Should tissues be subject to the margins of their optimal stress zones?

Kevin, I tend to agree in that the principle therapeutic effect of foot orthoses is due to the surface topography and the modification this has on the reaction forces beneath the weightbearing foot. I don't think I've ever said that the "function of orthoses can best be understood by trying to determine the energy transfer between the foot and the orthosis". However, there is no denying that foot orthoses play a role in energy transfer from the body to the ground and so shouldn't be ignored when a complete description of how foot orthoses "work" is being undertaken. I take on board your point re: shoe-stiffness, but shoes don't generally have the same congruence with the medial longitudinal arch that foot orthoses have, and I believe it is this area which should be targeted.

In you article on the orthoses deformation test you talked about deformations in the order of 2-3mm in the medial longitudinal arch area of the device. I suspect during true dynamic function such as running much greater deformations would be observed in the same devices which show 2-3mm deformation when performing this test. I think it should be possible to engineer devices which can deform more than this and in so doing, store and return greater energy to the foot. Such devices may be more compliant than many traditional foot orthoses and have stiffness values similar to those employed by McMahon. Modern manufacturing techniques will make this possible.

Here's a picture of a prototype I made up 3-4 years ago, inspired by McMahon's patent (image also attached). The device incorporates a wave spring http://www.smalley.com/wave_springs/about_springs.asp , tuned to my body weight to provide stiffness characteristics within McMahon's magic range- they were very "bouncy" :drinks Obviously using 3D printing the wave spring could be created as an integral part of the orthoses or you could employ a clip-lock system and insert different springs calibrated for different activities / different people. This prototype was the best I could do with the resources at my disposal at the time; I don't have multi-million pound funding like some projects have

Maybe we should just construct our foot orthoses out of D3o? http://en.wikipedia.org/wiki/D3o http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=84550

For those following, this paper gives a nice overview of the concepts of surface energetics: http://www.isss.de/conferences/Calgary2003/Nigg-Energy&Perform.pdf

 

Simon

Have you considered this:

Work = f*d and energy (k) = 1/2m*v^2

work energy theorem is this

There is a final velocity caused by work done and so therefore f*d = a change in kinetic energy.

Where Vo= initial velocity and V1 = final velocity then:

The equation to find the change in velocity is V1^2=Vo^2+2*a*d (where a = acceleration) Since Vo=zero then this becomes V1^2=2*a*d.

acceleration = f/m so V1^2=2*{[f*d]/m}

f*d = work so V1^2=2*[w/m] => m=V1^2 * [2*w] => w=[V1^2*m]/2
=> w= 1/2mv^2 i.e. work done = energy used.

Therefore if you know the work done then you can find the energy used and vice versa

Regards Dave

 

BTW - Work done = energy used --- doesn't mean that work is energy or equal to energy in the same way that if you observe a man lifting a weight then you could say that weight lifted = strength used but weight does not equal strength or weight is not the same thing as strength it is just the amount of strength used indicates, or is indicated by, the magnitude of weight.

Regards Dave

 

Simon

also BTW - in the case of energy used by an orthosis then you need to work in negative work because the direction of the force applied is opposite to the direction of the displacement i.e. you are braking the mass, which - a negative acceleration.

Maybe you've already worked through this and I'm teaching you to suck eggs DoH!

Dave

 

These are some notes I made on the topic some time ago, much of it based on the paper I linked above.

 

I still don't think that a foot orthosis will add much rebound energy to the foot unless the orthosis has a spring under the heel, or has, as part of its design, a "spring-like" topcover.

When the foot first hits the ground, the medial arch first flattens, stretching the plantar ligaments and plantar fascia, and then rebounds into a higher arch position, which will return energy to the body (Ker RF, Bennett MB, Bibby SR, Kester RC, Alexander RMcN: The spring in the arch of the human foot. Nature, 325: 147-149, 1987).

Now if we add an orthosis under the foot, that reduces the tensile force on these plantar ligaments, are we actually 1) adding energy to the body, or 2) reducing the elastic energy that comes from the plantar ligaments of the foot and not effectively changing the total energy returned to the body?

