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One of the more memorable public discussions that I had with one of the forefathers of Podiatric Biomechanics, Dr. William Orien, occurred at the Root Lab Biomechanics Meeting from about 16 years ago at Lake Tahoe, when Dr. Orien was lecturing to about 70 of us on the subject of treating feet with custom orthoses. During his lecture, he made the statement, "As we all know, gravity always causes pronation". After about 3 seconds of Dr. Orien making that statement, I raised my hand from the audience and said that gravity not only causes pronation but also causes supination, so that to say "gravity always causes pronation" was a misleading and innaccurate statement. [My thought process had been influenced heavily at the time by my observation that individuals with a laterally deviated subtalar joint axis would maximally supinate, rather than pronate, under the influence of gravity and the resultant ground reaction forces on the plantar feet.]

Getting a little annoyed with me, since I had already publicly questioned Dr. Mert Root for the last few years about some of his theories and ideas at his previous seminars, Dr. Orien said that he didn't know what physics books that I had learned from in school since everyone knew that "gravity caused pronation". I went on to tell Dr. Orien that he would need to wait for my book to come out before he may be able to understand about how gravity may cause both pronation and supination. Talking about this recollection from my past still brings a smile to my face due to the great learning experiences and discussions (some say arguments) I had while attending Dr. Root's lectures and seminars as a young podiatrist.

Having told this little story, I would be interested to know how many podiatrists still believe that gravity only can cause pronation since "the center of gravity is between the feet and will therefore tend to pull the calcaneus into eversion" (which is what I was taught in podiatry school at CCPM). In other words, how prevalent is this notion that gravity always causes a tendency for subtalar joint pronation and how often is it being taught within the international podiatric biomechanics community? I would be interested to hear from the students, recent graduates and more mature graduates of podiatry school to see whether these ideas are still being discussed and taught within our podiatric medical institutions.

Thanks in advance for your responses.

__________________
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
**************************************************

we are not taught about the gavity and root et al theary. We get taught that to be open minded to new thearies

To be honest i have not read much into the root theary much as we get taught it is dated, so i dont know too much about it, my lectueres seem to be very keen on the work Craig Payne is doing/has done

just my veiw on how it is taught in the UK at the moment, it would be intresting to see how it is taught in the USA as your system is so different from everyone elses?

cheers

nick

We were taught Root theory in second year in Queensland (Australia). It was raised at the time that much of Root theory had been superceded by other theories although these other theories were not examined until later. I do not remember hearing the statement that " gravity always causes pronation" although I am sure it was suggested that people pronated to the midline after contact. From discussing their study with our podiatry scholarship students it appears that the Root theory is still taught although with much less emphasis when I went through.

I think another factor might be the direction of force at contact. I have had some patients who had a wide base of gait and had a lateral movement of the foot prior to strike when running. This momentum caused lateral instability and supination after strike. Most runners will strike the ground in varus with the foot moving medially prior to strike to land under their bodies midline. This increases the force causing pronation.

Absolutes like Dr Orien's statement usually come undone. However I would agree that gravity causes a tendency for pronation... just not always.

Kevin,

In the earlies 90's, Dr. Root's paradigm was not taught in Spanish Podiatry Schools, neither physics actually; but common sense and a basic physics background tell us that when a force like gravity is applied to a body (foot) it will deform or resist deformation according to its structure (anatomy). Thus, I do not understand how it can be taught that "gravity always causes pronation" since it depends on each one anatomy. Also, GRF is the opposite force to the vector resultant or sum of the gravitational and inertial forces and its beneath your foot.

I am not involved in podiatry education in my country, but I hope it is not being taught.

Regards,

I can tell you that at CSU we get taught Root theory as the basis, with the reminder that there are other theories but we don't go into much depth. On placement however, we are exposed to, and have to integrate, our placement supervisor's different thought processes. And from personal experience, in our last few months in the course we seem to have been bombarded by a mix of sales people and well renowned pods giving us their two bobs worth.

