely directly related to a stable midfoot/forefoot as well and would be akin to what you are saying above.

What you don't say though, is why this scenario works within your theory of mechanics to which you subscribe. As Craig said a long time ago, there is a blending of theories. I agree. This is one of those areas within your theory of thought that does not fit. It makes perfect sense to me mechanically from my theoretical perspective. We're getting there, slowyl but surely I think.

Also, your "excellent example of the [b][i]neuromotor effect of foot orthoses" is really not a proven example but a theory. I am not discounting its likliness, just that it does need to be tested to be proven conclusively.
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Old 23rd October 2009, 07:45 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Bruce Williams View Post
Wow Kevin, you have a very interesting way of saying that the stabilization of the midfoot, from the digital wedging and FF valgus posting, activates the peroneals and thereby decreases the prolonged activation of the gastro-soleal complex.
Bruce:

Please define "stabilization of the midfoot".

What I did say was that the valgus forefoot wedge would increase the activation and duration of gastrocnemius-soleus complex activity. No where in what I wrote did I say the valgus forefoot wedge "activates the peroneals and thereby decreases the prolonged activation of the gastro-soleal complex."

Quote:
Originally Posted by Bruce
Also, your "excellent example of the [b][i]neuromotor effect of foot orthoses" is really not a proven example but a theory. I am not discounting its likliness, just that it does need to be tested to be proven conclusively.
I didn't say my example was "proven". I offered it as a theory just as all the proponents of sagittal plane facilitation theory offer their clinical examples simply as theories, without proof.
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Old 23rd October 2009, 09:32 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Bruce Williams View Post
Wow Kevin, you have a very interesting way of saying that the stabilization of the midfoot, from the digital wedging and FF valgus posting, activates the peroneals and thereby decreases the prolonged activation of the gastro-soleal complex.
Wow, Bruce you have an interesting way of interpreting Kevin's statement. I liked Kevin's version better because I can understand the term moments better than I understand the term stabilization. I'm also trying to figure out how stabilization of the midfoot activates the peroneal muscles when the peroneal muscles are activated by the central nervous system. And I'm trying to figure out how activation of the peroneal muscles decrease the prolonged activation of the gastroc soleus complex as the gastroc soleus complex is also controlled by the central nervous system.


I realize that what Kevin and I say may not have research to support our contentions, but at least it could be tested through the use of inverse dynamics and EMG analysis. I wouldn't know where to start doing research on stabilization because it is an undefined and unmeasurable term.

I don't mean to jump on you Bruce, but the concept of stability has always bothered me since I heard it used when I was a student. The word just sets me off.

Regards,

Eric
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Old 23rd October 2009, 11:38 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Robertisaacs View Post
I've been mulling the fascinating replies on the earlier thread on forefoot varus extensions. It's a puzzle which has been bouncing around in my head for a little while now and I've really enjoyed everyones view on it.

Of course the reason for its significance lies solidly in the realm of the applied biomechanics and the nitty gritty of where the bulk lies on an orthotic as well as the fascinating atom splitting which we all enjoy.

In that vein, at 3:30 am this morning, whilst attending to a flu ridden toddler (the swine), I mulled thus.

So a varus extention, or indeed a lump, can still reduce the pronation moment even if its lateral to the axis (by moving the COP nearer to the axis and reducing the lever arm).

So the significance of the axis is not that anything lateral to it will increase pronation moment (and vice versa for supination moment). That's certainly how I used to think of it.

BUT, there must be such a line, beyond which a mod stops reducing pronatory moment and starts adding to it, and it must involve the position of the axis.

If its about moving the COP nearer or further from the axis there must be a midpoint.

So we take the STA on one side, and the lateral most significantly weight bearing point (say 5th met). We draw a line in between the two. Thus



Theoretically, is it the case that any forefoot mod placed lateral to THAT line will pronate the foot?


OR


Is it simply the case that wherever the COP is in the forefoot, any mod placed Lateral to THAT line will create a pronatory moment, Thus



or thus



In terms of practical application, I'm thinking firstly in terms of where we put modifications on orthotics (as in forefoot varus extensions) and also in terms of where we find the bulk on a pre fab orthotic. It strikes me that this line, wherever it may be, would be a significant piece of information to know, if indeed it exists!

Would value everybodies thoughts.

Regards
Hope the child is feeling better. Got a bit of a flu bug myself I think

Also it is not a problem that the COP will be a moving target the whole time during gait. So if you could track the movemnt of COP and determine all appropriate positions, vectors etc it would then allow you to determine the best place for your forefoot posting. The problem here might be that the pad would also have to move in relation to COP to get the best results. I guess that also throws in axis movement as well into the equation.