I believe it is the latter, not the former.

 

We might be capturing and returning some of the energy which would otherwise be lost to the ground without over-loading the plantar fascia with too much energy. How many patients did you see with plantar fasciitis this week, Kevin?

I agree regarding orthoses design though, this is one of the reasons we need to start exploiting the technology available to us, and move our orthoses beyond the 1950's designs that many of them are, and into the 21st century.:drinks

 

But I kinda liked the '50's....

 

....can't forget the King of the 50's....

 

I'll get you a 1950's computer for Christmas then, I trust you have a free room in your house for its massive 0.5k RAM :santa::santa2:

 

Not fair, you're not allowed to bring the King into the argument. Thank you, very much. Brand new cadillac...

http://www.youtube.com/watch?gl=GB&hl=en-GB&v=6h0otRDNjG4
http://www.youtube.com/watch?v=prErnop1eiM

 

I'm not trying to play fair...just playing to win....here's Chuck at ya!

 

Is that a fake tattoo?

 

We used a "felt pen" to apply our "tattoos" for "Crankshaft and the V-8s"....

 

And whatever happened to number 95? A bit part in Planet of the Apes?

 

Curtis now is manager of a grocery chain here in Sacramento. We sang together all through junior high school in choir...he has a great voice and is a great guy.

 

Now, to get back to our original discussion.....

My point is that the medial longitudinal arch (MLA) deformation that the foot orthosis undergoes is stored in the foot orthosis as potential energy that will be released as kinetic energy when the orthosis MLA returns to its original shape. However, in absorbing this potential energy and then releasing this kinetic energy, the foot orthosis also will reduce the tension within the plantar fascia and plantar ligaments which will, in turn, reduce the potential energy of deformation of the MLA of the foot and also will reduce the kinetic energy of the MLA of the foot when it springs back into its original shape.

For example, let's say that with each running step the plantar fascia and plantar ligaments store and release 10.0 Joules (J) of energy. Now, we add a relatively thin polypropylene orthosis with good MLA congruity, with no rearfoot post and with no topcover to the shoe (let's assume the shoe returns no energy to make the example easier to understand). We then measure from the stiffness and deformation of the MLA of the orthosis that the orthosis stores and releases 4.0 J of energy with each running step (Ker RF, Bennett MB, Bibby SR, Kester RC, Alexander RMcN: The spring in the arch of the human foot. Nature, 325: 147-149, 1987). See paper below for all those still following along.

My belief is that, because of the reduction in tension force within plantar ligament and plantar fascia caused by the foot orthosis [the foot orthosis effectively reduces the stiffness and deformation of the MLA of the foot during each foot strike], the foot now will only store and release about 6.0 J of energy instead of the original 10.0 J of energy. However, since conservation of energy must occur (unless some energy is lost as heat by the intervention of using the foot orthosis inside the shoe), the total energy added to the center of mass (CoM) of the body will be the same whether running without the orthosis (10.0 J from the plantar ligaments/fascia) or running with the orthosis (4.0 J from the orthosis + 6.0 J from the foot = 10.0 J total).

Effectively, therefore, the foot orthosis does not add energy to the body as a whole, by decelerating and accelerating the CoM with each running step, but, rather, the foot orthosis reduces the total work done (or energy absorbed) within the plantar ligaments and plantar fascia with each running step. That, I believe, is how foot orthoses work....not by adding energy to the body....but rather by absorbing and then releasing some of the energy of each running step so that less work (remember work-energy equivalence) is done on the structures that prevent MLA deformation and effectively store and release energy within the human foot.

 

Kevin

Like it, Good thinking! No spring can out put more energy than was stored in it but the biological spring has a muscle attached to add energy. So if the orthosis allows attenuation of the energy impulse in the plantar aponeurosis at the load or braking stage but returns it at the propulsive stage when the muscle is also adding its energy could we then end up with a net increase in energy output?

