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I've cracked this off Mike's Kinetics vs kinematics thread because its not really relevant to that. This has been niggling around in my hindbrain for months, even years, and it suddenly crystalised this morning while I was fishing poo out of the bath (the kids not mine). This may be prophetic as to its nature.
I will preface this with this graph by Ian, which is almost certainly whats going to happen here.
Kevin said
Quote:
Orthotists have known for many decades that the best way to brace and limit motion of a joint is to design that brace to span the joint of interest to create counter force on both sides of the joint axis that limits its motion. Since foot orthoses do not cross or span the subtalar joint, but a high top boot or ankle-foot orthosis does span the subtalar joint, foot orthoses are mechanically limited in their ability to "brace" the subtalar joint. Therefore, high top boots and/or ankle foot orthoses are critical when increased "control" of subtalar joint motion is required such as in a patient with posterior tibial tendon dysfunction that has a highly medially deviated subtalar joint axis or in a patient with chronic lateral ankle instability.
I said
Quote:
I find my thoughts careening down a path which is at best heretical and possibly entirely wrong. For the first time in 5 years I'm questioning the applicability of SALRE in weight bearing function. I no longer believe the foot rotates around the sub talar axis in weight bearing!
You are going to need to expand upon this, Robert. But I should say that I've never believed that the foot rotates about the subtalar joint axis in weightbearing function; I do believe that the calcaneus and talus rotate about the subtalar joint axes; the talus and navicular rotate/ translate about the talonavicular joint axes; the navicular and medial cuneiform rotate /translate about their joints axes etc.
Right then
I've been thinking in SALRE terms. That is that in weight bearing, the lever arm of an externally applied force, GRF or ORF, is the distance between the vector of that force and the sub talar axis. In recent months, since the Kirby, Spooner, Smith paper, I've been thinking much more in terms of vectors, and that has caused me some problems.
The best demonstration I know for vectors around the sub talar axis in open chain, is to have a subject sit with lower leg hanging downward. Place your finger / thumb on the medial / planter aspect of the MLA, medial to the STAxis. If you apply a force directly up, the force vector exits the foot medial / superior to the STAxis and we have supination. If you apply a force in a medial -> lateral direction we get pronation because the force vector passes lateral / inferior to the STJAxis. Well and good.
But try that in weight bearing. If i stand a foot with any kind of an arch weight bearing, and apply the same medial to lateral force directly onto the medial side of the MLA, it supinates the foot. We have supination at the Sub talar axis. Thats how Surgical boots and AFOs, and to some degree UCBLs work. But how does that fit with our force vector? The force is still in the same place, still in the same direction, Still passing inferior to the STJaxis, but we have gone from that force causing a pronation moment to a supination one. Why.
Here is a thought. Perhaps we are looking at a brand spanking new axis when the foot is weight bearing.
An axis is a straight line around which a body rotates. When you supinate your foot weight bearing there is rotation around your STA, but the STA does move, dorsally and laterally as it does so. What then is the axis which does not move (or moves much less) about which the foot rotates?
How about the lateral side of the foot? A line between the lateral part of the heel and the centre of the 5th met head. If I stand still and supinate my foot it rolls on this line, the line stays still.
This fits with the change in effect of a medial ->lateral force on the medial arch from a pronation to a supination moment when the foot becomes weight bearing. If the new axis is on the ground (probably slightly above it on reflection) then this force vector passes over it. Unless the arch is completely flat, in which case it won't. Try putting the medial force on a pes pancake foot, it does nothing.
And in an ordinary foot, with an arch. Pull up on that medial point, supination moment. Move the force around to horizontal, the vector will cross the Sub talar axis from superior to inferior, but the force will STILL be supinatory. Keep moving the force around to the point where it is medial / downwards. It becomes a pronatory force when the vector crosses this lateral foot axis. This is the PNR, the point on no rotation. That suggests to me that the axis around which the foot rotates in weight bearing is not the STJ axis, but this one.
And thats what I mean buy ever orthotic being a 3 point system. If we assume the leg is fixed point one (relatively) and the lateral side of the foot is point 2 (fixed by friction) then to get the greatest supination moment I want my force vector to strike the Sub talar axis dead on, and normal to the line between the top of the leg and the lateral foot, applying the force in the middle of the bent column to straighten it.