Thats my thoughts
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Old 24th October 2009, 10:13 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Robertisaacs View Post
So a varus extention, or indeed a lump, can still reduce the pronation moment even if its lateral to the axis (by moving the COP nearer to the axis and reducing the lever arm).

So the significance of the axis is not that anything lateral to it will increase pronation moment (and vice versa for supination moment). That's certainly how I used to think of it.

BUT, there must be such a line, beyond which a mod stops reducing pronatory moment and starts adding to it, and it must involve the position of the axis.

If its about moving the COP nearer or further from the axis there must be a midpoint.

So we take the STA on one side, and the lateral most significantly weight bearing point (say 5th met). We draw a line in between the two. Thus


Theoretically, is it the case that any forefoot mod placed lateral to THAT line will pronate the foot?


OR


Is it simply the case that wherever the COP is in the forefoot, any mod placed Lateral to THAT line will create a pronatory moment, Thus

Would value everybodies thoughts.

Regards
What seems lacking here is any consideration of the influence of the structural relationship of the ff to rf and the functional influence of the MTJ. As an example, consider the impact of placing a wedge under the lateral forefoot in an individual with a forefoot varus as opposed to a forefoot valgus. How might this change the influence of the wedge? What about the difference in an individual with a fairly rigid mtj as opposed to an individual with a highly mobile mtj?

Respectfully,
Jeff
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Old 24th October 2009, 01:46 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by efuller View Post
Wow, Bruce you have an interesting way of interpreting Kevin's statement. I liked Kevin's version better because I can understand the term moments better than I understand the term stabilization. I'm also trying to figure out how stabilization of the midfoot activates the peroneal muscles when the peroneal muscles are activated by the central nervous system. And I'm trying to figure out how activation of the peroneal muscles decrease the prolonged activation of the gastroc soleus complex as the gastroc soleus complex is also controlled by the central nervous system.


I realize that what Kevin and I say may not have research to support our contentions, but at least it could be tested through the use of inverse dynamics and EMG analysis. I wouldn't know where to start doing research on stabilization because it is an undefined and unmeasurable term.

I don't mean to jump on you Bruce, but the concept of stability has always bothered me since I heard it used when I was a student. The word just sets me off.

Regards,

Eric
Kevin and Eric;

when I say stabilization I could also say stiffness. that might work out better for you both and will not apparently get Eric's panties in a bunch.

Nesters work is pointing out the stiffness and positional relationships between the midfoot and metatarsals. Biomechanics of the First Ray part v or VI by Rolling adn Christensen, shows a relationship between the peroneals being inhibited when the achilles complex is tight. Increase the rom of the AJ thru manipulation or stiffen the MF/FF complex and you will usually see activation of the peroneals in my experience. I did do an emg study on this as well that proved this but unfortunately was not published.

Ultimately neither one of you have really tried to explain the so called paradox that kevin alluded to initially in his SALRE theory. Why do these orthotic modifications work when according to the diagram that started this discussion off, they should most definitely not.

I have no qualms relating to a CNS effect from positional changes of the foot due to orthotic modifications. But if the theory says it won't work then how does the CNS suddenly know to paradoxically work they way it does? This is the part I'd like explained. I know you are tired of this and see it as sniping. That is not my intention, honestly.

We all seem to agree that the modifications in FF, digits and lateral MF work. I figured that you'd add DFion stiffness of the metatarsals into SALRE as it makes the most sense to me in why these mods do work. That is why I mention it regularly in this list and when I speak. I'd see the increased stiffness of the medial column affecting the STJ axis positioning as well, possibly enough to hit the midpoint that Robert started off this discussion with. This does not seem paradoxical at all to me, and also seems to almost work within the SALRE theory.

See you in Ohio Eric!
Bruce
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Old 24th October 2009, 05:39 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Bruce Williams View Post
Nesters work is pointing out the stiffness and positional relationships between the midfoot and metatarsals.
I thought Chris Nester's work has only been on kinematics. I don't believe Chris has ever measured the stiffness of the foot segments.

Quote:
Originally Posted by Bruce Williams
Biomechanics of the First Ray part v or VI by Rolling adn Christensen, shows a relationship between the peroneals being inhibited when the achilles complex is tight.
Jeff Christensten's (one of my CCPM classmates) study on the first ray was done on cadaver specimens. I don't believe he said that the peroneals were "inhibited" since these were dead feet. How does the central nervous system inhibit the peroneals when the individual is no longer living?

Quote:
Originally Posted by Bruce Williams
Ultimately neither one of you have really tried to explain the so called paradox that kevin alluded to initially in his SALRE theory. Why do these orthotic modifications work when according to the diagram that started this discussion off, they should most definitely not.
I did try to explain my theory in my last posting, which Eric seemed to have no problem understanding, but I guess that since you didn't understand it, there are probably many others that didn't also.