Dave

 

Kevin,
We are pretty much on the same page. But let's say the plantar fascia is storing 10J of energy, but it's energy threshold is 6J, as consequence the plantar fascia becomes injured. We now place our foot orthoses in situ such that plantar fascia now stores 6J and the foot orthoses stores 4J. The plantar fascia says thank you very much to the foot orthoses and feels much happier and healthy. The CoM continues on its way.

But, If there is no net gain in the total energy accelerating the CoM through variation in surface (orthoses) stiffness characteristics, how do we explain the work of McMahon and others? Dave may be on the money here.

 

And some springs will loose more energy in the form of heat etc. than others. Which begs the question which is more likely to "loose" energy in the form of heat etc. a foot orthoses or the plantar fascia? I.e. which is the more efficient spring?

 

The difference with McMahon's tuned track at Harvard that deforms and then rebounds with each running step and a foot orthosis with a well formed medial longitudinal arch that deforms and rebounds with each running step is the actual potential for lowering of the center of mass (CoM) by the track versus the orthosis when the foot strikes the ground. In other words, the magnitude of lowering in a tuned track is likely in the order of 20 mm with each step whereas the foot orthosis with no topcover has no ability, by itself, to lower the center of mass by its deflection (in fact, the orthosis may prevent the CoM from lowering as much as normal).

An energy return mechanism, whether it is hip flexion, knee flexion, ankle joint dorsiflexion, medial arch lowering, shoe midsole compression, or ground surface compression, all work by first allowing the CoM to lower toward the ground then raise it away from the ground. A traditional polypropylene foot orthosis, with no soft-springy topcover, has no ability to do this.

 

Like I said, maybe it's time to re-think those 1950's designs...

Moreover, given that leg stiffness is increased when running on a more compliant track, does the CoM actually lower any more than it would do when running with decreased leg stiffness on a stiffer surface? Indeed, Kerdok et al. report: "This suggests that runners compensate for variable ground stiffness without affecting the fluctuations in the motion of their center of mass." http://biomech.media.mit.edu/publications/Ground_Stiffness_Metabolism.pdf

This seems to question your contention, Kevin. Good discussion.

 

Yes, the runner's leg will stiffen on the tuned track, but the runner's leg also doesn't need to work as hard to spring the runner forward on to their next stride. In other words, the tuned track returns energy to the runner's center of mass (CoM), energy that the leg now doesn't need to provide as much of back to the CoM of the runner. This energy-returning mechanism to the runner's CoM is something that a conventional foot orthosis either can't do, or is not very good at doing.

Great discussion.:drinks

 

I'd agree that contemporary foot orthoses designs preclude the storage of much energy in the heel cup section of the device, I'm not so sure about the medial longitudinal arch section. What we really need to see is the degree of deformation that occurs in various types of foot orthoses during running gait.

According to Stefanyshyn and Nigg, a gymnasium floor which deforms about 5mm will store about 5J of energy. They go on to state that in running: "a return of 6J [from the surface] is still substantial representing over 3% of the mechanical energy per stride".

 

Can't agree with you here regarding medial longitudinal arch deformation of the orthosis returning energy to the body as I've already explained earlier.

Simon, please describe the mechanism by which the foot orthosis medial longitudinal arch can return energy to the center of mass of the runner's body if it is, in fact, reducing the energy return from the medial longitudinal arch of the foot. From what I can deduce, if it can do this, it is extremely limited in potential....much less than what a running shoe midsole can provide.

 

I'd say it can return the energy in the same way as the midsole of the shoe can and in the same way that a compliant running track can, Kevin. By reducing the energy lost to the ground, to heat, to sound etc. i.e., by reducing the net hysteresis of the system. Specifically, foot orthoses may improve the efficiency of the system by storing energy and then returning a proportion of that energy in the correct direction, at the correct time and in a superior manner to which the footwear alone can; but only if the orthoses are designed correctly. Or, just plain and simply by making the surface more compliant than it would be without the orthoses in-situ, allowing the leg to function with a higher stiffness. And since Kerdok et al. demonstrated that "lowering of the center of mass (CoM) by the track versus the orthosis when the foot strikes the ground" does not explain their results...

