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

I think you must have missed some parts of my other postings, Simon.

Here is what I said in post #40:

Here is what I said in post #46:

Here is what I said in post #61:

When I stated, quite a few times now over the last few days, that I thought that the medial longitudinal arch (MLA) of a foot orthosis has limited, if any, potential to add energy back into the center of mass (CoM) of the body during running by deforming, I was talking only about the MLA of the orthosis, not a topcover, or cushioned heel or forefoot of the orthosis. I agree that a soft, springy topcover on an orthosis and a soft, springy insole material inside a shoe do have potential to return energy into the CoM during running, which will largely be determined by their overall thickness and material stiffness.

However, if we now remove that topcover and only now have a non-compressible polypropylene orthosis plate sitting on top of the shoe insole, my previous statement still holds: "a relatively stiff polypropylene plate without a topcover being placed into a neutral running shoe with a thick midsole will probably not aid in energy absorption for the individual at all and may, by preventing the foot from sinking into the midsole upon impact, may actually decrease the shock-aborbing ability of the shoe."

Hope that helps clear up any confusion about what I have been saying over the last few days.

 

What about a relatively compliant polypropylene plate, or a relative compliant any material plate? With each iteration, more and more caveats. Kevin, as I said, we can agree to disagree on this one :drinks

 

Not more caveats, just saying the same thing over and over again.

We will need to agree to disagree since I am becoming tired of this discussion. Maybe someone else can explain these concepts more clearly than I can.:drinks

 

Guy's, Kevin and Simon

I think the reason why you two are going around without finding common ground id because you are trying to resolve a question that is unsolvable. In fact I think the question is a non sequitur.

I believe the question you are asking is something like 'what kind or design of orthosis / appliance will increased energy output at propulsion that will increase forward velocity in ambulation'?

I believe the problem is that you can't get more energy out than goes in. If you compress a spring during braking this uses energy which can be returned during propulsion but not 100% as there are losses. But even if you can return 100% of the energy in then this energy stored at braking phase decreases original velocity so the energy stored is then required to increase velocity to the original.

If we consider the muscular action then the muscles can be used to replace energy losses and maintain velocity. However to allow muscle action during propulsion there must be some joint flexion during braking that will attenuate peak force and elongate the force impulse thus maintaining a constant energy transfer. However, because of reduce peak braking force this means that springs in the orthosis cannot be compressed as much and so cannot return so much force or energy in the propulsive phase.

If the springs are made softer with a longer range of motion i.e. a longer frequency then this will allow more energy to be stored during braking when the joints are flexing and reducing peak force but this requires longer time so in this case the time between steps must increase and so each force impulse is further apart so even tho there is more time in energy output there is more time in energy storage. This means attenuated force impulses and this results in reduced rate of acceleration to regain original velocity.

The result is you can have bigger bounces or faster foot strike but not both. I believe the reason why tuned tracks reduce running time for a certain distance is because this reduces physiological energy use thru muscle action so maximum velocity can be maintained for longer but cannot be increased per se.

Another problem with springs loaded at braking is that as they unload they tend to push upward and give an increased bounce instead of an increased forward propulsion, which = a lot of wasted enrgy.

It seems to me whatever you do you can't get increased velocity by loading a spring during braking phase, if you could then each step would result inan increased velocity ad infinitum.

regards Dave

 

Dave:

Don't know if I can agree with you on this one. However, I do appreciate you trying to help us out since I think we need some new insights here in this discussion.

The tuned track is exactly that, tuned to the rebound frequency of the human leg while running. In other words, for the given weight of the runner, Tom McMahon designed the indoor track at Harvard so that the track itself would first depress upon foot impact from the runner and then elevate itself back to its original shape in the same time interval as the support phase of the runner [support phase of running is equivalent to the stance phase of walking].

