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I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this September 1997 Newsletter (you can buy the 2 books of newsletters off Precision Intricast):

There is a common misconception within the podiatric profession that foot orthoses act to limit excessive subtalar joint (STJ) pronation by methods which are either unexplainable or explainable only by vague, unscientific explanations. For example, I was taught that one of the ways that foot orthoses exert their control on pronation of the STJ is by “locking the midtarsal joint”. To this day, I don’t know what “locking the midtarsal joint” means. Opinions such as this from otherwise intelligent podiatrists frustrated me both during my podiatry education and during my years of practice since I believe that all observable physical phenomena are eventually explainable using scientific reasoning and logical thought.

Foot orthoses control STJ pronation by altering the effects of ground reaction force (GRF) on the plantar foot. Obviously, GRF will act only on the areas of the foot which directly are in contact with either the ground or shoe insole. GRF acting lateral to the STJ axis will cause a pronation moment across the STJ axis and GRF acting medial to the STJ axis will cause a supination moment across the STJ axis. Depending on the position of the STJ axis during that instant in gait and depending on the location where GRF is acting on the plantar foot, the magnitude of pronation or supination moments acting across the STJ axis will be variable (Kirby, K.A.: Methods for determination of positional variations in the subtalar joint axis. J.A.P.M.A. 77:5, pp. 228-234, May 1987).

The magnitude and position of forces acting on the plantar foot is altered when a foot orthosis is placed inside a patient’s shoe. Since the nature of the foot’s plantar interface changes from receiving forces from the ground to receiving forces from an orthosis, I introduced the terminology of orthosis reaction force (ORF) to describe the forces which a foot orthosis places on the plantar foot (Kirby, Kevin A., and Donald R. Green: "Evaluation and Nonoperative Management of Pes Valgus", pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992.) It is the change of magnitude and location of forces on the plantar foot with orthoses, from GRF to ORF, which cause the observable changes in gait and relief of symptoms that are expected from prescription foot orthoses. (Another term which I am introducing in this newsletter is plantar reaction force which is defined as a reaction force acting on the plantar foot which is generated by any object. Both GRF and ORF are plantar reaction forces.)

In regards to important design variables, the shape of the medial longitudinal arch (MLA) of the orthosis is one of the most important determinants of whether a prescription foot orthosis will succeed in decreasing excessive STJ pronation motion during weightbearing activities. Design parameters which determine the shape of the MLA of the orthosis include the height, length, sagittal plane contour and frontal plane contour of the MLA. In this newsletter, I will focus on the effects of increasing MLA height of the orthosis on its ability to exert pronation control on the foot.

Increasing the height of the MLA of the orthosis enhances the ability of the orthosis to exert pronation control by increasing the ORF along the MLA of the foot. Normally, the MLA of the foot does not receive any plantar force from GRF since it is a non-weightbearing structure. As the MLA height of the orthosis is increased, the ORF along the MLA of the foot is also increased. Since the summation of the magnitudes of plantar reaction force (PRF) does not change when comparing standing barefoot to standing on foot orthoses, then any increase in ORF along the MLA of the foot will necessarily be accompanied by a decrease in PRF elsewhere on the foot (Fig. 1).





Figure 1. Illustrated above is a frontal plane cross section through the midfoot at the level of the medial cuneiform (MC), intermediate cuneiform (IC), lateral cuneiform (LC), 4th metatarsal base (IV) and 5th metatarsal base (V). In a low arched orthosis (left), there is relatively more orthosis reaction force (ORF) on the lateral structures of the midfoot than on the medial structures. In a high arched orthosis (right), there is relatively more ORF on the medial structures of the midfoot than on the lateral structures. This shift in ORF toward the medial midfoot with the higher arched orthosis results in an increased supination moment and a decreased pronation moment across the subtalar joint (STJ) axis which increases the ability of the orthosis to control excessive pronation of the STJ.