It's all very confusing! As students we like things to be black and white - which biomechanics and orthoses mangement clearly isn't. However I'm sure there is common ground with most of the theories out there, I'd like to find them or be pointed in the right direction on how where they've already been found! In getting my head around everything biomechanics I'm currently working on a Keep It Simple Stupid summary of what we know about abnormal lower limb biomechanical function, and how we can use basic orthotic principles to address them.

As a novice I'm sure it'll be full on inaccuracies and inconsistencies, potholes and deadends, so I expect to get whacked for it but watch this space and I'll do my best!

Sincerely, Matt Kimball.

I thought that I should update that last post of mine following some more meaningful study of the literature. More specifically, upon reading Craig Payne and Adam Bird's article "Teaching clinical biomechanics in the context of uncertainty" http://www.japmaonline.org/cgi/reprint/89/10/525 . I've realised that clinical biomechanics shouldn't be black and white. It's as much the student's responsibility to digest and critically evalutate each viewpoint as it is the teacher's responsibility to present them. A KISS principle might be useful to begin with, but we earn our stripes as clinicians when we can call upon a broader base of knowledge, based on all the different theories, and adapt it to each individual case.

And to answer the original question, I believe we are initially taught that the foot will naturally pronate due to gravity and GRF occurring towards the midline of the body. However in retrospect, this doesn't really make sense, as it really doesn't explain the occurence of a weightbearing supinated foot - whereas the location of the STJ axis can.

The moment from ground reaction force is determined by the location of center of pressure of ground reaction force (the average point of ground reactive force) relative to the projection of the STJ axis onto the transverse plane.

All you need to understand the foot is knowledge of the anatomy and understanding of a some physics concepts.

One other thing for you students out there. An axis is an imaginary line and cannot limit motion, but it convenient to use in some situations.

Cheers,
Eric Fuller

OK

To confirm my ignorance.

If we are stood on both feet the centre of gravity may well be between the feet. However, in single limb stance or in single limb support (depending where you are in the gait cycle) does not the centre of gravity reach a point where it is more focused in the support foot. If this is the case then the STJ axis position would play a crucial role in the affect of gravity. Or am I confusing this with something else?

Ian

Ian:

Yes, subtalar joint (STJ) axis spatial location will greatly affect the rotational forces (i.e. moments) occurring across the STJ axis in both relaxed bipedal stance and during walking. The center of mass (CoM) of the body is aligned generally between the feet in relaxed bipedal stance. During the midstance phase of walking, the CoM is aligned slightly medial to the stance phase foot. As Eric said, the center of pressure acting on each foot, and the direction of the ground reaction force vector relative to the STJ axis, will determine the STJ moments that are being caused by ground reaction force. So in this way, yes, STJ axis location does play a crucial role in the effects of gravity on the mass of the body.

__________________
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
**************************************************

Kevin and all

1st gravity causes everything. In terms of gait and GRF force vectors there can be none without gravity.

The position of the COG implies nothing obout the direction or magnitude of the forces of origin and is just a convenient summation of all the force vectors.
The action of an individual segment in the mechanism is soley dependent on the forces acting on it at the point of interest.

Eric really summed it up neatly

.

How true

Dave

__________________
Descartes seems to consider here that beliefs formed by pure reasoning are less doubtful than those formed through perception.

Some say that pronation occurs because the navicular has to drop to accomodate the internal rotation of the leg.

Or is it that the leg rotates because gravity causes the navicular to drop?

Chicken? Egg? Egg? Chicken?

Regards
Robert

Robert:

Subtalar joint (STJ) pronation will only occur when the external STJ moments (i.e. ground reaction force) and the internal STJ moments (i.e. interosseous compression forces, ligament, tendon, muscle tensile forces) are added together and their summation equals a net STJ pronation moment. The net STJ pronation moment will then accelerate the STJ into a larger angular velocity of pronation.