In this fashion, the runner's lower extremity muscles would have to work less hard to impart an upward push to the runner's center of mass (CoM) with each running step. The result of running on the tuned track was a longer running stride for the same amount of metabolic energy that would have been required for running with a shorter stride on a hard, non-compliant surface. Once the runner had reached their steady state running velocity, the extra rebound energy from the tuned track would allow the runner to run with a longer stride which would equate to higher running velocities for equivalent efforts than when running on hard, non-compliant surfaces.

The rebound from this track only has to be vertically-directed to function properly since this extra vertical rebound from the track accelerates the CoM upward faster with each step which then results in a longer stride for equivalent effort (running is best modelled as a spring-mass model, or more simply, as a pogo stick or bouncing ball). In other words, the rebound doesn't have to necessarily be in the exact direction of travel of a bouncing ball in order to produce longer bounces in a bouncing ball, it only has to be one of the velocity components of the bouncing ball.

In addition, in many sports activities, in both upper and lower extremities, braking of a limb prior to acceleration of a limb is a very important part of optimal muscle performance. Braking a limb first before limb acceleration causes a preload (i.e. pre-stretch) of the elastic elements within the muscles and tendons (i.e. eccentric contraction) which will result in greater muscle contraction moment and power (i.e. concentric contraction) than if no braking or eccentric muscle contraction had preceded the concentric contraction. Without the braking phase of running, running would be much less efficient metabolically since, without braking and preloading the lower extremity muscles during the first half of the support phase of running, the second half of the support phase would produce less torque and less power to push off the CoM into the double float phase (i.e. both feet off ground).

This is why runners, at steady state running velocities at non-sprinting speeds need to have the feet contact the ground ahead of their center of mass to allow this desired braking phase to occur so that muscle preloading may also occur (contrary to many popular new beliefs from marketing of alternative running "techniques" such as Chi Running and Pose Running). By placing the foot slightly ahead of the CoM during running, optimal braking can occur simultaneously with eccentric musle contraction of the lower extremity muscles to allow optimal moment and power production during the second half of the supoort phase of running. Placing the foot directly under the CoM would accelerate the CoM forward (if normal braking also occurred during the first half of the support phase of running) resulting in the runner being pitched forward and likely falling flat on their face.

Still a good discussion.:drinks

 

You don't want MORE energy into the system for too long, the tissues wouldn't handle it well, ZOOS fans. And you can't run faster for the reasons David brings up..just more efficiently if you can return some of the energy lost. The key phrase Kevin said is "traditional polypro"...I think the COG about the MLA is wonderful to focus on with a greater range to exploit for energy than any other anatomic region of the foot...you just need the appropriate material properties in the most effective shape for each particular foot. Then be able to tune it to absorb energy at one point, then return the energy at another point in the gait cycle. ahh Parabolas and sine waves..Huzzah for the over-pronators of the world! Combine that shell with a non-attached, insole of a high-energy foam and an entropically favorable bottom cover, and you can maximize the limits of what energies can be either absorbed and returned or harvested and re-purposed inside a shoe...even using the friction generated at the shell/insole interface is possible. Piezoelectric generators have been implanted in orthoses but researchers are tight lipped. I don't think, Kevin, you would disagree that a shell could accomplish this feat, I just think you need proof or at very least, an evidence based explanation. Wish I had some to give...only anecdotal evidence..Look to the ski manufacturing industry for answers. As for novel orthotic design, 3D printing doesn't break the circle, in my opinion, just makes things cheaper...but this...

 

I'm speaking of in-shoe devices only. I've run on a tuned track. It works remarkably well and is a free feeling of flight. Zoooom!
http://rrg.utk.edu/resources/BME473/lectures/BME473_lecture_2.pdf

 

That I may not agree with you doesn't mean that I need the concepts to be explained more clearly, Kevin. As you know, I have been discussing these concepts at invited international meetings on and off for about 7 years now. I have employed finite element analyses to get a better understanding of the load/ deformation characteristics of foot orthoses in an attempt to unravel some of this. I do understand the concepts.