I'm trying to work out how to draw this, but I can't. Does this make sense to ANYBODY or have I completely lost the plot here.
I now sit back and wait for Simon to ask me the question which will show me and the rest of the world what a C**k I am. But the world would be a sad place if no one was willing to make a fool of themselves in the name of science. So be gentle with me.
This has been niggling around in my hindbrain for months, even years, and it suddenly crystalised this morning while I was fishing poo out of the bath (the kids not mine). This may be prophetic as to its nature.
To dig myself yet deeper, I tried to do a picture. I'm rubbish at pictures
So this is a frontal plane slice through the midfoot. Say, navicular level. The Sub talar axis, by this point, is hovering in the air somewhere above the foot.
Try this on a weight bearing foot.
Force A, is on the medial side of the arch aimed at the 5th met. It creates no moment one way or other, even though it is miles from the sub talar axis
Force B Is the force applied to the medial side of the arch. It passes well under the STJAxis and in non weight bearing creates pronation moment, but well above the new axis and thus in weight bearing causes supination
Force C is force directly upward, under the 5th met head. Medial to both axis, it always causes supination moment.
Force D is directly downward on the top of the MLA, medial to both in the other direction it always causes pronation moment.
Am I stating the blindingly obvious here? Only I can't remember ever seeing this axis, the lateral one, discussed anywhere.
During dynamic weightbearing function can the foot be accurately modelled as a single rigid body? This is the assumption upon which your proposition appears to be dependent, Robert. What the foot orthosis does is change the surface geometry at the foots interface, this will alter the moments about each of the foot joint axes, or if we chose to model it that way about a single net resultant foot axis. I talked about this before, I can't remember which thread. It's whether we want to look at the macro or the micro level. Personally, I think by modelling the foot as a single rigid body, too much of the important detail of how foot orthoses actually work will be lost in the dilution.
P.S. what you are talking about is not dissimilar to the "longitudinal axis of the foot" which was traditionally used in biomechanical modelling until the development of multisegment foot models.
Robert, an axis can only occur at a joint not mutiple joints, which is the whole debate between the Midtarsal joint v´s the talo-navicular and calcaneo-cuboid joint. So I´m notsure we can have a single foot axis....
An axis is created by motion at a joint, so how can the foot have a single axis, when we have mutiple motions occuring at different joints ?
An axis is created by motion at a joint, so how can the foot have a single axis, when we have mutiple motions occuring at different joints ?
Quote:
During dynamic weightbearing function can the foot be accurately modelled as a single rigid body? This is the assumption upon which your proposition appears to be dependent,
Obviously the foot is not a rigid body. But that is sort of how the thought experiments work isn't it. My contention is that a force applied to the arch of a foot causes a rotational moment at the lateral axis and the movement around that axis is accomodated by counterwise movements in multiple joints around it.
I know this is not an axis at a joint, but it is a theoretical line around which movement of a body (the foot) takes place. The body is not a rigid body, its a flexible body comprising many joints and changable morphology.
Taking the example of the medial force on the medial foot which changes from supination moment at the STJ to pronation moment at the STJ. How else can that be resolved?
Rather than try to do my own picture, I'll borrow Kevins
Quote:
In this second in a series of podiatric biomechanics thought experiments, I have created a model of the foot including the tibia and talus as one rigid body, the talo-tibial unit, which is attached to a rigid foot model by way of the subtalar joint axis, that, in this model is a hinge perpendicular to the frontal plane. The foot has only two weightbearing surfaces at the approximate locations of the 1st and 5th metatarsal heads. F1 is the ground reaction force (GRF) acting plantar to the medial weightbearing surface of the foot and F2 is the GRF acting plantar to the lateral weightbearing surface of the foot.
On this basis, if the axis of the sub talar joint ran slightly medial to the First met head (a not unusual scenario in vivo) Force F1 would now exert a pronatory moment. How many feet will pronate more if a shaft pad is placed under the 1st met head?
This is prime heresy. Consider it a hypothetical question to stimulate debate. I know the STJ axis is never medial to the foot at the heel.