Anyway, what is the average height from the supporting surface to the peak of the medial longitudinal arch section of a foot orthosis? What is the thickness of the midsole material of a shoe at this point?

By the way you didn't answer my question- which is the more efficient "spring" the plantar fascia or a foot orthotic- which has the potential to return more energy and which might loose more energy to heat etc.? You seem to be making the assumption that it's more efficient to store the energy in the plantar-fascia than within a foot orthoses- is it?

Certainly there is a limit to the energy we can store in the plantar-fascia without causing injury, perhaps gait behaviour is modulated to avoid pain and as such certain limits on performance are placed on the body by the CNS in an attempt to avoid pain/ injury. By transferring some of the energy to an external store, then maybe the CNS allows the body to perform at higher levels. So without orthoses the plantar-fascia can store 10J without pain and the body attempts to limit the energy within the fascia to 10J by constraining gait characteristics. When that limit is reached, velocity etc. is red-lined and the limiter kicks in; with an orthosis in situ the plantar fascia can still only store 10J without pain, but since the orthoses is now taking some of the energy transferred through the foot, the system can perform at a higher energy state without pain in the plantar fascia and the "limiter" kicking in e.g. 20J total with the plantar fascia storing 10J and the orthoses storing a further 10J. Just an hypothesis.

What if the foot orthosis doesn't change the kinematics at the medial longitudinal arch?

To re-iterate, we got to think past those dated orthoses designs, Kevin. I've got a design concept which I call "HERO", which stands for High Energy Return Orthoses, if anyone is interested?

 

Further thoughts:

As we've pointed out earlier in this thread, the tissues of the body are visco-elastic. Thus, their (spring) stiffness is dependent upon the rate of loading: they will be stiffer (i.e. less deformation for a given load) when loaded more rapidly. The amount of deformation in a tissue is quadratically related to the energy it can store and potentially return- less stiff = more deformation potential = more energy storage. Foot orthoses have been demonstrated to reduce the rate of pronation and viz. the rate of loading on the plantar fascia. Therefore, foot orthoses by virtue of their influence on the rate of loading on the plantar fascia and ergo their influence on the stiffness of this tissue, could allow greater energy to be stored and potentially returned from the plantar fascia than when the foot orthosis is not in-situ, increasing performance or making matters worse if the plantar fascia is already over-loaded with energy.

Get out of that and stay fashionable with your 1950's music style and felt-tip tattoo, Prof. Kirby :drinks :santa2::

Haven't thought this hard about foot biomechanics in ages. double Brand new cadillac, thank you, very much....

 

Simon

Fascinating conversation. Can you explain the above, please? Not sure I quite understand this.

Cheers

 

Read this Mark http://www.isss.de/conferences/Calga...gy&Perform.pdf

Basically (and it's a nightmare trying to write equations on podiatry arena)
Energy (surface) = 1⁄2 k x>2
where:
k= stiffness
x= deformation


The x term is squared, hence it has a quadratic relationship with surface energy.

What Kevin will come back with is: that despite the tissue having a lower k value due to the decreased rate of loading, the actual deformation of the plantar fascia produced by the ground reaction force will be reduced also by virtue of the foot orthoses being in-situ. However, this will be speculation upon his part. Since, as we know, rearfoot pronation range of motion is often reported as being unaltered by foot orthoses and midfoot kinematics are rarely reported at all.

 

Thanks Simon - can you check the link please - I'm getting an employee benefit scheme!

 

That's weird. I hit on the link I'd previously embedded and it took me to the right place, I copy and pasted and it took me to sh!t...

Try this: http://www.isss.de/conferences/Calgary2003/Nigg-Energy&Perform.pdf

 

Really? I would have thought that was one of the primary goals of some devices! Cheers for the link and the read. I'd like to consider this and get back after the weekend.