Generally, the increase in ORF along the MLA of the foot is accompanied by a decrease in the PRF acting along the more lateral areas of the plantar foot which are normally weightbearing, such as the shafts of the fourth and fifth metatarsals. Therefore, in this scenario of increasing MLA height of the orthosis, the mechanical effect of the foot orthosis is to transfer ORF from the more lateral aspect of the plantar midfoot and forefoot to the more medial aspects of the plantar midfoot and forefoot.

The effect of shifting ORF from lateral to medial on the foot has a very significant effect on the ability of the foot orthosis to exert pronation control on the foot. As more ORF is shifted from the lateral to the medial aspects of the plantar foot with increasing MLA height, there are decreased pronation moments acting across the STJ axis because of the decreased ORF acting on the lateral midfoot and forefoot. In addition, there are increased supination moments acting across the STJ axis because of the increased ORF acting on the MLA of the foot. This decrease in pronation moment and increase in supination moment causes a net increase in supination moment across the STJ axis when the MLA height of the orthosis is increased. Therefore, one of the reasons why a foot orthosis with an increased MLA height is able to exert increased pronation control on the foot during weightbearing activities is due to this net increase in supination moment acting across the STJ (Fig 1).
In addition to the ability of increased MLA height of an orthosis to transfer PRF from a laterally located GRF to a medially located ORF, increased MLA height of an orthosis also has other mechanical effects. Other than the direct effects on the moments acting across the STJ, increasing the MLA height of the orthosis directly alters the forces and moments acting across the midtarsal joints and joints of the medial column. These topics will require a more detailed discussion in next month’s newsletter.

[Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 39-40.]

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I wrote this newsletter over nine years ago and the concepts are still important to the practitioner who is interested in the mechanical nature of how foot orthoses produce their effect on the foot and lower extremity. Orthosis medial arch height has a huge impact on STJ pronation/supination moments since shifting the plantar reaction force more medially, by increasing medial arch height of the orthosis, will increase the magnitude of STJ supination moment and better help to "control pronation".

Any questions on the concepts involved?

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

e-mail: kevinakirby@comcast.net

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This is an interesting newsletter. If we view the two diagrams we see that in the higher arched shell the inclination angle of the shell to the horizontal is increased with increaing arch height, thus while the CoP may be shifted medially the vertical and horizontal components of the CoF will also be altered by trigonmetric functions.

What effects do you believe this will have on the overall angulation of the force vector and the moment produced about the STJ axis?

Do you think that the angle of the CoF vector could be altered in association with more highly arched orthoses such that its line of action relative to the STJ axis results in reduced net supination moment and/ or may in fact now act laterally to the axis?

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Obviously, we have no idea of the 3D orientation of the orthosis reaction force (ORF) vectors at any location on the orthosis surface, especially given out current state of technology. However, since we do know that frictional forces exist at the orthosis-foot interface, then we can assume that the ORF vectors are not always perpendicular (i.e. normal) to the orthosis surface. Therefore, we can no more assume that the higher medial arched orthosis will have more laterally directed ORF than the lower arched orthosis will, since the frictional forces may be greater in the higher arched orthosis. We simply don't have enough information to know exactly how these ORF vectors are oriented within space.

To change the subject to something that we can hang our hat on regarding ORF vs ground reaction force (GRF), look at this new illustration. Which of these two models, one with an orthosis under the foot, one with no orthosis under the foot, would you expect to have more STJ pronation moment? [Both models illustrate a frontal plane cross section through the foot at LisFranc's joint with corresponding GRF and ORF.] It becomes readily evident when modelling ORF at each level of the foot, that this concept of considering the ORF vs GRF is very helpful in comprehending how foot orthoses may alter the internal forces/moments in the foot and lower extremity during weightbearing activities.

Attached Images

Orthosis Reaction Force vs.jpg (94.3 KB, 448 views)

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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

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Agreed.Your final point here is what I really wanted to highlight. We could re-write your statement above as: therefore, we can no more assume that the higher medial arched orthosis will have more medially directed ORF than the lower arched orthosis will, since the line of action of the force vector may have altered. That is we cannot assume that just because CoP is seen to move medially that there is improved pronation control. Further, tracking CoP and/or force vectors is of limited value unless we can also track the axial position.