Here is a short version:

Ground reaction force (GRF) acting lateral to the subtalar joint (STJ) axis on the lateral forefoot will have only one direct mechanical effect on the STJ axis: it will cause an increase in external STJ pronation moment. However, since some feet supinate in late midstance and propulsion with a valgus forefoot wedge added to the orthosis, then, using subtalar joint axis location and rotational equilibrium (SALRE) theory, there must be some other internal STJ supination moment acting during late midstance and propulsion to cause the STJ supination acceleration. This, I propose, comes from the central nervous system (CNS) sensing that the additional STJ pronation moment coming from the lateral forefoot from the forefoot valgus wedging will offer enough resistance to STJ supination to allow it to send increased magnitude and duration of efferent stimuli to the gastrocnemius-soleus complex (GSC) to allow increased activation and duration of GSC activity during propulsion, without causing STJ supination instability.

As you know, the CNS uses joint position receptors, plantar skin receptors and tendon stretch receptors in addition to visual and inner ear balance afferent input to determine the specific muscle firing pattern which will be best for the individual's optimal bipedal locomotion. The example I have given you above is one of the examples of neuromotor effects of foot orthoses which I have written about and will soon publish in my October 2009 Precision Intricast Newsletter.

The SALRE theory does not, Bruce, preclude including the internal STJ supination and pronation moments from muscular (which originate from CNS activation) and from joint compression forces within the equilibrium calculations. In fact, I think you will find that even my first paper on rotational equilibrium across the STJ axis that I wrote over two decades ago (Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989) discusses how internal STJ supination moments from posterior tibial tendon tensile forces and sinus tarsi compression forces may alter STJ rotational equilibrium.

I have attached my first rotational equilibrium paper for your review so you can see how muscle function is already integrated into SALRE theory, whether it arises from CNS efferent output or from passive muscle-tendon stretch effects.

Quote:
Originally Posted by Bruce Williams
We all seem to agree that the modifications in FF, digits and lateral MF work. I figured that you'd add DFion stiffness of the metatarsals into SALRE as it makes the most sense to me in why these mods do work. That is why I mention it regularly in this list and when I speak. I'd see the increased stiffness of the medial column affecting the STJ axis positioning as well, possibly enough to hit the midpoint that Robert started off this discussion with. This does not seem paradoxical at all to me, and also seems to almost work within the SALRE theory.
The concept of metatarsal ray stiffness/compliance that I have tried to introduce into the podiatric lexicon is simply my attempt to bring the clinical world of podiatric medicine into using the same terminology that is already in wide use within the international biomechanics community (Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009, pp. 75-84). I have already recently written a Precision Intricast Newsletter in April 2009 on how increased first ray dorsiflexion stiffness may increase the external STJ supination moment by causing a shift in center of pressure medially. This newsletter, along with my direct mechanical/neuromotor newsletter will probably be published in my fourth book, in about 5 years.

Therefore, as you so rightly point out, it is relatively easy to integrate the load-deformation characteristics of the metatarsal rays into SALRE theory and, when I lecture, I also try to combine the two concepts so that a more biomechanically-sound approach is presented to the audience.

Good discussion.
Attached Files
File Type: pdf Rotational Equilibrium Across the STJ Axis.pdf (1.28 MB, 22 views)
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Old 25th October 2009, 09:12 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Jeff Root View Post
What seems lacking here is any consideration of the influence of the structural relationship of the ff to rf and the functional influence of the MTJ. As an example, consider the impact of placing a wedge under the lateral forefoot in an individual with a forefoot varus as opposed to a forefoot valgus. How might this change the influence of the wedge? What about the difference in an individual with a fairly rigid mtj as opposed to an individual with a highly mobile mtj?

Respectfully,
Jeff
www.root-lab.com
I feel that forefoot to rearfoot relationship is not needed in this discussion because we are looking at a foot post compensation for its deformity, if any existed. We are looking at the foot in the position that it is in in rest. To move the foot out of its rest position forces will have to be applied to it no matter what the relationship of the forefoot to rearfoot was in the non weight bearing neutral position.

Respectfully,

Eric Fuller
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Old 25th October 2009, 09:43 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Bruce Williams View Post
Kevin and Eric;

when I say stabilization I could also say stiffness. that might work out better for you both and will not apparently get Eric's panties in a bunch.
My panties are most of the way out of the bunch. To fully unbunch them you'd have to talk about the direction of the stiffness. I am assuming you are referring to stiffness in dorsiflexion?


Quote:
Originally Posted by Bruce Williams View Post
Ultimately neither one of you have really tried to explain the so called paradox that kevin alluded to initially in his SALRE theory. Why do these orthotic modifications work when according to the diagram that started this discussion off, they should most definitely not.