The key to our discussion is how stiff the medial longitudinal arch of a "traditional" foot orthoses is and for that matter, how stiff any other area of the device is. Obviously this will influence the energy stored in this area of an orthosis since: Energy (surface) = 1⁄2 k x>2
where:
k= stiffness
x= deformation

As I said early on, we don't know how stiff the medial longitudinal arch area is in any given device nor how much deformation occurs in this area in-vivo. Since neither you nor I know the answer to this Kevin, neither of us can talk with certainty on the issue. But since you have reported figures in the region of 2-3mm in your orthoses deformation test, I would say it shouldn't be too unreasonable to expect deformations in excess of 6mm in your "traditional" devices during running.

We don't know how much energy storage and return by an orthoses should be considered as significant. Stefanyshen and Nigg have suggested that 6J could be considered as making a significant contribution to running locomotion. Since we don't know how much "rebound energy" the foot orthoses is adding how can we know whether this is significant? I understand that if some of the energy that is normally stored in the plantar fascia is stored in the orthoses instead then there may be no net increase in energy, however, it is the efficiency of these energy stores, how the foot orthoses modify the efficiency of the biological stores and the energy stored within them, and how the energy which would otherwise be lost from usable form can be stored and ultimately returned efficiently which I am interested in.

I think another problem is in translation because your "traditional" foot orthoses are probably a lot stiffer in this area of the orthoses than mine, Kevin. On reflection, and thinking back to meeting with Paul Rasmussen and looking at his devices, they were very stiff, much stiffer than the devices I generally manufacture. Hence my "traditional" polypropylene foot orthoses probably have a lower medial longitudinal arch stiffness than yours. Indeed, from my experiences of teaching in Portugal, the Portuguese "traditional" orthoses are more compliant than those which I generally employ. So it all depends on the orthoses stiffness and what we understand as being "traditional".

Clearly one cannot just say a "traditional polypropylene device" since, as pointed out, one mans "tradition" may not equate to another's, moreover, the stiffness of the orthoses in this area will be dependent not only upon the type of material it is made from, but also it's thickness and geometrical form; a 4mm thick polypropylene device with a relatively low medial longitudinal arch curvature may have the same stiffness here as a 2mm polypropylene device with a slightly higher curvature. So to talk about "traditional stiff polypropylene orthoses" is rather unhelpful; how stiff is "stiff"?

When we are talking about orthoses stiffness at a certain point of the orthoses, it's important to understand that the orthoses shell has a certain stiffness at this point (kshell), and the top-cover has a certain stiffness too (kcover) together we are effectively placing springs in series, so the net orthoses stiffness (kortho) at this point will be:

kortho = (1/kshell + 1/kcover)>-1

Of note here is that my "traditional" polypropylene orthoses have a 3-4mm EVA top-cover as standard, while I'm guessing that yours have a vinyl top-cover as standard, Kevin.

The shoe adds another spring in series (kshoe), so too might the floor (kfloor). So together we could call the combination of orthoses, shoe and the floor, the surface stiffness (ksurf). As we know, the body modulates the leg stiffness (kleg) in response to the ksurf to maintain the centre of mass (CoM) displacement pathway. Certain Ksurf's will allow decreased muscle activation, higher performance, lower metabolic cost and decreased risk of injury, since they are returning energy to the CoM that would otherwise be lost from usable form and should need to be replaced by the muscles, other Ksurf's may increase the need for muscular energy input to maintain the CoM displacement pathway. But it must be recognised that whenever you put an orthosis in a shoe, regardless of it's construction, it has the potential to alter the ksurf and thus the relative (proportional) energetics of the system.

Dave, I am not trying to get more energy out of an orthoses than has been put in! I am interested in the concept of whether the materials that orthoses are constructed of show more or less hysteresis than biological tissues, i.e. do they offer more efficient storage vessels than biological tissues? Whether some of the kinetic energy which is ordinarily transferred to the ground and lost to forms which cannot aid in locomotion such as heat and sound can be scavenged so that as much of the available energy as possible is stored within the orthoses/ shoe and I'm interested in how orthoses design can aid in returning as much of the energy in a useful form, in the right direction and at the right time- which probably comes back to the need to move away from the "traditional" orthoses design: see figures 4 and 5 and the accompanying text from the paper I linked to in post number 71.

