Midtarsal Joint Equilibrium Theory

A deformable body still has to obey the laws of physics. It is not really possible to get just deformation when a force is applied to a deformable body. Imagine an underinflated beach ball. As you poke it with your finger it will deform and accelerate. F=ma. If there is a wall behind the ball and you apply a force toward the ball, the ball will deform in the direction of the force and it won't accelerate toward the wall because the wall will apply a force in the opposite direction so there net force applied is equal to zero. The same should be true for the foot. This may be why you can act like a deformable body is a rigid body when forces are applied.

Cheers,

Eric

 

skspooner@blueyonder.co.uk

I'm with Kevin, please try to answer my questions here rather than sending me to read the paper, what if I read the paper and still have questions? What does a 45 degree vector angle infer? Is it that for every degree of rearfoot segment motion, the forefoot segment moves by the same amount, i.e. in an angle, angle plot (x, y plot) with a perfect correlation (1:1 ratio) and the line passing through the origin, the line would be angled at 45 degrees. Or is it something completely different?

What are the kinetics of in-phase motion during midstance? If we take two blocks one for the forefoot segment and one for the rearfoot segment and rotate them, then plot the angle of each block against the other. In order for a 45 degree line in the plot, the ratio of the angular displacement of the two blocks has to be 1:1- right? So in other words they are effectively acting as a rigid body in the direction of rotation? So the net moments acting on each block in the direction of rotation are the same- wait a minute, what about the difference in the masses of each segment?

 

But as Ryan stated they had a range of 22.5 degrees - 67.5 degrees with pure being 45 degrees being considered in-phase so in realitly it can´t be ratio 1 : 1 can it ?

Also Ryan, you stated somewhere that he used the medial and lateral segments in this testing procedure. If so there would navicular-cuboid motion, which should have shown up in the testing, what did you do with these numbers as the medial and lateral columns maybe moving anti-phase to each other ? so why was it important to do this if you then compare segments which I´m reading as rearfoot-forefoot or ?

 

45 degrees would be perfect in-phase motion. Moving away from this the segments may still be moving in the same direction but not at the same velocity.

 

I hope this thread is not about to die ..... To many questions left unanswered.

Over the last few days I´ve been doing some more reading and thinking ....

Why was it decided that 2 joints ie the calcaneocuboid and the talonavicular joints would be referred to as the midtarsal joint, seems with more a more reading and with some of the discussion between Eric and Kevin re rigid and deformaable body diagrams the best thing would be to model the Midtarsal joint as 2 seperate joints with 2 different axis. The calcaneocuboid and talonavicular joints as we know from bone studies do have some movement and however small is movement.

This movement can be I guess described as in-phase or anti-phase so the " midtarsal joint" may move in-phase mostly but there will be some anti-phase.

I know this is old ground somewhat, But I keep coming back to it.

 

We discussed this in another thread on the midtarsal joint regarding 1 or 2 axis models... seek and you'll find it.
This is why the Van Langelaan work is interesting because he found the helical axes for the joints independently.

 

Heres the thread Simon mentions The midtarsal joint Ive read a few times and working my way through Van Langelaan ( not the lightest read) at the moment.

I know from some previous discussions with some of the foot and ankle reasearch team (Swedish bone pin studies group) they see the motion between the cuboid and navicular as significant. I look forward to the group latest study ( when it´s published )on wedges be intersting to see what happens at the midtarsal region.

 

Yeah, that's the one. Funny how I still believe now what I wrote then; not always the case.

 

I think the major reason for treating the midtarsal joint as a single joint is that the cuboid does not move much relative to the navicular. In the cadaver work that I've done, there is just not much movement between the cuboid and navicular. Between the talus and calcaneus there is quite a bit of motion. There are some pretty firm ligamentous attachments between the cuboid and navicular. There has to be, because of the pull of the posterior tibial tendon. If that ligament could tear easily you would commonly see an "avulsion" of the navicular from the cuboid.