I have no qualms relating to a CNS effect from positional changes of the foot due to orthotic modifications. But if the theory says it won't work then how does the CNS suddenly know to paradoxically work they way it does? This is the part I'd like explained. I know you are tired of this and see it as sniping. That is not my intention, honestly.
Kevin's explanation is quite plausible. I'd like to try a sligthly different version.

A lot of feet exhibit late shance phase pronation of the STJ or pronation about an axis described as the oblique axis of the MTJ. The phenomenon that you are asking us to explain is why do these feet with late stance phase pronation not have this pronation when a forefoot valgus wedge is added to a device.

To have late stance phase pronation of the STJ there has to be Range of Motion available at the STJ. Although Root Orion and Weed described subluxation of the STJ in the direction of pronation, I don't see how this can happen anatomically. When the lateral process of the talus hits the floor of the sinus tarsi there is no where for the STJ to sublux to, in the direction of pronation. So, these people who exhibit late stance phase pronation have heel contact, lateral forefoot loading and then medial forefoot loading and then stop a position of the STJ where it is not maximally pronated. Often this occurs because there is a more laterally positioned STJ axis that can provide a direct supination moment to stop the pronation that started at forefoot loading. (those feet with a more medially deviated STJ axis will continue to pronate until they reach the end range of motion of the STJ because there is not sufficient supination moment to stop the initial contact phase pronation.)

So, you have a foot with a laterally deviated STJ axis that is in gait just after full forefoot loading. It is not maximally pronated. The tibia rotates forward and activity of the Gastroc soleus begin to increase to create a plantar flexoin moment. The gastroc soleus simultaneously adds a supination moment at the STJ. The individual feels this increased tendency toward supination and does not want this to occur because that would tend to cause a sprained ankle. So to prevent this supination they "choose" to use their peroneal muscles to prevent a sprained ankle. This is my explanation of why late stance phase pronation occurs. I've also called this pronation from muscular causes as opposed to being caused by ground reaction force.

A forefoot valgus wedge could prevent this muscularly caused pronation in a couple of ways. When the foot lands on the wedge in gait, the STJ will have to pronate farther than without the wedge to get force on the medial forefoot to have the forefoot stop pronation. This further pronation will tend to internally rotate the talus and move the STJ axis to a more medially angled position. Both of these potential effects may make it feel that the ankle is less likely to sprain and therefore there will be less peroneal activation and potentially allowing resupination later in gait.

Let me know if there is any of the above that does not make sense to you.

Regards,

Eric
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Old 26th October 2009, 01:34 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by efuller View Post
My panties are most of the way out of the bunch. To fully unbunch them you'd have to talk about the direction of the stiffness. I am assuming you are referring to stiffness in dorsiflexion?




Kevin's explanation is quite plausible. I'd like to try a sligthly different version.

A lot of feet exhibit late shance phase pronation of the STJ or pronation about an axis described as the oblique axis of the MTJ. The phenomenon that you are asking us to explain is why do these feet with late stance phase pronation not have this pronation when a forefoot valgus wedge is added to a device.

To have late stance phase pronation of the STJ there has to be Range of Motion available at the STJ. Although Root Orion and Weed described subluxation of the STJ in the direction of pronation, I don't see how this can happen anatomically. When the lateral process of the talus hits the floor of the sinus tarsi there is no where for the STJ to sublux to, in the direction of pronation. So, these people who exhibit late stance phase pronation have heel contact, lateral forefoot loading and then medial forefoot loading and then stop a position of the STJ where it is not maximally pronated. Often this occurs because there is a more laterally positioned STJ axis that can provide a direct supination moment to stop the pronation that started at forefoot loading. (those feet with a more medially deviated STJ axis will continue to pronate until they reach the end range of motion of the STJ because there is not sufficient supination moment to stop the initial contact phase pronation.)

So, you have a foot with a laterally deviated STJ axis that is in gait just after full forefoot loading. It is not maximally pronated. The tibia rotates forward and activity of the Gastroc soleus begin to increase to create a plantar flexoin moment. The gastroc soleus simultaneously adds a supination moment at the STJ. The individual feels this increased tendency toward supination and does not want this to occur because that would tend to cause a sprained ankle. So to prevent this supination they "choose" to use their peroneal muscles to prevent a sprained ankle. This is my explanation of why late stance phase pronation occurs. I've also called this pronation from muscular causes as opposed to being caused by ground reaction force.