I don't know if I quite understand what you are trying to get at here but maybe I can help. If you were to make a free body diagram of the foot as you are trying to do in the first post of this thread, all of the internal and external force vectors, their magnitudes and their points of application acting on the foot (at an equilibrium position) would need to be illustrated. This should include external forces from ground reaction force and all orthosis reaction forces. This should also include any significant internal forces coming from muscle force, ligament force and joint contact forces.
It is fine to model the foot simplistically as I have done in my papers and books over the past 20+ years as long as you realize the inherent limitations of modelling. In other words, models do not accurately describe the complete workings of a mechanical entity but attempt rather to focus on one or more aspects of the mechanical entity in order to gain useful information about how it works, hopefully without being so innaccurate that the model is useless.
One other thing, you must be very careful in describing a joint axis (i.e. between two objects, such as between the talus and calcaneus at the STJ) relative to a movement axis of an object (e.g. movement of that object relative to the ground) that in comprised of multiple joints. A joint axis and a movement axis may not be always the same axis since one describes the set of points around which two bones rotate and the other describes the set of points around which an object (possibly containing multiple joints) rotates relative to another object.
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
So You're saying that force B, our medial force creates a lateral GRF from friction (equal and opposite) which being further from the STJ axis than force B represents a larger supination moment...
In the above example my foot rotates relative to my leg about the subtalar joint axis, but my foot rotates relative to the floor about an axis between my shoe and ground, pretty much where the crack in the floorboards is in the pictures. It's a frame of reference thing that I think you are trying to describe.
Would'nt be so bold as to make a statement about what a foot orthoses is trying to do. Thats up to the person who makes the device. But I am saying that looked at that way things make sense to me which did not make sense when modeled in the absence of that axis. There are a few lever arms I may have to recalculate and a few puzzles now solved. Its an element I had not considered.
What are your thoughts on the change in effect of the medial lateral force when going between non weight bearing and weight bearing?
So.... does the orthotic change the moments at the foot relative to the ground at it's interface axis (IA), or does the the orthotic change the moment at the subtalar joint axis, or both? Think about the period when the whole of the foot is on the ground (Say the first 4 frames in my images above). Then think about the periods when the foot is not entirely on the ground (the last 4 frames above). Hint, go back to that pdf I linked to earlier. If we have foot orthosis which modify ground reaction force....
Would'nt be so bold as to make a statement about what a foot orthoses is trying to do. Thats up to the person who makes the device. But I am saying that looked at that way things make sense to me which did not make sense when modeled in the absence of that axis. There are a few lever arms I may have to recalculate and a few puzzles now solved. Its an element I had not considered.
What are your thoughts on the change in effect of the medial lateral force when going between non weight bearing and weight bearing?
Would'nt be so bold as to make a statement about what a foot orthoses is trying to do. Thats up to the person who makes the device. But I am saying that looked at that way things make sense to me which did not make sense when modeled in the absence of that axis. There are a few lever arms I may have to recalculate and a few puzzles now solved. Its an element I had not considered.
What are your thoughts on the change in effect of the medial lateral force when going between non weight bearing and weight bearing?
Robert:
It looks like you are trying to describe how an orthosis may limit rearfoot pronation by using a "motion axis" of the foot relative to the ground during the contact phase of gait. This certainly is a valid approach and one that I tinkered around with during my biomechanics fellowship about 25 years ago. Unfortunately, even though such a model can be effectively used to describe how an foot orthosis may work during the contact phase of gait it doesn't take into account, for example, the alterations in moment arms for the posterior tibial muscle at decelerating that STJ pronation when the subtalar joint axis is more medially deviated.
Actually there was a paper done in 1978 in JAPA that describes something similar to what you are trying to describe here (Downing JW, Klein SJ, D'Amico JC: The axis of motion of ther rearfoot compex. JAPA, 68:484-99. 1978.) I don't think anything ever became of this concept.....maybe it is time for you to resurrect it from the ashes.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Unfortunately, even though such a model can be effectively used to describe how an foot orthosis may work during the contact phase of gait it doesn't take into account, for example, the alterations in moment arms for the posterior tibial muscle at decelerating that STJ pronation when the subtalar joint axis is more medially deviated.