If you think of the midtarsal joint as a planar joint with an envelope of motion then the observations seen make the most sense. A planar joint can have an infinite number of axes of rotation. Nester's work show that there were several different axes of rotation that were different from the classic LMTJ and OMTJ axes. A joint with an infinite number of axes can have, at any particular instant time, each of those described axes of motion. The take home message of this is that axis of motion just describes the motion seen, but does not offer predictive value of what the next motion will be for the midtarsal joint.

Using free body diagrams to examine forces across the midtarsal will reveal much more about what tissues are stressed. There was a paper by Scott and Winter that calculated the force in the Achilles tendon could be as high as 11 times body weight in a slow jog. That will create quite a bit of plantar flexion moment at the ankle and at at the midtarsal joint when that load is resisted by ground reaction force on the forefoot. So, if you want to understand the midtarsal joint, follow the force.

Regards,

Eric

 

That was an excellent discussion we had back then. I wonder what ever happened to Kevin Miller and all the tensegrity research data he was going to shake up the world with......"we are on the verge of knocking out foot mechanics"? Goes to show you.....when the promises seem too good to be true....they probably aren't true.

 

If you still have questions, I will be happy to answer them. I don't log onto PA regularly, but will try.

A 45 degree coupling angle means that the rearfoot and forefoot segments (not joints) rotated in the same direction with respect to the global coordinate system. The rotational magnitude for both segments was equal (e.g. both rotated 3 degrees eversion wrt to the global). You could call this a 1:1.

A 135 degree angle indicates that one segment rotated in one direction, while the other went in the other, in phase (e.g. forefoot eversion and rearfoot inversion.) This is also a 1:1.

A deviation from 45, for instance 67.5 degrees means more rotation of one segment wrt the other.... so a 2:1.

Yes, the bin definition has an effect of how you infer these data.

The traditional orthogonal directions (0, 90, 180, 270) are single segment movements. (e.g. forefoot everion but no rearfoot rotation).

With the way that these coupling angles are computed, we cannot infer the magnitude of the velocity, unfortunately. That information is certainly inherent to the angle angle plot, but not in the coupling angle. You could get any idea of velocity by determining the LENGTH of the vector between data points. I have thought about incorporating that info, but have not done so formally.

 

The vector coding method is kinematics only, no kinetics are computed. I really want to compute them, but there are significant difficulties. I'm not totally clear on what you mean here... so i don't think my answer is what you are looking for.

Sometimes it can be 1:1, but the way we categorize them, not "all in-phase" motion is precisely 1:1. If we set all the bin tolerances to nearly zero (i.e. exactly 45 degrees = in-phase), i think the number of in-phase movements would be miniscule... but that's not a great approach because the concept of anti-phase should have some tolerance. We can certainly play with the bin sizes, but if we make them smaller, then we would be left with unclassified movement patterns/data. We prefer to use 45 degree bins so that all data could be classified.

 

Ok follow the force, it appears that free body diagrams will be important in understanding the MTJ.

Ive posted some paper Winter re kinetics in slow running and walking and some bone pin studies in walking and slow running - maybe it will help ?


Ps Simon and Kevin does Ryans paper help was apples and apples or apples and oranges

 

Full text of Pohl's PhD thesis here (it's a big file):

http://etheses.whiterose.ac.uk/812/1/uk_bl_ethos_424496.pdf

 

Went looking for some stuff found this sorry no full text IG no go either maybe someone else will have luck, but it seems to indicate a change in GRF readings with plantar fasciits patients to those without - which may come back to Ryans in-phase and anti-phase changes with plantar fascia related issues,as there must be changes in GRF reading with different forefoot motion patterns.

Will try and read Pohl tonight with a couple of Jars

 

Michael, you should be aware that Wearing et al. did not control for gait speed in this experiment. In fact they have no measure of walking speed whatsoever. It is absolutely crucial to control walking speed especially when comparing two groups like this. It is quite possible that the PF group walked slower than the healthy group because of pain.

 

Thanks Ryan seems a fairly big whole in the experiment design.