This does highlight the importance of the co-efficient of friction of the top covers employed in orthoses manufacture- me I don't like slippy vinyl.


I think you know that I understand the point you are making, and I know that you know the point I am making is valid.
:)

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My impression is that the more varus slope the orthosis has, the more the orthosis (or shoe) must push medially on the plantar foot (i.e. shear forces) to keep it on the orthosis which helps negate the more laterally directed ORF that would come from a frictionless orthosis surface that is higher in the medial arch.

Good research project for you Simon!

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Kevin

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Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

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Not until technology catches up

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Simon:

While on my morning run a few hours ago, I thought of a way to perform a little experiment that may allow us to determine whether a varus-angulated surface redirects the GRF vector more lateral to the STJ axis, thus causing a STJ pronation moment, or redirects it medially, thus causing a STJ supination moment.

Here is the project: Fabricate a flat varus angulated surface of about 45 degrees under the rearfoot or midfoot so that a perpendicular from the flat surface is pointing lateral to the STJ axis. Then have the subject stand on this flat surface and see whether the foot supinates or pronates. If the point you are making is valid, that we should possibly be considering only the vectors perpendicular to the orthosis surface when we determine orthosis effects then the foot should pronate with this type of wedge. However if the vectors are more vertically oriented, then the STJ should supinate. Why would this experiment not be ample support for my contention that a 45 degree varus wedge effect is best modelled as being a vertically oriented rather than being 45 degrees oriented to the transverse and sagittal planes??

By the way, Simon, good to see you back to contributing again. I missed your input and comments.

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Kevin

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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

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The point you make is good and I concede that if I stand upon the inclined plane in all likelyhood the foot would supinate to rest upon this plane. Unless perhaps the co-efficient of friction between the foot and the plane was sufficently low to allow the foot to slide down the plane. However this single plane, static analysis cannot possibly reflect the 4D dynamic function of the foot upon an orthoses shell. Hence my posting which you quoted above regarding technology. I assumed that the original models in your newsletter showing orthotic reaction forces at the midfoot were supposed to be quasi-static representations at a given instant in time during the gait cycle with dynamic function. Therefore, I don't think this experiment that you suggest is comparable.

The original diagrams in your newsletter show orthotic reaction forces as acting purely vertical. In my opinion, this is incorrect. Moreover, the original two diagrams show no change in STJ axial position between the two orthotic conditions. If the STJ either supinates or pronates more in response to the higher arched orthotic, then surely this should be reflected in a change in axial position within the models? (rhetoric question, see point re: kinematics versus kinetics below)

I didn't say this, nor do I think this is good sense. The point I make is that the orthotic reaction forces do not act vertically as illustrated in your original diagrams, not that they act perpendicular to the supporting surface since clearly the foot-orthoses interfaces are not frictionless. If your diagrams were supposed to illustrate the vertical components in isolation then this is fine, but this is not stated within the newsletter.

Kinematics versus kinetics (you taught me this!). In other words, only if there is sufficent net moment to cause change in joint position. The fact that we do not see a change in joint position to favour either pronation or supination at this point in the gait cycle does not indicate that the moment from the orthoses in isolation has or has not shifted in one way or the other. But to reiterate, without knowledge of the absolute spatial orientation of the axes at this time it is somewhat difficult to calculate the joint moments.


Not sure where the idea comes from that the forces should be normal to the supporting plane? This may be your contention but it is not mine. I think a better model should include all the forces acting- this includes friction. I just don't think vertical or for that matter normal is accurate.

Nice to run around the stump with you again Kevin. :)

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I agree that supination motion may not occur if the varus wedge added supination moment but there was an existing pronation moment resisting it. You know this and I know this, but how many other podiatrists know this??? I probably should have been more careful in my wording.

