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Ive been thinking again ...........

Ive taken Kevin´s picture from Variable mechanical effect of anterior tibial muscle contraction thread hope thats ok Kevin.

3 points.

point 1 - I know Simon and Robert have touched on this - but still think it worth going over again. In the above thread Kevin and Eric have posted re the Tibialis Anterior muscle can pronate - supinate or have no effect on motion at the subtalar joint. I get this and have discussed this with other many times.

But we are viewing the foot in the transverse plane so in reality the Subtalar joint axis may have no effect of transverse plane motion at the subtalar joint but cause moments in the Sagittial and Frontal planes. To truely say it have not motion we must view the foot and Subtalar joint axis in all 3 body planes or instead of saying when the subtalar joint is medially or laterally deviated and therefore the action of that muscle or even Gound reaction force will cause this triplance moment at the Subtalar joint we should infact be discussing in terms of single plane motion. Or am I thinking too much again..........

point 2 - When we are discussing Point 1 we always use the inserton points, should we not also be looking at other points that the muscle has a mode of action,say the Posterior Tib. Contraction of this muscle will attempt to shorten the distance between the origin and insertion, this increased tension will cause a moment at the Subtalar joint not just at the insertion point but also as it crosses under the medial mall. Right ? is this important?

I can picture where the Subtalar axis may run in between to points of action of a single muscle is this important?

point 3 when we are considering moments aound axis created from muscle do we need to consider the line of action of the muscle not just the point of insertion.

thinking too much ? or worth having a look at ?

Attached Images

STJ axis and TA.jpg (22.5 KB, 152 views)

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Mike Weber.



Crazy Mary

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Hey Mike.

It's a Bugger, but we mostly learn on paper, which is two dimensional. Often therefore we have to simplify into biplanar models and yes, that can cause problems. If we spend too long looking at 2 dimensional stuff its easy to forget the 3 dimensional bit. There's a video I did on youtube for a student which shows, in a very crude way with a foam box to represent the foot and arrows (Easton 2016 XX75 gamegetter if anyone is interested) to represent force vectors which shows this.

I think you're right. If the recent Kirby, Spooner Smith paper showed anything its the importance of vectors. This applies just as much to muscles pulling as ORF pushing.

I have another problem with this concept (the Tib ant function variation) related to the Interface axis thing I bounced earlier, but thats a long thread I've been saving for a rainy day

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Robert Isaacs
Specialist in Biomechanical Therapies
www.Footprintspodiatrysolutions.co.uk

small, yellow, leech-like, and probably the oddest thing in the universe

Semper in excretum sum sed alta variat

The tensile force within the anterior tibial tendon can cause moments across the subtalar joint (STJ) axis. The force vector for this tendon will likely follow very closely to the actual tendon, with the STJ moment being determined by where this tensile force vector from the tendon is relative to the STJ axis.

Try not to make this relatively uncomplicated mechanical concept any harder than it should be. If you are really stuck, make a model with the STJ axis connecting two blocks of wood and a rope that you can pull on to simulate the anterior tibial tendon so that you can play with it yourself.

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Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
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Kevin I´m pretty sure you know that I get the concepts but I looking to break it down a bit more, maybe there is no need, but it´s sometimes fun and educational.

A vector created by a muscle contraction to have no effect at a joint must the exact opposite to that joints axis in all 3 body planes.

is the above statment right, wrong or .... ?

and if the the vector created by the muscle is not exactly opposite the axis of the joint, then there will be a moment, if we are discussing a joint which has the ability of a triplane motion ie the Subtalar joint then if the vector is opposite in only one of the body planes then moments will occur in the other two body planes.

is the above statment right, wrong or .... ?

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Mike Weber.



Crazy Mary

As long as the muscle crosses a joint axis, its contractile activity will cause some mechanical effect on that joint, regardless of orientation of its force vector relative to the joint axis. If the force from the muscle has a moment arm to the joint axis, then it will create a rotational force (i.e. moment). If the force from the muscle has no moment arm to the joint axis, then it will create only a compression force at the joint. Generally, in most real situations, the muscle will create both a rotational force and a compression force at the joint.