A forefoot valgus wedge could prevent this muscularly caused pronation in a couple of ways. When the foot lands on the wedge in gait, the STJ will have to pronate farther than without the wedge to get force on the medial forefoot to have the forefoot stop pronation. This further pronation will tend to internally rotate the talus and move the STJ axis to a more medially angled position. Both of these potential effects may make it feel that the ankle is less likely to sprain and therefore there will be less peroneal activation and potentially allowing resupination later in gait.

Let me know if there is any of the above that does not make sense to you.

Regards,

Eric

Eric;

Once again I have to point out that according to the model put forth in the SALRE theory, the one that started out this entire discussion, any lateral posting will cause an increase in the pronation moment. I don't see how the example above has a medial STJ axis. I have not once been talking about anything but the example above.

This quasi leap of faith that you and Kevin ask us to believe completely disregards the model that started this discussion, and now you ask me to think that the MF/FF valgus posting is pronating the STJ more which then allows the CNS to kick in and make it all better? So all we have to do is maximally pronate the STJ and everything will be better?

Seriously, are you guys just messing with me here? Good on you if you are, I'll buy and we can start to have a real discussion about foot function then!

The benefit of this modification, lateral MF/FF Valgus post in conjuntion with a varus heel posting or skive is that the RF is positioned in such a way that it resists some of the pronation moments from the AJ equinus (Kevin says it may act as a heel lift as well and I have to agree) and benefits even more from the supination moment that starts at heel unweighting and allows the shortened lever arm of the PL to stabilize the 1st ray in a PF'd position below the lesser rays. The Plantar Fascia medial band will kick in too as the hallux extends adding even more supination moment. ( use kogler as a reference here).

I respectfully ask you guys to work within the context of the model that started out this discussion if we are going to continue to talk about the plausible ways of this working. I don't mind working it as a functional model even though it is a static image, I think we can agree to that.

Let me know if I'm buying the beer, I could use a good laugh!
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Old 26th October 2009, 01:48 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Kevin;

I am not, repeat NOT, trying to get you to admit that sagittal plane theory, or any other theory is better than yours! I promise!

There is a middle ground and I am trying my darndest to work towards it. I think that pretty much everything I've tossed at you will increase the supination moment of the STJ, let me know if I'm wrong.

I think I'm really trying to explain to you and others why modifications such as I use may indeed work within SALRE. I don't disagree with you CNS or muscle activation references only insomuch as they are not supported within the realm of the model that started out our discussion, not to my satisfaction anyway.

Work with me here. I like the idea that the skive is working somewhat as a heel lift which will combat AJE in this patient. That is a plus to the supination moment right there. Add in some manipulation to to increase the AJ rom and we are really on a roll!
Bruce
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Old 26th October 2009, 05:47 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Bruce Williams View Post
Kevin;

I am not, repeat NOT, trying to get you to admit that sagittal plane theory, or any other theory is better than yours! I promise!

There is a middle ground and I am trying my darndest to work towards it. I think that pretty much everything I've tossed at you will increase the supination moment of the STJ, let me know if I'm wrong.

I think I'm really trying to explain to you and others why modifications such as I use may indeed work within SALRE. I don't disagree with you CNS or muscle activation references only insomuch as they are not supported within the realm of the model that started out our discussion, not to my satisfaction anyway.

Work with me here. I like the idea that the skive is working somewhat as a heel lift which will combat AJE in this patient. That is a plus to the supination moment right there. Add in some manipulation to to increase the AJ rom and we are really on a roll!
Bruce
Bruce:

I don't know how to make my explanations any simpler than I already have. I could care less if my theory is proved wrong, if good scientific reasoning and rational thought processes are behind the proof.

It appears as though you are under the assumption, for reasons that I can't even imagine, that subtalar joint axis location rotational equilibrium (SALRE) theory states that there is no possibility of central nervous system control of muscular activity and/or no possibility that there can be internal muscular forces which can overcome the external pronation/supination moments coming from the foot orthosis. Please show me in any of my following papers or books, or any of my 3,000+ postings here on Podiatry Arena, where I have stated such, and I will admit to being corrected.

Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987.

Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989.

Kirby KA: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997.

Kirby KA: Biomechanics of the normal and abnormal foot. JAPMA, 90:30-34, 2000.

Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.

Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002.

Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009.
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Old 26th October 2009, 06:43 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Kevin Kirby View Post
Bruce:

I don't know how to make my explanations any simpler than I already have. I could care less if my theory is proved wrong, if good scientific reasoning and rational thought processes are behind the proof.

It appears as though you are under the assumption, for reasons that I can't even imagine, that subtalar joint axis location rotational equilibrium (SALRE) theory states that there is no possibility of central nervous system control of muscular activity and/or no possibility that there can be internal muscular forces which can overcome the external pronation/supination moments coming from the foot orthosis. Please show me in any of my following papers or books, or any of my 3,000+ postings here on Podiatry Arena, where I have stated such, and I will admit to being corrected.
Kevin;

Considering where a reverse morton's extension or FF valgus post would be placed on the diagram that started out this entire thread I do not see how the CNS or muscles would react as beneficially as you state it would according to SALRE theory.