Have a good one.

 

But, the orthosis can't return energy the same way that a shoe midsole and a compliant track, can, Simon. That is the whole point. The foot orthosis has no way of decelerating the plantar metatarsal heads and plantar calcaneus in its descent toward the ground during foot strike in the way that a shoe midsole and compliant track can, unless the orthosis has a cushioned heel pad and/or a cushioned forefoot and rearfoot topcover, for example. Just because the orthosis flexes at the medial longitudinal arch (MLA) does not mean it is absorbing and returning energy to the center of mass (CoM) of the body. In order to absorb and return energy to the CoM during running, the orthosis would need to decelerate the CoM downward during the first half of the support phase of running and then accelerate the CoM upward during the second half of the support phase of running. Traditional foot orthoses simply can't perform that function to any significant degree.

Those are all theoretical possibilities but, in my opinion, none of them make good mechanical sense. Maybe you could explain how supporting the MLA of the foot allows the foot orthosis to store and return energy to the CoM since, so far, I'm not being convinced by your argument.

It depends on the foot and type of shoe in question.

I believe the plantar ligaments and plantar fascia of the foot are more efficient mechanisms of transferring energy back into the CoM than are a traditional orthosis since the plantar ligaments and plantar fascia attach directly to the osseous components within the foot and, as a result, are more efficient because of their "direct in-bone" connnection to the foot skeleton. Since the foot orthosis only supports the MLA of the foot and compresses the plantar soft tissues, it can never, I believe, be as efficient as the "direct in-bone" tensile-load bearing attachments of the plantar ligaments and plantar fascia.

Sounds like an interesting hypothesis, Simon. However, I think you are reaching a bit too far on this one.

However, I do agree that modifying the traditional orthosis design into something different may be better at returning energy to the foot during running. Maybe having a foot orthosis with a compressed carbon dioxide cartridge built into the MLA that is connected to an accelerometer that precisely times carbon dioxide gas release downward so that this jet propels the foot upward during the latter half of support phase would be a unique energy-return orthosis that one could patent as Orthosis Jets.

 

Not pure speculation....rather....extraordinary speculation!

Conservation of energy must be maintained when an orthosis is added to the shoe and reduces the tensile force in the plantar ligaments and plantar fascia. The orthosis can't add energy to the system if it is also simultaneously reducing the tensile forces within the plantar ligaments and plantar fascia.

 

So foot orthoses like the ones used in a recent study that incorporated a 6mm poron +3mm EVA cannot act in a similar manner to a cushioned running shoe in your opinion, Kevin? We have a potential for something approaching say 7-8mm of deformation in these orthoses at the metatarsal heads and centre of the heel, in the medial longitudinal arch it will be greater due to the topography of the device. : A comparison of customised and prefabricated insoles to reduce risk factors for neuropathic diabetic foot ulceration: a participant-blinded randomised controlled trial
(Joanne S Paton, Elizabeth A Stenhouse, Graham Bruce, Daniel Zahra and Ray B Jones Journal of Foot and Ankle Research 2012, 5:31 doi:10.1186/1757-1146-5-31).

That you seem to believe that adding a foot orthoses of any design does not have the potential to change the surface stiffness and thus the leg stiffness seems incredible to me.

You do not think that some of the body's kinetic energy being transferred as sound and heat during impact with the ground makes good mechanical sense; really? Kevin, as you know I have a reasonable handle on physics, I'm not saying the orthoses create energy as this would break the laws of thermodynamics, what I'm saying is that they may prevent some of the energy being lost to forms which cannot be used by the body to assist in locomotion, as well you know. They store and return energy by virtue of their load-deformation cycle during loading and un-loading, in the same way that the plantar fascia does.

Perhaps we need to go back a step: when the body impacts the ground it has a certain amount of kinetic energy, some of that energy is stored in the body's tissue such as the plantar fascia as elastic-strain (potential) energy, what happens to the rest of it?

Indeed it does, with the potential for quite a bit of deformation into that space between the orthoses and the shoe.