I don't understand your question above, Mike. However, I think I know what you are trying to get at. In a highly constrained joint axis, such as the subtalar joint (STJ), then even an oblique pull from a muscle will cause a moment across the STJ axis, as long as the muscle force vector does not pass directly through the STJ axis. Because the STJ is a triplanar axis, and because the STJ is one of the most tightly constrained joint axes in the foot, then an internal force or an external force can act in any direction across the STJ and still cause a moment across the STJ, as long as the force vector does not pass directly through the STJ axis.

For example,in open kinetic chain examination of a foot, if you were to push on the plantar calcaneus with your thumb with a force vector that was directed medially in a line of action that was at the intersection of the transverse and frontal body planes (directly medial), the STJ would still rotate in a supination direction with the calcaneus plantarflexing, inverting, adducting and anteriorly translating on the talus. As long as your thumb's pushing force did not go directly through the STJ axis, then it would tend to rotate the STJ in a triplane direction.

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Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

Mike:

Here is a better illustration for you from my personal collection of 1,100+ drawings, all done on CorelDraw.

Attached Images

Anterior tibial tendon-STJ axis PPT.jpg (393.1 KB, 149 views)

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

Ok. Here's a question.

In the above foot, (GOT to get me corel draw) the axis is medial to the medial cuniform and the force vector we shall call vertical. And this, we say, exerts a pronation moment.

If I made an L shaped piece of metal and slid the short end under the base of the first met, and exerted an upward pull, the force, which we'll call BBOMF, would be lateral to the axis and the vector basically vertical. Much as in the above.

I cannot imagine how that force could ever pronate the foot more. In vivo that is always going to supinate the foot.

Why.

Obviously my answer involves the interface axis but I'd be interested in everyone elses thoughts.

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Robert Isaacs
Specialist in Biomechanical Therapies
www.Footprintspodiatrysolutions.co.uk

small, yellow, leech-like, and probably the oddest thing in the universe

Semper in excretum sum sed alta variat

Because while you may be exerting a pronation moment about the STJ axis at this point, your force application may well result in an increase in supination moments elsewhere.

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"Science is the antidote to the poison of enthusiasm and superstition."

Robert:

CorelDraw won't make an individual who lacks artistic talent become an artist. However, CorelDraw will enable the individual who has artistic talent to create a wide variety of images that may have been difficult to create otherwise and, especially, will allow easier manipulation of existing graphics to edit or create new graphics.

In other words, if the individual can't draw a foot free-hand so that it looks like a foot, then CorelDraw probably won't help them draw a better looking foot.

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

Ah. That first one would be me. The only thing I can reliably draw is blood!

Ok. Where and how?

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Robert Isaacs
Specialist in Biomechanical Therapies
www.Footprintspodiatrysolutions.co.uk

small, yellow, leech-like, and probably the oddest thing in the universe

Semper in excretum sum sed alta variat

Robert:

You will need to draw a picture of this since I am not following you or your description.

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

Anything that it medial to, starter for ten: calcaneocuboid joint axis.

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Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Amazing Kevin, Do you have a similar view on the STJ axis and Tib Ant. from a Sagittal and Frontal plane ?

I guess what I´m getting at is in your brilliant picture we would say normal position of the STJ axis contraction of the Tib Ant will cause a supination moment at the STJ axis and in the medial deviated view we would say contraction of the Tib Ant. will cause a Pronation moment at the Subtalar joint.

But as we are only looking at the view of the foot in one plane, we may not get pronation/supination moments but we may have a number of different moment groups triplane (not always pronation and supination ), bi plane, single plane or no motion at the Subtlar joint depending on the spatial location of the STJ axis within all 3 body planes in relation to the force created by contraction of the Tib Ant. muscle in this case.