Devil's advocate: does it have to? That is, does this model describe how foot orthoses work better or as well as a STJ axis centric model? Lets take the example of an over-stressed posterior tibiaiis tendon, does this foot centric model predict reduction in stress in this tendon with decreased eversion moments of the foot? If we are talking about how orthotics work, why is the STJ axis centric model, better than a foot axis centric model?
Devil's advocate: does it have to? That is, does this model describe how foot orthoses work better or as well as a STJ axis centric model? Lets take the example of an over-stressed posterior tibiaiis tendon, does this foot centric model predict reduction in stress in this tendon with decreased eversion moments of the foot? If we are talking about how orthotics work, why is the STJ axis centric model, better than a foot axis centric model?
If the "foot centric model" did not account for alterations in different spatial locations of the STJ axis, then it would still be able to predict that a reduction in posterior tibial tendon tensile force with foot orthoses would occur, but it would not be able to predict the magnitude of tensile force necessary within the posterior tibial (PT) tendon to maintain an equilibrium position with the medial aspect of the forefoot not in contact with the ground.
In other words, knowing the length of the PT tendon moment arm relative to the STJ axis along with the length of the ground reaction force (GRF) moment arm relative to the STJ axis will allow us enough information to be able to calculate the magnitude of PT tensile force necessary for the foot to maintain an equilibrium position as long as we know the magnitude, point of application and direction of the GRF vector on the plantar foot. The "foot centric model" is too simple to allow this calculation of internal forces, but does allow us to determine qualitatively that PT tendon tensile force will be dimished with an inverted foot orthosis.
Here is a drawing I did 10 years ago which somewhat illustrates this concept.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
If the "foot centric model" did not account for alterations in different spatial locations of the STJ axis, then it would still be able to predict that a reduction in posterior tibial tendon tensile force with foot orthoses would occur, but it would not be able to predict the magnitude of tensile force necessary within the posterior tibial (PT) tendon necessary to maintain an equilibrium position with the medial aspect of the forefoot not in contact with the ground.
In other words, knowing the length of the PT tendon moment arm relative to the STJ axis along with the length of the ground reaction force (GRF) moment arm relative to the STJ axis will allow us enough information to be able to calculate the magnitude of PT tensile force necessary for the foot to maintain an equilibrium position as long as we know the magnitude, point of application and direction of the GRF vector on the plantar foot. The "foot centric model" is too simple to allow this calculation of internal forces, but does allow us to determine qualitatively that PT tendon tensile force will be dimished with an inverted foot orthosis.
Here is a drawing I did 10 years ago which somewhat illustrates this concept.
No argument with this, but does this model better illustrate the effect of foot orthoses...
No argument with this, but does this model better illustrate the effect of foot orthoses...
Only for the MASS-minded podiatrist that thinks that all pathologies are caused by excessive pronation of the foot....
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Only for the MASS-minded podiatrist that thinks that all pathologies are caused by excessive pronation of the foot....
C'mon Kevin, lets assume that a foot orthoses designed to increase pronation moment might result in a net foot to floor rotational axis along the medial side of the foot to ground interface. So if we go back to our orthosis designed to increase supination moment of the foot, does modelling about the STJ axis, provide grossly different answers to modelling about the net foot to ground axis on the lateral side of the foot?
I suspect the key, as Robert has intimated, is in vectors. That is when we have a centre of pressure position which lies on one side of the STJ axis, yet the line of action of that force passes to the other side of the STj axis. The same is obviously true of the net foot to ground rotational axis, but which axis should give the greater number of erroneous answers if we looked just at the CoP position relative to each axis?
C'mon Kevin, lets assume that a foot orthoses designed to increase pronation moment might result in a net foot to floor rotational axis along the medial side of the foot to ground interface. So if we go back to our orthosis designed to increase supination moment of the foot, does modelling about the STJ axis, provide grossly different answers to modelling about the net foot to ground axis on the lateral side of the foot?
It all depends on if you are only worried about the mechanism of how a foot orthosis might restrict/decelerate STJ pronation motion and not concerned about the magnitude of internal forces within the foot that restrict/decelerate STJ pronation motion.
