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I have one question about mid-tarsal joint biomechanic.

Does subtalar joint supination directly lead to mid-tarsal joint pronation(osseous locking eversion) from mid-stance to propulsive toe-off phase?

Or Does subtalar joint supination 'just' decrease range of motion of mid-tarsal joint, then indirectly lateral ground reaction force(GRF) leads to mid-tarsal joint pronation(eversion)?

Or both subtalar joint supination and indirect GRF contribute on mid-tarsal joint pronation from mid-stance to propulsive toe-off phase?

With textbooks, I still found not clear about this concept.

I am guinue who is 2nd year Podiatry student.
I have one question about mid-tarsal joint biomechanic.
Does subtalar joint supination directly lead to mid-tarsal joint pronation(osseous locking eversion) from mid-stance to propulsive toe-off phase?
Or Does subtalar joint supination 'just' decrease range of motion of mid-tarsal joint, then indirectly lateral ground reaction force(GRF) leads to mid-tarsal joint pronation(eversion)?
Or both subtalar joint supination and indirect GRF contribute on mid-tarsal joint pronation from mid-stance to propulsive toe-off phase?
With textbooks, I still found not clear about this concept.
Can anyone help me?
Thanks
Regards Guinue

What is a Guinue? is it a 1st year who's been given a good seeing to by the sports master behind the bike sheds? Gooey Newy oh no can't be since your a second year, maybe its a novel recipe for a chocolate brownie topped with caramel, are you a baker?. Or maybe you come out of a magic lamp when rubbed ah yeah and if your rubbed to much you'll be a gooey newy Genie eh! MAGIC!

Sorry couldn't resist,

Forget about locking, forget about joints, imagine a very wide plastic 30cm rule. Thru one axis it is very compliant and flexible and thru the other it is very stiff and rigid. Lets say you hold it parallel to the ground so it is flexible to bending moments in the saggital plane and rigid in the transverse plane and you can twist it in the frontal plane by applying torsion at either end.

As you twist the rule it becomes stiffer to the applied torsion and at some point your maximum applied force will be matched by the stiff torsional resistance of the rule i.e. max applied moments = maximum resisting moments. But now also the stiffness to bending will be increased in the saggital plane and the stiffness to bending in the transverse plane will decrease.

This is because of changes in the second moment of area(sma), which is changed by the twisting action, as the thin or narrow sma gets greater the wide sma reduces.

Wiki explains 2nd area of moment well enough so:
Second moment of area means: The second moment of area, also known as "moment of inertia of plane area", "area moment of inertia", or "second area moment", is a property of a cross-section that can be used to predict the resistance of a beam to bending and deflection around an axis that lies in the cross-sectional plane. The stress in, and deflection of, a beam under load depends not only on the load but also on the geometry of the beam's cross-section: larger values of second moment cause smaller values of stress and deflection. This is why beams with larger second moments of area, such as I-beams, are used in building construction in preference to other beams with the same cross-sectional area.

So the larger the second moment of area the stiffer the beam is to bending moments applied. (I beam T beam ) Bend your rule (which is a flat beam) about different axes and discover which has the greatest moment of area. Can you say which is the largest sma? Yes! good.

Apply this to the foot and you'll get the basic theory on how it gets stiffer and more flexible depending on how its loaded relative to its second moment of area at the time of interest.

Regards Dave Smith

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

The Following 3 Users Say Thank You to David Smith For This Useful Post:

7Pod7 (20th September 2012),
pod29 (19th September 2012),
Simon Spooner (19th September 2012)

What is a Guinue? is it a 1st year who's been given a good seeing to by the sports master behind the bike sheds? Gooey Newy oh no can't be since your a second year, maybe its a novel recipe for a chocolate brownie topped with caramel, are you a baker?. Or maybe you come out of a magic lamp when rubbed ah yeah and if your rubbed to much you'll be a gooey newy Genie eh! MAGIC!

Sorry couldn't resist,

Forget about locking, forget about joints, imagine a very wide plastic 30cm rule. Thru one axis it is very compliant and flexible and thru the other it is very stiff and rigid. Lets say you hold it parallel to the ground so it is flexible to bending moments in the saggital plane and rigid in the transverse plane and you can twist it in the frontal plane by applying torsion at either end.