Does that make more sense.

or does the position of the STJ axis in the transverse plane give us a solid indication of where the spatial location of the STJ axis is in relation to insertion points of muscle within the frontal and sagittal body planes ?

I would say it gives us an idea in real world mechanics - as Simon and Your paper indicates - The Subtalar Joint Axis Locator - A Preliminary Report

But if we modelling the foot would this be enough ?

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Mike Weber.



Crazy Mary

Video of the above STJ axis locator in use. For those that have not seen it.

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Mike Weber.



Crazy Mary

link to video doesn't work, Mike.

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Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Weird did for me when I checked anyway here is the weblink Subtalar joint axis locator you tube Vid

__________________
Mike Weber.



Crazy Mary

Did for me as well. Your end perhaps Simon.

Gaahh. You know I have the artistic ability of a cluster of colourblind hedgehogs in a bag!

Ok, I'll put it another way. Consider the supination resistance test. Assume that the fingers are placed under the cuniform / base of the first met and upward force is applied. Does the supination resistance test become a pronation resistance test if the axis lies medial to the point at which the fingers are placed? Will the force applied then pronate the foot?

Ok, so bearing in mind that the axis for the CCJ is a movable feast, lets say that force creates an inversion moment at the CCJ. How do I get from there to supination moment?

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Robert Isaacs
Specialist in Biomechanical Therapies
www.Footprintspodiatrysolutions.co.uk

small, yellow, leech-like, and probably the oddest thing in the universe

Semper in excretum sum sed alta variat

This is where is gets a bit interesting....

Motion creates axis. So if the foot is standing in midstance there will be no motion around the joint so no axis.

Then your 2 fingers lift up under the cuniform/base of 1st met and create motion and therefore an axis, so I don´t believe a supination resistance test can cause a pronation moment in this case.

But if someone is walking on a treadmill barefoot and you stuck your fingers in under the Cunifrom/base of the 1st met, depending on the StJ axis spartial location .......

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Mike Weber.



Crazy Mary

Which axis?

Why not?

I understand the concept that motion defines the axis, I don't understand how, when that axis is created and assuming it is the sub talar axis, that the force lateral to it can create a supination moment.

__________________
Robert Isaacs
Specialist in Biomechanical Therapies
www.Footprintspodiatrysolutions.co.uk

small, yellow, leech-like, and probably the oddest thing in the universe

Semper in excretum sum sed alta variat

I recall we had this discussion before. If memory serves you even identified a foot in which this occurred, Robert.

Presumably you mean supination moment at the subtalar joint? In which case via inversion of the calcaneus.

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Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Any axis of any joint that has moved



No I take it back there could be a pronation moment at the STJ created by supination resistance testing - uncommon but possible.

pesplanpancakeis foot type, in standing no movement but STJ is 90% pronated so base of 1st met/cuneform joint is lateral to the where the STJ axis just before it stopped moving, You lift motion = Pronation moment.

__________________
Mike Weber.



Crazy Mary

Yes, I had a negative windlass foot in front of me once. But only once. Whereas I've had dozens or hundreds of feet in front of me with Axes medial to the medial cuniform which supinate when you pull upward under the cuniform.

So the force under the cuniform creates inversion moment at the CCJ which creates supination moment at the STJ via the constraint mechanisms of the CCJ. That makes a lot of sense to me.

In which case:-

1. Returning to the tibialis anterior action, could that have a similar effect in weight bearing? If the Tib ant is pulling up on the cuniform, will it behave differently to if the fingers are pushing up?

2, Is there a rotational equilibrium equation to be considered between the pronation moment created by such a force in the classical way, and the supination moment exerted through the constraint mechanisms of the Mid Tarsal Joint?

Is it though? As I said to simon I've only seen one foot which pronated more with upward force under the cuniform and I've seen loads with STJ axes medial to that point. I'm sure you do the supination resistance test with your pts Mike, in those pespancakus feet, which are uncommon but not rare, how many times have you seen it pronate the foot?