The newsletter was meant to be a quasi-static representation of a foot on two types of foot orthoses. I agree that the orthosis reaction force (ORF) is not probably directly vertical and also that the ORF is not exactly normal (i.e. perpendicular) to the dorsal surface of the orthosis. The frictional component of ORF may be substantial or the lateral heel cup ORF may provide a substantial medially directed force plantar to the STJ axis (with an increased medial arch height orthosis) so that the combined effect of all the points of ORF on the orthosis is, in effect, similar to what I have illustrated in my newsletter. Time will tell with better technology how close I am to reality....hopefully while I am still around to see it.

Indeed, the STJ axis would change in spatial orientation with STJ rotational motion however this was not what I was trying to show in the illustrations. I wanted to show how an increase in medial arch height in an orthosis would shift more ORF medially (and decrease ORF laterally) and would therefore increase the STJ supination moment and decrease the STJ pronation moment from the effects of plantar reaction forces. Glad to see that this 9 year old newsletter still can stimulate some interesting discussion.

Thanks for chewing the fat with me, Dr. Spooner.

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Kevin

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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:
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If the person were to stand on one foot on top of any orthosis the vertical compmonent will be very close to body weight. The horizontal component from the othosis in the diagram will create a medial to lateral force that will tend to accelerate the foot unless it is opposed by some other force. With a higher arched orthosis, and no additional muscle activity, the assumption of increased force medially would seem to be a good one.
Cheers,
Eric Fuller

I'm missing something here

Surely by tipping the foot outward by 45 degrees you are also moving the axis laterally thus even if the GRF vector is perpendicular to the WB surface it stays medial to the axis and the moment is a supination one still.

Please forgive the crude illustration and sorry if i'm being dense here!

Respectfully

Robert

Attached Images

orfshif2.jpg (13.0 KB, 305 views)

But the vector will not be vertical and you don't know where the joint axes are in relation to it. What effect would this have when the shear forces that occur during gait are considered too?

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You don't know this. I've come back and edited this posting as I was a little short previously. Robert, take your third diagram here where you show the old and new axial positions, you make a quantum leap in faith in deciding your old and new positions BTW . Taking this diagram above: one possible scenario is the one you've illustrated in which the axis moves by a distance (lets call this distance d) and remains lateral to the CoP, but see what happens when the axis moves by 0.5d or 0.25d- completely different- right?

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Kevin

This is how I would show the wedge effect of the of the orthotic arch.

I moved the vectors up to the knee for clarity but the effect on the STJ would be the same.

Attached Files

wedge effect.pdf (17.6 KB, 56 views)

Not sure this adds clarity Dave. Perhaps you could provide some description to go along with your pictures? Are you saying that the vector will be inclined at the same angle given the two orthotic conditions, low arched and high arched, If so can you explain please.

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Robert:

You are missing something. I never said I was "tipping the foot". I said I was having the person stand on a 45 degree varus wedge. This may or may not cause a change in STJ axis position. A change in STJ axis position will only occur with a rotational motion of the foot in response to standing on that wedge. Rotational motion of the STJ may not occur if the STJ supination moment from addition of the 45 degree varus wedge to the plantar foot does not have sufficient magnitude to counterbalance any preexisting STJ pronation moment from ground reaction force (i.e. a foot with a significantly medially deviated STJ axis).

The other important point here is that a STJ supination moment does not necessarily need to be applied to the foot in order to have a reduction in pronation-related symptoms (e.g. sinus tarsi syndrome, posterior tibial tendinitis/dysfunction). All one needs to do to relieve pronation-related symptoms in the foot/ankle/lower extremity is reduce the magnitude of STJ pronation moment, not necessarily cause a STJ supination moment. In other words, the orthosis only needs to press on the foot less lateral to the STJ axis in order for the orthosis to have the mechanical effect of reducing pronation-related symptoms. This is one of the reasons orthoses can work for a patient to relieve pronation-related symptoms even though their STJ axis is severely medially deviated...the orthosis is decreasing the magnitude of STJ pronation moment by shifting ground reaction force more medially on the foot (via conversion of GRF to an ORF).

Hope this helps.