Since you do not seem to want to discuss this further I suppose we'll have to leave it where it is.

I hope that I might have been able to add some potential insight to those who initially posted to this thread and would be happy to discuss this further with them if they wish.

Bruce
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Old 26th October 2009, 06:51 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Robertisaacs View Post
I've been mulling the fascinating replies on the earlier thread on forefoot varus extensions. It's a puzzle which has been bouncing around in my head for a little while now and I've really enjoyed everyones view on it.

Of course the reason for its significance lies solidly in the realm of the applied biomechanics and the nitty gritty of where the bulk lies on an orthotic as well as the fascinating atom splitting which we all enjoy.

In that vein, at 3:30 am this morning, whilst attending to a flu ridden toddler (the swine), I mulled thus.

So a varus extention, or indeed a lump, can still reduce the pronation moment even if its lateral to the axis (by moving the COP nearer to the axis and reducing the lever arm).

So the significance of the axis is not that anything lateral to it will increase pronation moment (and vice versa for supination moment). That's certainly how I used to think of it.

BUT, there must be such a line, beyond which a mod stops reducing pronatory moment and starts adding to it, and it must involve the position of the axis.

If its about moving the COP nearer or further from the axis there must be a midpoint.

So we take the STA on one side, and the lateral most significantly weight bearing point (say 5th met). We draw a line in between the two. Thus



Theoretically, is it the case that any forefoot mod placed lateral to THAT line will pronate the foot?


OR


Is it simply the case that wherever the COP is in the forefoot, any mod placed Lateral to THAT line will create a pronatory moment, Thus



or thus



In terms of practical application, I'm thinking firstly in terms of where we put modifications on orthotics (as in forefoot varus extensions) and also in terms of where we find the bulk on a pre fab orthotic. It strikes me that this line, wherever it may be, would be a significant piece of information to know, if indeed it exists!

Would value everybodies thoughts.

Regards
Robert;

I'd say its a combination of the proximal to distal line along the 2nd ray and the last diagram with a more lateral location of the STJ axis line. Maybe if you were to bring the most proximal aspect of the STJ axis in the last diagram towards the heel more and the more distal aspect near the 2nd or 3rd mpj? My gut feeling is that will be the closest axis during late midstance into propulsion.

nice post, I hope I might have at least stimulated some thought on this.

Bruce
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Old 26th October 2009, 09:11 PM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Bruce Williams View Post
Eric;

Once again I have to point out that according to the model put forth in the SALRE theory, the one that started out this entire discussion, any lateral posting will cause an increase in the pronation moment. I don't see how the example above has a medial STJ axis. I have not once been talking about anything but the example above.
Bruce, I read through my post once again and I never saw where I said the example had a medially deviated STJ axis. I described a foot with a laterally deviated axis that was pronating because of muscular action. Does this help?

The model that started this discussion was one where a forefoot varus wedge was placed under the forefoot. This is a different beast than what I described above. I would only put a forefoot valgus wedge under the foot that started this discussion if most of ground reaction force was on the medial column. The foot would stay maximally pronated with additional wedge, but after heel off the force on the forefoot would be more evenly distributed. This wouldn't necessarily cause resupination, but would be more comfortable. Does this help?

Regards,

Eric
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Old 28th October 2009, 08:39 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Bruce Williams View Post
Kevin;

Considering where a reverse morton's extension or FF valgus post would be placed on the diagram that started out this entire thread I do not see how the CNS or muscles would react as beneficially as you state it would according to SALRE theory.

Since you do not seem to want to discuss this further I suppose we'll have to leave it where it is.

I hope that I might have been able to add some potential insight to those who initially posted to this thread and would be happy to discuss this further with them if they wish.

Bruce
Bruce:

Sorry for not responding sooner. Been very busy lately.

The whole reason I have been promoting over the past 20+ years that the podiatric profession uses the term "change moments" to describe what orthoses do to the foot, rather than using the term "change motion" as most of the rest of the podiatry profession has been saying orthoses do to the foot, is because my strong belief that orthoses change moments, rather than change motion. This allows for my subtalar joint axis location/rotational equilibrium (SALRE) theory to encompass the wide range of internal changes in STJ moments that may occur in response to the mechanical pushing effect from the orthosis on the plantar foot. In other words, SALRE theory takes into account that a moment applied to the foot does not necessarily mean that the foot will rotate in the same direction as the direction of the applied external moment.