As Kevin notes, it's partially about matching the surface frequency with the CoM frequency through material selection and design intent. Unlike Kevin, I don't think this requires "jets".

 

Simon

That seems consistent with what I was trying to convey, I need to dig out my Winter books and look at how he describes energetics. (David Winter that is, not the books reserved for winter reading )

Regards Dave

 

Agreed.

 

Simon:

I was never questioning your knowledge. In fact, you are one of the few people that I could have such a discussion with. When I wrote that possibly someone could hopefully explain these concepts more clearly, what I meant was that I was probably not being clear enough in my explanation, not that you didn't have the intellectual capacity to understand these concepts. Sorry for the misunderstanding.

However, with my limited time, I think I've said all I can say on this subject for now since I feel I've gone around this stump too many times already. Maybe, the next time we talk in person, we will tackle this subject again and, I'm sure, I will be able to explain myself better.

Hope you and your family are having a nice holiday season.:drinks

 

Simon

Have you considered evaluating energy characteristics of orthoses in terms of power?

Power = work/time and so, as shown earlier work = energy then work/time = energy/time So Power = energy/time

Power = work/time and work = force * displacement and displacement / time = velocity and therefore Energy / time = (force * displacement )/time

Or KE/t - (fd)/t = 0

Dave

 

Have I considered it? Yes. Have I done it? No.

 

Simon
in your paper attached earlier to this thread:

So, are you saying what you want, or what would be ideal for maximum efficiency in ambulation, is an orthosis with a stiffness characteristic that allows the maximum deformation by the force applied by GRF V's BW in the time of 1/2 of one stance phase i.e the braking phase and the able to return approaching 100% in put KE in the time of one propulsive phase?

Regards Dave

 

Question

If for this question energy is added by a device, could there not be an issue if energy is returned at the wrong time?

 

Sure could if you were interested in maximum output for minimal physiological energy input i.e. highly efficient ambulation.

Dave

 

Was thinking in terms of niggs muscle vibrations,
But maybe the body would adjust stiffness to dampened the vibrations

 

I think so, if I understand your statement correctly. I don't think you'll get anywhere near 100% as useable energy though. Ostensibly, you are trying to match the surface frequency to the CoM frequency as Kevin stated previously. There appears to be a "magic" range of Ksurf which decrease metabolic cost and increases performance, so it would seem reasonable to shoot for this range as a starting point. If the surface is too compliant then the metabolic cost starts to go back up. The key would seem to be in storing as much energy in the surface as possible then releasing the stored energy from the surface back to the body at the right time, right place and in the right direction.

Short on time at the moment.

 

It's all so easy when you put it like that eh!?

So, do you think that you can equate the linear compression of material in a single plane (like a foam sheet) with the deformation of a plastic orthosis even if you only consider the as 2D arch shape? 3mm of displacement at the arch apex is approaching zero displacement at the extremities and so change in energy approaches zero too and if the arch is not symmetrical then this distorts energy calculations. How about considering the arch as a truss with a hinged apex and the displacement in radians and the radius as the distance from apex hinge to each end of the truss. Then 1 radian displacement is constant throughout the whole of each side of the system even if there is asymmetry. Then you can find powers or f.v (f.d/t) at each node of interest and from that calculate energy used. I.E. change in K energy = change in f.d = work done with time as a constant of either side.
I think that works anyway but maybe that's not where your going anyway!?

Dave

 

Your point re: the 3D shape of foot orthoses and the point to point variation in energy are well made. This is one of the reasons I began to explore orthoses design using 3D finite element analyses, Dave. To tune the entire surface is going to require more processing power than I can muster, hence i opted to look to tuning the heel cup section with my prototypes.

I'll need to give your ideas some more thought when I have the time. At the moment there are a couple of members of my family who need and deserve my attentions more than this does.