As you twist the rule it becomes stiffer to the applied torsion and at some point your maximum applied force will be matched by the stiff torsional resistance of the rule i.e. max applied moments = maximum resisting moments. But now also the stiffness to bending will be increased in the saggital plane and the stiffness to bending in the transverse plane will decrease.

This is because of changes in the second moment of area(sma), which is changed by the twisting action, as the thin or narrow sma gets greater the wide sma reduces.

Wiki explains 2nd area of moment well enough so:
Second moment of area means: The second moment of area, also known as "moment of inertia of plane area", "area moment of inertia", or "second area moment", is a property of a cross-section that can be used to predict the resistance of a beam to bending and deflection around an axis that lies in the cross-sectional plane. The stress in, and deflection of, a beam under load depends not only on the load but also on the geometry of the beam's cross-section: larger values of second moment cause smaller values of stress and deflection. This is why beams with larger second moments of area, such as I-beams, are used in building construction in preference to other beams with the same cross-sectional area.

So the larger the second moment of area the stiffer the beam is to bending moments applied. (I beam T beam ) Bend your rule (which is a flat beam) about different axes and discover which has the greatest moment of area. Can you say which is the largest sma? Yes! good.

Apply this to the foot and you'll get the basic theory on how it gets stiffer and more flexible depending on how its loaded relative to its second moment of area at the time of interest.

Regards Dave Smith

Other than the fact that I'm wondering if you've been on the happy tablets today, Dave- this above and the running economy gag, both in the same day , I like what you are saying. I'm reminded of the twisted beam model which, if memory serves, Saraffian discussed. He modelled the foot as a flat beam with a twist within it. I'll pull out my copy of Saraffian tomorrow (thanks Labguy) and take another look as it's been a while since I read this. But if the starting position has a "twist" in the beam, we unwind in one direction of motion and increase the "wind" in the other direction. Just want to check the direction of twist that Saraffian gives for the relaxed foot.

Is there any literature on biomechanics out there where I can revisit what you are teaching here. It sounds very realistic model ie applied science.
The locking/unlocking of MTJ never sat quite comfortably with me. A little too abstract.

Teaching is not my objective in that I don't say this is what you must believe but rather presenting you with an idea for you to evaluate and decide how useful it is for your purposes.

This model, when applied to the foot, was from my own inventive mind and in response to Guinue's question. As you can see, Simon has referenced Saffarian as another proponent of this model and with luck Simon will post some literature soon.

The general principle is one that can be found in many engineering texts and one taught by the excellent Prof Nicol at Strathclyde Uni using large foam blocks with lines drawn on it to intuitively show the cross sectional deformation.

Obviously this simple model allows the question to be explored in an intuitive way without introducing confusing variables (even tho in the real world model they may be very important) This could be likened to a statistical regression model in many ways. Whereby we manipulate the variables to a convenient curve in order to predict or explain a generally expected outcome or trend.

Guinue wanted to understand the influence of torsional moments or twisting, i.e. GRF versus muscle action, in the longitudinal axis of the foot in terms of the deformation of the foot about the joints of interest i.e. the mid tarsal joints.

Guinue was also asking if there is a change in the foot mid tarsal stiffness when the pronated STJ is compared to the supinated STJ in open chain i.e. non weight bearing. This introduces confusing concepts of reference frames and which reference frame you are talking about when assessing stiffness.

If you invert the STJ open chain then effectively there is little change in the stiffness using a reference frame that rotates with the foot but if you use the fixed reference frame of the ground with a fixed direction of applied force then the stiffness does change. This would be the same with the 30cm rule example and is what we see in the example earlier.

However, we cannot assess bending or torsional stiffness with only one applied force, as in the open chain model, we need two applied forces at least. As soon as we apply two forces we are, to all intents and purposes, assessing as a closed chain model.