This simplified method of mechanical analysis is the best way to understand the complexity of mechanical variables that may produce pronation and supination motions of the STJ in the human foot during both non-weightbearing and weightbearing activities. When compared to the bewildering assortment of "foot and lower extremity deformities" and other confusing biomechanics theories that I was taught during my student and biomechanics fellowship years at the California College of Podiatric Medicine, this simplified method of mechanical analysis is far superior to both understand and to teach at the podiatry student and clinician level, as long as the student and clinician already comprehend the elementary concepts of Newtonian physics.

__________________
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
**************************************************

Kevin,

Is this a personal theoretical assumption or a known, scientifically proven fact, when it comes to foot function that is?

__________________
Graham Curryer

None of us know what we are doing, but some of us know more about what we are not doing than others!::

Graham:

It is equally as theoretical as the supposition that using in-shoe pressure analysis gives you a good indication of the function of the foot and gives you a good idea of the internal forces occurring within the foot and lower extremity during weightbearing activities or that functional hallux limitus causes flattening of the longitudinal arch of the foot or that functional hallux limitus causes subtalar joint pronation.

Good to have you back nipping at my heels, Graham.

__________________
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
**************************************************

Kevin,

My pleasure! No offence meant but frank statements made by prominent members such as your self, without clarification such as, "in my opionion", or, "theoretically it would suggest that", suggests absolute truth, and can be missleading. Similar to saying my orthoses are better than yours!

Which in my opinion they are, theoretically.

__________________
Graham Curryer

None of us know what we are doing, but some of us know more about what we are not doing than others!::

You are right. I hope no one thinks that anything I say should be taken as an "absolute truth"......just my best educated guess at this instant in time.

__________________
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
**************************************************

Kevin,

And always stimulating and a pleasure to read and think through.

Thanks Kevin.

__________________
Graham Curryer

None of us know what we are doing, but some of us know more about what we are not doing than others!::

Will you two stop with the ? You're livers won't hold.

Right with you on the moments Kevin. The tricky question (for me) is knowing where those moments come from.

A movement occurs. Pronation. The foot everts, the navicular drops and the leg rotates.

Is the internal rotation of the leg something which creates pronation moments across the joint? Or is it the case that the leg is moveing in response to internal rotational moments at the tibia generated by grf moments in the STJ causing pronation. In other words does the leg push the foot into pronation or does the foot pull the leg into rotation?

Also glad to have you back graham. Have a coffee after all that beer .

Or else....

Regards
Robert

To add my two cents:

Be careful of the distinction between the center of mass, center of pressure and center of gravity. The center of mass is the average point of mass. The center of gravity is projection of the center of mass onto the ground.

When describing how forces act on an object you must carefully define the object. So, for the following discussion the foot is defined as the talus and all bones and stuff distal. The center of pressure is the average point of force, in this case, under the foot. Gravity, will affect the foot as the mass is supported by the leg. So the force of gravity (gravitational attraction of the earth and person) will be applied to the foot from the bottom of the tibia. The center of mass does not apply a force to the foot the bottom of the tibia does.

Ground reaction force occurs because gravity would accelerate the body downward. The ground prevents the body from accelerating downward, therefore ground reaction force is exactly equal to the pull of gravity. This is one area that has confused people trying to understand the biomechanics of the foot for a long time. There is a force from above and a force from below when you examine the foot. The force from above is very close to the STJ axis. The force from below can vary its position relative to the STJ axis.

One of the mechanical principles you need in addition to the anatomy is free body diagram analysis. This helps you identify where the forces are coming from

Regards,

Eric Fuller

Hi Robert,

It depends on the input forces and starting position/conditions. If you are standing there and someone grabs your femur and internally rotates it, it will cause and internal rotation moment on the talus. If the foot is held steady, by friction for example, then the pronation moment could come from the leg. On the other hand, if you had a little trap door under the first metatarsal head. You are standing there and the trap door is released, this will cause a lateral shift in the center of pressure under the foot causing an increase in pronation moment from the ground. If range of motion is available, there will be STJ pronation with internal talar rotation. The internal talar rotation would create a moment on the lower leg causing it to internally rotate if the mometn was unopposed.