For example, if I place a varus wedge under the heel of a foot, an increase in external STJ supination moment will be placed on the foot. However, this increase in external STJ supination moment from the rearfoot varus wedge does not necessarily mean that STJ supination motion will occur as a result of the wedge since it is possible also that internal STJ pronation moments may have been generated within the body in response to the externally applied moment from the rearfoot varus wedge.

Physics tells us that there are three possible rotational mechanical effects of the varus rearfoot wedge on the subtalar joint (STJ):

1. No STJ motion.
2. STJ supination motion.
3. STJ pronation motion.

Now, if we can assume that the varus rearfoot wedge has added 5 Nm of external STJ supination moment, what do we now know about the magnitude of change in internal STJ pronation and/or internal STJ supination moments that have occurred simultaneously with application of the varus rearfoot wedge to the foot in each of the three conditions? If you can answer this question, then you will develop a better understanding of why SALRE theory makes full allowance for all the constantly changing internal STJ rotational forces that occur within the human body which may indeed, at times, be opposite in rotational direction to the rotational direction that the orthosis is mechanically pushing the foot.
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Old 28th October 2009, 10:05 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Kevin,

Quote:
The whole reason I have been promoting over the past 20+ years that the podiatric profession uses the term "change moments" to describe what orthoses do to the foot, rather than using the term "change motion" as most of the rest of the podiatry profession has been saying
It's probably both. The question for me is which comes first?

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Old 28th October 2009, 10:28 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by efuller View Post
I feel that forefoot to rearfoot relationship is not needed in this discussion because we are looking at a foot post compensation for its deformity, if any existed. We are looking at the foot in the position that it is in in rest. To move the foot out of its rest position forces will have to be applied to it no matter what the relationship of the forefoot to rearfoot was in the non weight bearing neutral position.

Respectfully,

Eric Fuller
Eric,

My comment was in response to Robert who said
Quote:
So the significance of the axis is not that anything lateral to it will increase pronation moment (and vice versa for supination moment). That's certainly how I used to think of it.
My point is that an everted forefoot wedge, which is lateral to the STJ axis may simply act distal to the MTJ and could potentially have no influence on the STJ. For example, if an individual on non-weightbearing examination displays a plantarflexed 1st ray and as a result has an everted forefoot to rearfoot relationship, the foot may compensate during relaxed stance via closed chain forefoot inversion bringing the mets parallel to the supporting surface with no change in STJ position. You can place a valgus wedge under the lateral forefoot that will evert the forefoot but all of the influence could potentially be anterior to the MTJ.

Conversely, if the same patient happened a have perpendicular forefoot to rearfoot relationship and as a result had no further range of closed chain MTJ eversion available when you applied a valgus wedge under the lateral forefoot, then the influence of the ff wedge at the STJ would be different as it would likely create a greater STJ pronation moment. In other words, the retrograde influence of the same wedge can be influenced by the ff to rf relationship and the potential rom at the MTJ.

If you really what to have some fun with this discussion, try superimposing a hypothetical MTJ axis on the drawings, then tell me what happens at the STJ. A much more complicated model and discussion would result.

Respectfully,
Jeff
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Old 28th October 2009, 10:46 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

Quote:
Originally Posted by Jeff Root View Post
Eric,



My point is that an everted forefoot wedge, which is lateral to the STJ axis may simply act distal to the MTJ and could potentially have no influence on the STJ.
Any change in force beneath the foot will simultaneously influence the moment about the STJ axis since forces don't "flow". You are making the assumption of kinematic change, which I believe was the point Kevin was making in his last post to this thread.
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Old 28th October 2009, 11:16 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Jeff Root View Post
Eric,

If you really what to have some fun with this discussion, try superimposing a hypothetical MTJ axis on the drawings, then tell me what happens at the STJ. A much more complicated model and discussion would result.

Respectfully,
Jeff
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Thank you Jeff. This is exactly what I've been trying to do in this thread.

The real question is will the moments inside and outside the STJ change significantly as the CoM of the body, not the foot, move past or anterior to the STJ, especially depending on what FF/MF postings are utilized?

As the heel lifts, or begins to lift, off the ground which joint(s) in the foot will now govern its reactionary movement(s)? Once the heel starts to unweight it should be much more susceptible to supination moments if the mpj's start to extend. If they do not then the PF will not be able to impart its drastically needed use in the way it seems to have been intended.

Utilizing a FF valgus posting could very well at least allow the 1st ray to stay PF'd in relation to the lesser rays causing an increase in DFion moments at the 1st mjp, and imparting an increase in supination moments back at the STJ as well.