You might be think, well when you invert/supinate the STJ the calcaneo-cuboid joint rotates around the talo-navicular joint and this change in relative position means that the joint is stiffer if a force is applied to dorsiflex the mid foot. Now first you have to determine what you mean by dorsiflex and what direction that is relative to the new foot position as compared to the original position of a everted or pronated STJ. This is confusing and if the force is applied from the same direction on both inverted and everted positions then what reference frame are you using and where are the axes or joints of interest.

The 30 cm rule example makes all this much simpler and I believe represents a sufficiently realistic model as to be useful for practical application.

regards Dave Smith

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

This model, when applied to the foot, was from my own inventive mind and in response to Guinue's question. As you can see, Simon has referenced Saffarian as another proponent of this model and with luck Simon will post some literature soon.

I had a quick look in Sarrafian this morning and he basically talking about the "lamina pedis" which as you can see is Dave's 30cm ruler with a twist in it. http://books.google.co.uk/books?id=I...edis&f=fals e

I believe both Eric Fuller and I have also commented on this idea that the midtarsal joint dorsiflexion stiffness within the sagittal plane is largely determined by the dorsal to plantar thickness of the midtarsal-midfoot ioints, rather than by some imaginary crossing of "joint axes" as suggested by Elftman in 1960 (Elftman H: The transverse tarsal joint and its control. Clin. Orthop., 16:41-44, 1960).

Here is an excerpt from a Precision Intricast Newsletter I wrote in May 2008 regarding the dorsiflexion stiffness of the midtarsal joint and the concept that increased dorsal to plantar thickness of the midtarsal/midfoot will increase midtarsal joint dorsiflexion stiffness (Kirby KA: Foot and Lower Extremity Biomechanics III: Precision Intricast Newsletters, 2002-2008. Precision Intricast, Inc., Payson, AZ, 2009, p. 128).

Quote:

Originally Posted by Kevin Kirby, 2008

The most important passive plantar soft tissue structure that helps to maintain the proper biomechanical function of the foot during propulsion is the central component of the plantar aponeurosis (i.e. plantar fascia). Since the plantar fascia inserts onto the bases of the proximal phalanges, the plantar fascia is wound around the plantar aspects of the MPJs during propulsion. John Hicks first described how this winding of the plantar fascia around the larger diameter first metatarsal head created a very important effect on the foot, which he called the “windlass effect”. Hicks described that dorsiflexion of the hallux caused increased medial longitudinal arch height, subtalar joint (STJ) supination, and tibial external rotation. He also noted that the windlass effect did not rely on contractile activity from muscle since it also occurred in both the living and the cadaver foot (Hicks JH: The mechanics of the foot. II. The plantar aponeurosis and the arch. J Anatomy. 88:24-31, 1954).

The combination of the STJ supination moments arising from the gastrocnemius, soleus and deep flexor muscles, along with the medial arch raising effects from the windlass action of the plantar fascia, will normally cause supination of the STJ during propulsion. STJ supination makes the foot into a more rigid lever since, during supination, the talar head rotates laterally so that the talonavicular joint (TNJ) becomes more superiorly located relative to the calcaneocuboid joint (CCJ). The more superior location of the TNJ to the CCJ that occurs as the foot supinates creates a greater dorsal-plantar thickness of the midfoot which, in turn, makes the foot less flexible. Therefore, the “rigid lever” effect caused by STJ supination allows the forefoot to better resist dorsiflexion relative to the rearfoot so that when GRF is transferred solely to the forefoot during propulsion, the forefoot will be better able to transmit propulsion force to the ground than would have been possible in a more flexible foot.

This topic really deserves a complete paper of its own. I'll consider this when things slow down a bit.

__________________
Sincerely,

Kevin

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

If one thinks of the foot as a piece of lumber that simply resists deformation when loads are applied, then the dorsiflexion stiffness of the foot will increase with the cube of the dorsal to plantar thickness of the foot at the midtarsal-midfoot.

Quote:

CHAPTER I.

DETERMINING THE STRENGTH OF WOODEN BEAMS

Many persons doubtless think that the strength of wooden beams is a matter of conjecture and not of mathematics, but except for a slight variation in the strength of the wood, due to different conditions inherent in the tree and also in the degree of seasoning, the strength of a given beam can be very accurately determined by simple calculations. Even with the variation due to the wood, it is possible to determine the maximum load that it is safe to put upon a beam, which is usually the information desired.