Regards,

Eric Fuller

Think of a seesaw motion and the associated axis...

Does gravity cause one side of the seesaw to rise, or does it only make the opposite side lower?

Or is it GRF's?

I say both...

Remove gravity and what is the result?

Quite. So in the absence of small people grabbing femurs and twisting them how much of this rotational force exists originating proximally during gait?

I suppose one way we could find out would be to chop somebodies foot off, replace it with a round ended stump ball, put them on a low friction surface (so that the ball could rotate freely on the ground but there was no grf causing rotational movement) and then see how much rotation there was in the femur during gait.

Any volunteers ?

Its possible that this might affect other gait parameters as well but you get the idea.

This is connected to my ongoing headache about what volume of supination moments are generated by the glutes and by inference, how significant they are in foot posture.

Put it this way. Shortly after the the stance leg is planted the contralateral leg starts traveling forward relative to it. Thus the pelvis rotates around that limb and presumably generates some EXTERNAL rotation moments through the hip. This, presumably again, is why the tibial starts to externally rotate just after Foot flat.

During the first part of stance phase when the tibia is internally rotating is this driven by an active muscular force? Because if not, based on the performance of the tibia after foot flat, one might suppose that the internal rotation is a passive movement.

Thanks for bearing with me guys.

Regards
Robert

The external hip rotators obviously have a potential to increase the subtalar joint (STJ) supination moments during the stance phase of gait by their contractile activity. However, I would imagine they have much less potential to increase STJ supination moment compared to the posterior tibial muscle. In addition, in a flatter arched foot, where there is a low inclination angle of the STJ axis, the external hip rotators would have a much more difficult time in generating STJ supination moment compared to a higher arched foot due to the decreased STJ supination moment arm available for rotational forces from GRF within the transverse plane to cause STJ supination moment.

In other words, the external hip rotators probably play only a minor role in generating STJ supination moment in most individuals.

__________________
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 Always been my position. It would be nice to have a way to measure. I kow a good number of respected colleagues who feel differently!

Regards
Robert

It's been a while since I've looked at Winters' inverse dynamic papers. He did measure transevers plane moments. I don't recall off the top of my head what they were.

Regards,
Eric

Hi Guys,

If we take a loose-wired skeletal foot and set it upon a table top (to save our knees) and hold it in a position close to subtalar neutral we can observe that:

a) it will display a bony lateral border which in life will be not far removed from the ground surface, and

b) a raised medial aspect, beneath which is a considerable amount of airspace.

When we release the model it will invariably (with apologies to Schroeder - and his cat!) fall inwards - i.e. it will pronate upon the surface. This it does under the effect of gravity.

Now, attach ligaments, muscle, fascia and a leg. Can I ask - is it now different? Has gravity finally been overcome in some biogalactomechanical takeover?

I sometimes have moments of 'inspiration' in which I beleive that Einstein got it wrong, such is my open-mindedness! Am I having one of those attacks right now? Was Root right, after all? Am I going nuts?

To mutilate the words of Issac Newton,
"If I have seen further than most it is because I have seen between those big fellas standing in front...."

Measure navicular height in RCSP, contract external rotators and re-measure.

__________________
Science is the antidote to the poison of enthusiasm and superstition

My location

Graham

Kevin wrote
you replied
In human terms there are no absolute truths but when it comes to theory and the probability that the theory in question is true then this one is about as probable as it gets. The chances of of rejecting this hypothisis has a probability of <0.0000000000000000000001.

Someone, PodAus I think, mentioned a see saw, well the probability that gravity will accelerate one side or the other is within the constraint of distribution of mass about the pivot. How the mass is distributed will determin the forces and moments about the pivot. IE the side with the greatest force and therefore moments, goes down.