That's not really what I was asking, neither running tracks nor shoes are attached to the bones; trampolines and spring-boards aren't attached to the bones either but they are capable of storing energy from, and returning it to the body rather well. Rather, I was asking which would show the greater hysteresis in a typical load-unload cycle: the plantar fascia, or polypropylene, polyurethane, EVA, carbon fibre etc. foot orthoses? Viz. which would loose more energy? How does the shape of the material influence it's spring-like behaviour?

So the CNS ignores pain, pain has no influence upon gait function in your opinion, and the tissues can store unlimited energy without experiencing damage, Kevin?

Yes, Kevin. I think you'd better stick to your 1950's designed orthoses and leave the energy returning foot orthoses designs to someone else.

Can orthoses change the rate of loading of tissues? If they can, then they can change the tissue stiffness; if the stiffness of the tissue can be changed then in so doing it's ability to store and return energy can be altered: Energy (surface) = 1⁄2 k x>2
where:
k= stiffness
x= deformation

Does this mean they put more energy into the system? No, of course not. Does this mean they may alter the body's tissues ability to store and return energy? Yes. Does this mean they may prevent some energy being lost to forms which the body cannot use to assist in locomotion or simply dissipated to the ground? Yes, they can.

I suspect we'll need to agree to disagree on this one, Kevin. You seem to be stuck on the idea that orthoses are putting energy into the body, whereas I am happy that they can modify the tissues ability to store energy and prevent the loss of, and return some usable energy to the system.

Here's a picture of a couple of contemporary insoles from a recently published study referenced above, my contention is that these orthoses can and do store and return energy to body, Kevin's contention seems to be that "foot orthoses simply can't perform that function to any significant degree": 6mm poron + 3mm EVA= what do you think?

 

Interesting then that Mundermann reported a decreased vertical loading rate with "traditional" posted and moulded orthoses... http://www.orthotek.gr/pdf/Foot orthotics affect lower extremity kinematics (2003).pdf

Obviously this could be due to several reasons, but maybe the orthoses decelerated the "plantar calcaneus in its descent toward the ground during foot strike"? They certainly decelerated the CoM.

 

This got my attention. I'm not sure I can contribute to the degree Kevin and Simon are managing - fascinating as it is to follow, but anecdotally from a mere clinician's point of view and experience, I've found that some materials work in a way that is quite unexpected. Thinking about the concept of energy absorbtion and one material in particular, this may go someway to explain its success in clinical application.

Not long after I graduated wnd working with a number of chronic RA patients with significant forefoot problems, I started using what I think is a low density EVA - from 5 season Karimats (specialist high altitude campling mats). Typically 10mm thick and very lightweight - I fit a full-length insole into a suitable shoe for the patient to wear for a couple of weeks. When they return the material will have deformed to contour the plantar surface of the foot. On high pressure areas the material will have completely "bottomed out", less so with decreased GRF. At this stage I usually add a 3mm Poron layer to the base and any other mods like rearfoot posts between the layers.

I've found that with patients with plantar plate pathology - perforation, tears, atrophy - as well as the RA group - this material performs incredibly well compared to other traditional materials such as poron (all densities), sorbothane and many of the clinical EVAs we use. What is remarkable is the resolution time of plantar lesions where present - even chronic lesions respond favourably, usually resolving completely within 8 weeks. I'll upload some photographs later of a typical insole at supply and 4 weeks - but it is the properties of the material and their potential for energy absorbtion that seems key here. Certainly suggests to me that high absorbtion reduces much of the damaging tissue stress.

The other advantage is cost - typically £11 for a 1x2m sheet.

Just my observation.

 

Sounds good - Where is it available from - Get me some tout suite sil vous plait.

Dave

 

Hi Dave

Your local climbing/outdoor shop should stock them - or try here http://www.needlesports.com/Catalogue/Camping-Equipment/Camping-Mats/Closed-Cell-Foam-Mats

 

Oh yeah I know what you mean now. cheers Mark

Dave