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

I don't understand your diagrams.

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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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The force couple effect of a laterally directed ORF in the medial arch/medial heel acting in concert with a medially directed ORF at the lateral heel cup probably plays a very significant role in producing an increase in STJ supination moment (or a decrease in STJ pronation moment) with a higher arched/medial heel skive type orthosis. Unless we can determine how to plot multiple orthosis reaction vectors from the plantar foot so that both normal and shearing forces may be more accurately determined at the orthosis surface, we will simply be making educated guesses as to how foot orthoses actually produce their mechanical effects. I'll be sure to leave that question for the next generation of podiatrists/researchers.

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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
**************************************************

I think this will come with 3D transducers applied to the kind of insoles used in inshoe pressure measurement.

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Phew! I thought it was just me being a bit thick.

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Kevin and Simon

Thanks for the clarification. Simon i know i'm making big fat presumptions that the Axis will move and how far (based on the highly unscientific method of standing with my lateral malleolus against the wall and inverting the foot). As you say i'm taking it on faith, its just a way in which things could be which would explain why the foot supinates even if the orf is perpendicular to the surface. Starting with a presumption and trying to find a way to make the facts fit is BAD science. We live and learn.

And now i think i see what i was missing. Thanks for helping me to keep up guys. I suspect there are some other lurkers who have benifited from your clarification as well!

Respectfully

Robert

Robert:

Things were much simpler for the podiatrist when they were being taught that all they needed to do was draw a line on the heel, take some measurements of the foot and leg, and balance the foot orthosis vertically so that all abnormal compensations would be eliminated and the patient would get better. Increased knowledge just makes life much more complicated....living in ignorance is a much more blissful experience.

Welcome to the world of increased knowledge, frequent frustration, and some rewarding moments when one may be able to catch a glimpse of the clear blue sky. :)

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Sincerely,

Kevin

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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
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Thanks. I used to think i knew stuff!

As the song says:-

"Can it be that it was all so simple then?
Or has time re-written every line?
If we had the chance to do it all again
Tell me, would we? Could we?"

If Reading grants knowledge, knowledge is power and power corrupts does that mean we should avoid reading to remain uncorrupted?

Respectfully
Robert

Kevin and Simon


Simon asked Kevin
and
Kevin and Simon said "I don't understand your diagrams"

I was trying to show the effect of force vectors when considering horizontal forces. Is this more clear?

see attached.

Attached Files

MLA vectors.pdf (17.9 KB, 39 views)

Kevin,

I don't know if things were much simpler. There are some highly complex explanations of observations in the old ways. There were many studentts, who in studying the old ways, felt like they did not understand biomechanics and yet got high scores on tests of their knowledge of the subject. If you ignored the explanations then things were simple. We can create a recipe in using the tissue stress approach where things can be quite simple. However, following a recipe is different from understanding the subject.

Cheers,

Eric Fuller

Kevin,

Good point. However, the maximum distance between these horizontal forces is the height of the medial arch. (For others, a refresher on force couples. The moment created by a force couple is equal to the magnitude of the force times the distance between the point of application of the forces.) These horizontal forces will be quite small in relation to the magnitude of ground reaction force. I will concede that when we are able to measure these forces we will know whether there is a larger contribution from the shift of location of the center of pressure in the transverse plane versus the moment from the force couple described above. I'd bet $10 that the shift in the transverse plane is more important. Someday, we shall see.

Regards,

Eric

Kevin, the higher the lateral HEEL flange, what ORF it does exert wouldn't it be applying less supinatory STJ moments and possibly more supinatory moments (frontal plane). Wouldn't a lateral midfoot? flange (with 'significant moment arm) apply supinatory STJ moments worthy of consideration?



I can comprehend the reduced pronatory STJ moments but would it necessarily increase STJ supinatory moments?



Beautiful, if only i had read you guys 20 years ago

Interestingly, its about 20 years since Kevin published his first paper on rotational equilibrium, so you could have. Me, I was still at High School then and more interested in becoming a rock star.

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