Before giving any rules, however, it will be well to consider some of the facts relating to the strength of beams. The strength of a beam depends upon its size and shape, its span, (or if a cantilever, the projection beyond the point of support), the kind of wood and its condition, and also the manner of loading. The following facts are also true of all rectangular wooden beams:

1. The strength of a beam decreases in the proportion that its span is increased. Thus the strength of a given beam, with a span of 10 feet, is one-half that of the same beam with a 5-foot span. With a span of 12 feet the strength will be five-sixths what it would be with a span of 10 feet. Or if we have a beam with a span of 20 feet and place a support under the center we just double the strength.

2. The strength of a beam increases exactly as its breadth or thickness is increased. Thus a beam 2 inches thick is twice as strong as a beam 1 inch thick, provided the other conditions remain the same.

3. The strength of a beam increases in proportion to the square of its depth. A 2 x 8 inch beam will be four times as strong as a 2x4 inch beam, and a 2 x 12 inch beam will be nine times as strong as a 2 x 4 inch beam, the square of four being 16, and of twelve 144, or nine times as great.

It follows from the second and third paragraphs that the strength of a rectangular beam is in proportion to the product of the breadth by the square of the depth if the span remains the same. A knowledge of these facts is very important for the wise use of timber.

A beam 8x8 contains 64 square inches in cross section, and a beam 6 x 10 contains 60 square inches, yet their strength will be in the proportion of 512 (8 X 8 X 8) to 600 (6 X 10 * 10), the 6 x 10 beam being the stronger. The strength of a 6 x 8 inch beam on edge in proportion to the strength of the same beam laid flat wise is as 6 x 8 x 8 to 8 * 6 x 6, or 384 to 288.

Deep beams are also very much stiffer than shallow beams, the resistance, of a beam to bending increasing in proportion to the cube of the depth. The stiffness therefore of a 2 x 12 inch beam and a 2 x 10 inch beam is in the proportion of the cube of 12 to the cube of 10, or 1728 to 1000. This property of stiffness is very important in floor joists, where the span in feet is usually greater than the depth in inches, but for shorter beams it need not be considered.

In speaking of the strength or stiffness of beams the breadth of the beam always refers to the thickness measured horizontally, and the depth to the height of the beam as it sets in place, without regard to which is the larger dimension. When a beam is supported at each end the distance between supports is called the span. The distance which the ends rest on their support is called the bearing.

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

Many Thanks for the visual and Text Reference. I assume one of you at least has the text book-how strongly do you recommend it in context of biomechanics of the foot. Thinking of acquiring it.

Many Thanks for the visual and Text Reference. I assume one of you at least has the text book-how strongly do you recommend it in context of biomechanics of the foot. Thinking of acquiring it.

Thanks,

7Pod7

It's not a biomechanics book per se, but it is probably the best book on anatomy of the foot.

It's not a biomechanics book per se, but it is probably the best book on anatomy of the foot.

I noticed ch10 about function of foot and maybe cross sectional area would be about biomechanics. I'm thinking that a great revision of anatomy would spruse up my diagnostic skills as well. What do you recon?

I noticed ch10 about function of foot and maybe cross sectional area would be about biomechanics. I'm thinking that a great revision of anatomy would spruse up my diagnostic skills as well. What do you recon?

If you can afford it, you can't have enough books in your library.

If you can afford it, you can't have enough books in your library.

Right in both accounts. Just want to splurge a little from a Tax Refund.
One shop wants $400, another just under $200.
One more question;
Any excellent Biomechanics texts to check out on the web you're familiar with?

Right in both accounts. Just want to splurge a little from a Tax Refund.
One shop wants $400, another just under $200.
One more question;
Any excellent Biomechanics texts to check out on the web you're familiar with?

Right. I saw these advertised on PodArena and meant to check them out a little while ago. Had other goals then and now they disappeared. Are they available in any other outlets?

Right. I saw these advertised on PodArena and meant to check them out a little while ago. Had other goals then and now they disappeared. Are they available in any other outlets?