Bruce
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Old 28th October 2009, 11:22 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Kevin Kirby View Post
Physics tells us that there are three possible rotational mechanical effects of the varus rearfoot wedge on the subtalar joint (STJ):

1. No STJ motion.
2. STJ supination motion.
3. STJ pronation motion.

Now, if we can assume that the varus rearfoot wedge has added 5 Nm of external STJ supination moment, what do we now know about the magnitude of change in internal STJ pronation and/or internal STJ supination moments that have occurred simultaneously with application of the varus rearfoot wedge to the foot in each of the three conditions?
1. Change in external+ internal supination moments is exactly matched by change in internal + external pronation moments.
2. Internal pronation moment may have stayed the same, decreased, or increased in response to the wedge. However, if the internal pronation moment did increase in response to the wedge, it's magnitude + the magnitude of the external pronation moment are less than the magnitude of the external + internal supination moments.
3. Increase in internal + external pronation moments in response to the wedge is greater than the increase in external + internal supination moments.

Something like that, anyway.
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Old 28th October 2009, 11:22 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by efuller View Post
Bruce, I read through my post once again and I never saw where I said the example had a medially deviated STJ axis. I described a foot with a laterally deviated axis that was pronating because of muscular action. Does this help?

The model that started this discussion was one where a forefoot varus wedge was placed under the forefoot. This is a different beast than what I described above. I would only put a forefoot valgus wedge under the foot that started this discussion if most of ground reaction force was on the medial column. The foot would stay maximally pronated with additional wedge, but after heel off the force on the forefoot would be more evenly distributed. This wouldn't necessarily cause resupination, but would be more comfortable. Does this help?

Regards,

Eric

Eric;

My bad if I said your example had a medial STJ axis.

Still, I think it has been very obvious to me, though not to you and Kevin, that I don't want to talk about any other example other than the one that started this thread.

I appreciate that you at least talked about the example that I wanted and that you did talk about what will happen after heel lift.

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Old 28th October 2009, 11:23 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Originally Posted by Simon Spooner View Post
Any change in force will simultaneously influence the moment about the STJ axis since forces don't "flow".
Simon,

So let's look at another example. If the hallux is bearing weight in relaxed stance and if I then add a 5 degree Cluffy wedge under the hallux which then dorsiflexes the hallux five degrees, have I then increased the net dorsiflexion forces at the ankle joint because the ankle joint axis is proximal to the hallux or have I just changed the relative position of the hallux? What if I severed all the tendons to the hallux?

Now, if I put a wedge under the lateral forefoot and evert the MTJ, have I increased tension in the plantar fascia? Does that fascia insert medial to the STJ axis? What is the net effect on moments at the STJ when you consider all the of supination and pronation moments (net moments) resulting from a valgus forefoot wedge? Can we leave the MTJ and ff to rf relationship out of the discussion?

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Old 28th October 2009, 11:25 AM
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Default Re: Sub talar Rotational Equilibrium and Movement of COP

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Bruce:

Sorry for not responding sooner. Been very busy lately.

The whole reason I have been promoting over the past 20+ years that the podiatric profession uses the term "change moments" to describe what orthoses do to the foot, rather than using the term "change motion" as most of the rest of the podiatry profession has been saying orthoses do to the foot, is because my strong belief that orthoses change moments, rather than change motion. This allows for my subtalar joint axis location/rotational equilibrium (SALRE) theory to encompass the wide range of internal changes in STJ moments that may occur in response to the mechanical pushing effect from the orthosis on the plantar foot. In other words, SALRE theory takes into account that a moment applied to the foot does not necessarily mean that the foot will rotate in the same direction as the direction of the applied external moment.

For example, if I place a varus wedge under the heel of a foot, an increase in external STJ supination moment will be placed on the foot. However, this increase in external STJ supination moment from the rearfoot varus wedge does not necessarily mean that STJ supination motion will occur as a result of the wedge since it is possible also that internal STJ pronation moments may have been generated within the body in response to the externally applied moment from the rearfoot varus wedge.

Physics tells us that there are three possible rotational mechanical effects of the varus rearfoot wedge on the subtalar joint (STJ):

1. No STJ motion.
2. STJ supination motion.
3. STJ pronation motion.

Now, if we can assume that the varus rearfoot wedge has added 5 Nm of external STJ supination moment, what do we now know about the magnitude of change in internal STJ pronation and/or internal STJ supination moments that have occurred simultaneously with application of the varus rearfoot wedge to the foot in each of the three conditions? If you can answer this question, then you will develop a better understanding of why SALRE theory makes full allowance for all the constantly changing internal STJ rotational forces that occur within the human body which may indeed, at times, be opposite in rotational direction to the rotational direction that the orthosis is mechanically pushing the foot.
Kevin;

it is very obvious that you don't want to talk about what I keep asking you about regarding the example foot that started this thread.

I'll let you get back to your busy schedule then!

Bruce
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