Some questions for you, Dr. Spooner:
1. How would this "foot centric model" be able to predict that some feet would be more likely to develop sinus tarsi syndrome due to the increased interosseous compression forces that would develop between the lateral process of the talus and the floor of the sinus tarsi of the calcaneus?
2. How would this "foot centric model" be able to predict that using an inverted orthosis might actually cause an inversion ankle sprain in some feet but not in other feet?
3. How would this "foot centric model" be able to predict that using an inverted orthosis might cause chronic peroneal tendinitis in some feet but not in other feet?
4. How would this "foot centric model" be able to predict that some feet might not respond well to an inverted orthosis and may actually need a high top boot or ankle foot orthosis in order to adequately reduce the pronation moments sufficiently to make the patient feel and function better?
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
1. How would this "foot centric model" be able to predict that some feet would be more likely to develop sinus tarsi syndrome due to the increased interosseous compression forces that would develop between the lateral process of the talus and the floor of the sinus tarsi of the calcaneus?
Since it is a model related to orthotic therapy and not to foot pathology per se, I don't think it could, nor does it need to. However, I suspect there might be a relationship between the spatial location of this axis and the force required to bring about a kinematic change in foot position about this axis.
Quote:
Originally Posted by Kevin Kirby
2. How would this "foot centric model" be able to predict that using an inverted orthosis might actually cause an inversion ankle sprain in some feet but not in other feet?
Too much supination moment from the orthotic about the net foot to ground axis.
Quote:
Originally Posted by Kevin Kirby
3. How would this "foot centric model" be able to predict that using an inverted orthosis might cause chronic peroneal tendinitis in some feet but not in other feet?
Same answer as above.
Quote:
Originally Posted by Kevin Kirby
4. How would this "foot centric model" be able to predict that some feet might not respond well to an inverted orthosis and may actually need a high top boot or ankle foot orthosis in order to adequately reduce the pronation moments sufficiently to make the patient feel and function better?
I guess through variation in the spatial location of the net foot to ground rotational axis and the force required to bring about a kinematic change at this axis. Same answer as to number 1. really.
What does the supination resistance test, test? The force required to invert the foot about the foot to ground interface axis? The force required to invert the foot about the subtalar joint axis? Both? Or just medial longitudinal arch stiffness?
Here's a thought, has anyone described a pronation resistance test in which the examiner places fingers or a force gauge under the cuboid and attempts to pronate the foot? Measuring load/ displacement should give a measure of pronation stiffness too. What if we measured the ratio of supination stiffness to pronation stiffness??
Here is a drawing I did 10 years ago which somewhat illustrates this concept.
Would a more medially positioned foot to ground interface axis provide a similar explanation? Growing horns, hooves and a tail, is it warm in here, or just me?.... does all of this come down to the wet footprint test? That is if we bisect the lateral section of the footprint between the heel and the forefoot, is this an approximation of the foot to ground interface axis? In a flat foot, this area will be wider and thus the bisection will be more medial; in a cavus foot this area will be more lateral and thus provide a more lateral interface axis. Indeed, in a highly cavus foot this section of the print might not exist, in which case we'll make something else up...
It looks like you are trying to describe how an orthosis may limit rearfoot pronation by using a "motion axis" of the foot relative to the ground during the contact phase of gait. This certainly is a valid approach and one that I tinkered around with during my biomechanics fellowship about 25 years ago. Unfortunately, even though such a model can be effectively used to describe how an foot orthosis may work during the contact phase of gait it doesn't take into account, for example, the alterations in moment arms for the posterior tibial muscle at decelerating that STJ pronation when the subtalar joint axis is more medially deviated.
Actually there was a paper done in 1978 in JAPA that describes something similar to what you are trying to describe here (Downing JW, Klein SJ, D'Amico JC: The axis of motion of the rearfoot compex. JAPA, 68:484-99. 1978.) I don't think anything ever became of this concept.....maybe it is time for you to resurrect it from the ashes.
Here's the paper....(Downing JW, Klein SJ, D'Amico JC: The axis of motion of the rearfoot compex. JAPA, 68:484-99. 1978.)
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Another Axis. A 3 point model.
Linear Mode