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Robert Isaacs
Specialist in Biomechanical Therapies
www.Footprintspodiatrysolutions.co.uk

small, yellow, leech-like, and probably the oddest thing in the universe

Semper in excretum sum sed alta variat

You need to think about the phasic activity of tibialis anterior too, much of the time it is firing, during the first part of stance, the forefoot is still off the ground.

The foot is highly complex, it is important to understand that the external and internal forces acting on the foot may simultaneously exert moments about several joints and their structural components.

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Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Robert:

I think I see where the problem is now: you are talking weightbearing and I am thinking non-weightbearing.

In the weightbearing situation, such as in performing the supination resistance test, you must remember that any additional force from your fingers under the foot will decrease the total ground reaction force (GRF) under the foot and will also alter the location of GRF, thus altering the subtalar joint (STJ) moments from GRF. For this reason, the supination resistance test can't be used to accurately determine the STJ axis spatial location but will be a good indicator of how medial or lateral it is in each foot, assuming there are no abnormal internal forces coming from the extrinsic invertors and evertors of the foot during the test.

For example, if you press lateral to the STJ axis in the medial longitudinal arch of the foot with your fingers in the supination resistance test, you may still get a supination motion of the foot if the force from your fingers decreases the magnitude of STJ pronation moments from the mechanical action of GRF acting on the plantar foot. This is very similar to how the support of the medial longitudinal arch of a foot with a foot orthosis can cause an effective increase in supination moment even though the medial longitudinal arch of the foot is mostly lateral to the STJ axis. It must be remembered, in this regard, that a decrease in STJ pronation moment has the same mechanical effect as an increase in STJ supination moment.

Hope this helps.

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

In 3-d, to calculate the moment from a force (tendon pull) you need to know the perpendicular distance (d) from the line of action of the force to the axis of motion. You also have to know the component of the force vector perpendicular to the axis. So, if you draw a 2-d representation and either the force is pointed toward you, or the axis is pointed toward you, you can calculate the lever arm of the force. So, you don't necessarily have to draw all 3 plains. Especially if d = 0.

Now you are looking at equal and opposite moments. There are three things to consider here. There is the body proximal to the joint, the body distal to the joint and the joint/joint axis. So, tension in the posterior tibial tendon will create a supination moment from the leg acting on the foot about the STJ axis. It will also create a supination moment (most easily seen as external rotation) from the foot acting on the leg. This is the same moment but acting on different bodies.


Yes. A good example is the peroneus longus muscle. The line of action, at its insertion at the base of the 1st met is not the line of action we should be using for calculating the moment at the STJ. The cuboid acts as a pulley for the peroneus longus muscle, so its line of action is changed by the pulley. For the STJ we care about the line of action between the lateral side of the notch of the cuboid and the lateral malleolus.

Eric

A foot that pronates with attempted activation of the windlass is a rare foot indeed. I tried to explain how it works in my windlass paper.

You have to think center of pressure. Someone is standing and they have a center of pressure from ground reaction force. Most of the time this is creating a pronation moment. When you grab under the cuneiform and lift upward, you will be changing the center of pressure to a more medial location.

Now, if you had a foot that had its center of pressure medial to where you lift upward on the cuneiform, you could theoretically shift the center of pressure more laterally and hence cause a pronation moment.



It makes sense, but it doesn't work that way. If you were able to rotate the CCJ without effecting the rest of the foot, you could apply a force with the beak of the cuboid in a more medial location. If that location was not medial to the axis then that force would still create a pronation moment. However, as above, that force may shift the total center of pressure.

It should behave the same. Or said differently, there is a place you could push with your fingers that would get the same effect as the ant tib tendon if there were no other changes in other applied forces.

There's always a rotational equilibrium equation to be considered.

The way to break this down would be free body diagram analysis of the ligaments and bones of the midtarsal joint.