The STJ or any joint of interest is no more complicated than that. The only force (vector) that one needs to consider in terms of foot mechanics is the GRF vector. Whatever other more proximal forces are acting they will determine the position and magnitude of the GRFV and the GRFV will indicate the 3D external moments acting about the joint. Because there is always equilibrium, then all the other forces will act to cause moments to balance the momnets by GRFV. This is why it is important to accept that there is alway equilibrium of forces and moments in a mechanism whether it is in motion or stationary.

The intuitive visualisation of the GRF vector is difficult, so intuitively, in clinic, we tend look at body motion and attempt to make some assumption about internal forces but because of the many and various variables, I belive this is also difficult and leads to misinterpretation and misunderstanding of basically simple and reliable mechanical principles.

Dave

__________________
Descartes seems to consider here that beliefs formed by pure reasoning are less doubtful than those formed through perception.

Dave:

Good reply to Graham.

A few weeks ago, I gave a 25 minute lecture at the PFOLA meeting in Vancouver titled "Subtalar Joint Kinetics: A 24 Year Journey of Determining Subtalar Joint Axis Location" in which I reviewed the history of the scientific literature on STJ axis location determination, reviewed the importance of STJ axis location to the kinetics of the foot and lower extremity and to foot orthosis therapy, and reviewed some recent research I am participating in on determining the STJ axis location. After the lecture, I walked out of the lecture hall to get a cup of coffee and one of the keynote speakers at the seminar, a PhD biomechanist that is very well known, said that he had listened to my lecture and said the following to me:
"I thought your lecture made complete sense and was very straightforward, Kevin. Do podiatrists disagree with the concepts you presented since I can't see much to disagree with?"

My reply:

"The problem is that many podiatrists just don't have the background knowledge in biomechanics and are so locked into their older theoretical notions as to how the foot works that they often struggle with the concepts I have been lecturing on for over the past two decades."

The PhD biomechanist then shook his head in agreement.

This is a relatively common occurence that a PhD biomechanist has heard that my ideas are controversial within podiatry, goes to hear my lecture for the first time, and then comes up to me and says he doesn't understand why other podiatrists find my ideas so controversial since they make complete biomechanical sense.

The recent episode here on Podiatry Arena, where an older podiatrist that doesn't understand basic biomechanics terminology tried to convince us that regressing to static descriptions of foot shape using archane architectural terms was somehow an improvement in biomechanics, is a good example of the way many podiatrists view the biomechanics of the foot. It is my firm belief that the way forward for podiatry is not to regress back to the Root model or try to come up with another classification scheme based on external deformities of the foot but rather to try to understand foot mechanics using the same proven concepts and methods that biomechanists and engineers would describe any mechanical system.

I have always considered that if I have used Newtonian mechanics to base my theories on, then I will be standing on much more solid ground than the others that have not adhered to these very solid mechanical principles when they have conjectured as to how the foot functions. Twenty five years from now, it will be interesting to see which podiatric theory of foot function is still being taught and which theories have been discarded.

__________________
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
**************************************************

Hi Johnpod
You describe what happens with rotational equilibrium quite well. The foot model is sitting there in equilibrium. You add a supination moment to place it in neutral positoin. It supinates to a new position where there is more weight bearing laterally. The lateral weight bearing creates a pronation moment that is balanced by your input supination moment. When you let go, your input supination moment is removed, but there is still a pronation moment from the ground. So, in response the pronation moment from the ground causes the STJ to pronate until equilibrium is again achieved. Gravity is involved, but it is a pronation moment that casued the foot to fall back to its original position. Most feet, when you maximally invert them and apply force from the tibia and the ground the foot will tend to stay supinated, because the location of ground reaction is medial to the STJ axis.

Your description also illustrates a problem with neutral position. Root et al, described neutral position as a position of stability. In this example neutral position is inherently unstable, because you have to hold it in that position.
There is no good reason to start the description of the foot in what Root et al, described as neutral position. They were right in that to compare feet you should start in the same position of the joint for each foot, but that position does not have to be neutral position.

Regards,

Eric Fuller