Eric

Isn't that the whole point: by rotating the ccj we are affecting other parts of the foot? Moreover, that the tibialis anterior doesn't just produce a moment about the subtalar joint axis, but rather due to the line of action of it's pull at the medial cuneiform will directly create translation and rotational forces across all of the inter-tarsal joints? While I understand the point regarding change in net external moment, surely the change in internal moments cannot be discounted with the such a broad sweeping brush as "it doesn't work that way"?

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"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Lol. Like death and taxes. Always with us.

__________________
Robert Isaacs
Specialist in Biomechanical Therapies
www.Footprintspodiatrysolutions.co.uk

small, yellow, leech-like, and probably the oddest thing in the universe

Semper in excretum sum sed alta variat

I wanted to bump this thread one year later because I have thought about it for quite a while and having seen Paul Harradine lecture at the Foot Health Conference on Tuesday, it jogged my memory that I wanted to talk about it.

I lose this thread at post 17. I can't think that out in 3 dimensions so I'm going to have to ask for some help.

I understand that Kevin makes the distinction between weight bearing assessment of the sub talar joint axis(STJ) and non weight bearing but more often than not I measure the STJ axis location in weight bearing(statically) which is something that Paul mentioned he did on Tuesday.

Rereading this paragraph written by Kevin, in practical terms I get this but I cannot explain it to anyone else


What I am trying to ask, I think, is; when we use a foot orthosis that has a medial longitudinal arch support on a foot where some of the arch support is lateral to the spacial location of the STJ axis it still has a supinatory effect on the foot.

Is that because , although the line of action of the orthosis reaction force(ORF) in the transverse plane is lateral to the joint axis, the point of application relative to the STJ axis in the other planes may not have the same net effect on the foot.

So the ORF is perpendicular to the surface of the orthosis. If that ORF is being applied lateral to the transverse plane of the STJ axis it will create a pronation moment about the sub talar joint axis. However, if the direction of the force is such that it applies a supination moment to the STJ axis in the sagittal plane then the net moment will be different and dependant on the direction of force being applied

I am pretty sure that someone will come along with a much better explanation of what I am trying to say. Does this make sense to anyone?

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An easier way to think about this is center of pressure. The center of pressure is a weighted average of the location of ground reaction force. So barefoot on the ground, or in the shoe, has one location of center of pressure. Put the orthotic under the foot and there is now more contact more medial (in the arch.) This will bring the average point of force more medial and hence increase the supination (which is saying the same thing as decrease pronation moment.) This assumes that no muscle activation was changed.

Breaking it up into planes, in my opinion, makes it harder. This is a three dimensional relationship. If you want to draw it on paper... Take a picture with either the force pointed toward you or the axis that you care about pointed at you. Then, trace that picture and draw in the the other thing (force or axis.) Now by inspection you can see the lever arm relative to the axis. Ground reaction force, in static stance, is vertical. This is why we use the transverse plane most of the time. Ground reaction force is perpendicular to the transverse plane, so when you draw the projection of the axis in the transverse plane and the location of center of pressure in the transverse plane you can see the perpendicular distance from the line of action of force to the axis. This is the lever arm of the force.

The downside, of just translating the 3-d to 2-d is that you can't calculate the moment. (Which is fine clinically, because we don't need to calculate the moment, we just need to know where to change the location of force to change the moment in the desired direction.) To calculate the moment we have to know the angle of the axis to, in this case, the transverse plane. We can use this to calculate the moment because only the component of the ground reaction force vector that is perpendicular to the axis will create a moment about the axis. For example, if the STJ axis were vertical then there would be no moment from a vertical ground reaction force. If the STJ axis is angled 45 degree from the transverse plane then only half of the magnitude of ground reaction force should be used to calculate the moment. We can still skip the calculation step clinically because the force stays vertical (mostly) and the percentage of the force relative to the axis stays the same.

Hope this helps,

Eric