MASS Discussion

So, you use fscan, a pressure mapping system, to define force, and length in pressure plates vs force to derive an angle between force and distance? Am I reading you right?

A lot has to be assumed when converting pressure mapping to altitudes. It's only a guess though.

This doesn't suggest a stable or repeatable process.

Straw man argument? Generally, when referring to angles, degrees are pretty much the standard unit of measurement, although computers often deal in radians, easily converted internally for simplicity to the operator. Specifying sine, cosine, tangent of a single angle, instead of one specific calculation suggests an absence of a mathematical axiom. My one hope is that you're not depending on any pressure mapping system to produce a 3 dimensional device, especially when the most important data, the actual arch, is missing.

 

Great question,
We did a study (never published) where we measured the power output on 135 runners using an ergometer. We measured each on their own bike. they were measured at 100, 80 and 60% of max heart rate. Overall there was an increase of 2.5% in power and in the competitive cyclist many were 11% or more.
Why the increase. Cycling, like sprinting, is a continuous propulsive phase event. You are transferring energy from the achilles tendon (gastrocsoleus) to the pedal. The stiffness of the lever is a major determinant of efficient force transfer. Cycling shoes are quite rigid and slightly plantarflexed which contributes to the transfer. If the foot in supiated within the shoe, the talus externally rotates on the level anterior facet which locks the talus against the calcaneus in the sagittal plane. This creates a more efficient transfer of force. If the foot is pronated; unlocked in the sagittal plane, then the force of the gastroc causes a spongy, inefficient transfer of force to the pedal.

I hope that this is helpful.

Thanks,
Ed Glaser, DPM

 

How much of golf is swinging and how much is just walking?
Ed

 

Thanks Jeff,
These are some excellent examples of where Merton Root and I differ in our approach to foot biomechanics. It certainly helps with the whole "newness" argument. Remember calibration is for force and flexibility. This answers Eric as well. We do expect the orthotic to move when the downward force of the body exceeds the upward force of the orthotic. Newton's law of inertia.

Ed

 

The F scan is for research only. We do not get that data from our docs for every patient because few docs have the F scan. I am sorry, I do not understand the question.

Altitudes? You mean 3D wireframes? To my knowledge pressure mapping data cannot be used to create a 3D model. ....other than maybe a rrelief map of the pressure itself.

We take 3 measurements of each device to assure repeatability.

No absence of axiom....just pointing out that the same mathematical conclusions can be derived using various paths. You can call it checking your work if you like. Again, no models are made from F scan data.

I hope this helps you.
Ed

 

Brings us back to what we mean by fitting the arch.

As I said earlier

 

Yeah tried the elastic band thing. There is an easier way...

I use a Chatillon force gauge to measure vertical stiffness of the orthosis at a number of points. These measurements tell me about the stiffness of the orthosis. The "proper resistive force of the orthosis" is another question. For running we may want to aim to produce a device which has stiffness characteristics in the range of the stiffness values reported in the literature to decrease metabolic cost etc. However, we must also take into account the stiffness characteristics of the shoe and the supporting surface, so we need to physicaly test the devices in situ within the target footwear. The other main issue is at what part of the orthotic shell is the stiffness needed to be within this target range? During rearfoot strike running this is probably the heel cup since the leg stiffness will be moderated by the CNS at the time of strike. In order to significantly alter the orthosis heel cup vertical stiffness we need a relatively high amount of vertical displacement in the heel of the device, not necessarily the medial longitudinal arch section, this is why McMahon used cone springs under the heel in his patent.

This takes us back to the first point re: various sports and the differences in ground reaction forces they produce, if the CNS is moderating leg stiffness in response to it's impact loading forces at heel strike and immediately after, and the spring mass model holds true, there should be very little time for changing that vertical leg stiffness (kleg) throughout the compressive phase, so I'm not sure how much impact the stiffness of the medial longitudinal arch section of the device should have in moderating Kleg, in running at least.

Not many, I don't have the time or the staff to truly explore this to the level I should like. So it is still at the experimental stage. The problem is, as I stated above, the orthosis and the shoe are additive in the spring mass model, so we should really need to model both the shoe and the orthosis. The FEA gets more complicated when you add in more laminates and particularly when we have non-linear behaviour. I don't use the same software as you are attempting to, so I can't comment on the relative merits of it. As I've also stated we know the body will moderate the kleg in response to different surface stiffness, we also can hypothesise that there will be a zone of optimum leg stiffness (ZOOLS) for each individual during each task. The Problem as I see it is we are trying to alter the surface stiffness to allow the leg to function within it's ZOOLS, but we don't know what that ZOOLS is. Pissing in the dark as one of my colleagues said.

Forces don't flow like waves, Ed. I can't go into too much detail here until our paper is published, but the problems with in shoe-pressure measurement in this situation are probably much bigger than you think. Suffice to say, we make a device, measure the loading on top of that device, plug the numbers into the FEA model, alter the shape/ stiffness of the device in the FEA model- the numbers are no longer valid, so we have to make another device....

As someone else said, I would be very cautious about your interpretation of success based on customer returns.

I'll come back to the rest of your post latter., but I did want to explore this point. If contour and orthosis stiffness are key, then we can make two devices one from a weightbearing cast; one from a MASS position cast and we can design the two devices to have the same stiffness characteristics. Both will be congruent to the foot, both will have the same "calibration", so why should one be better than the other in terms of relieving pathological forces? As Robert is pointing out congruence tells us little unless we know the position of the foot at congruence.

 

Surely it depends upon when the forces are reaching pathological levels? If it's during the swinging you got to target that phase; if it's during the walking you got to target that phase.

 

Ed 2 of the major/common problems with cyclists are related to frontal plane position of the knee and loss of sensation in the foot which can be caused by compression of the dorum of the foot by the cycling shoe. Thats why forefoot posting or wedges screwed under the shoeand cleat have been used for 50 years and will be used for a few more. By filling up the arch there will be more compression on the dorsum of the foot.

But don´t worry about an answer to be honest I´m more intersted in your discussion with Simon. Leg stiffnes is a huge interst of mine.

 

Ed, there is a point of minimum stiffness (MS) required for all orthoses. If a device is too compliant (i.e. stiffness falls below the minimum stiffness threshold), then the device will not resist pathological forces sufficiently and the patient’s symptoms will not respond. Any device that functions above the required level of MS will resist pathological forces.

At some point an orthosis can become too stiff for some people. Let’s not forget that there was a time when spring steel orthoses were not uncommon and many people benefited from them. These were very stiff orthoses. If an orthosis becomes too stiff, it can restrict motion which is necessary for comfort or tolerance purposes. An excessively stiff orthosis can also reduce motion that is necessary for functional purposes, and the individual can develop other symptoms as a result. Therefore, there is a range of acceptable orthotic stiffness that exists between too compliant and too stiff. This range is somewhat individual.

Your casting technique accentuates the height of the medial longitudinal arch because you advocate supinating the midtarsal joint or plantarflexing the forefoot during casting. Your casting technique does not replicate the normal anatomical height of the medial longitudinal arch that occurs when both the forefoot and rearfoot are in contact with the supporting surface. The only time that the foot normally achieves your supinated arch height during gait is after heel lift occurs as the foot continues to supinate at the STJ during propulsion.

Your orthoses (technically they’re custom arch supports) have a smaller range of acceptable stiffness than functional orthoses because they do not represent the anatomical shape the medial arch that should occur during the first two thirds of the stance phase of gait. The only time that the foot should achieve this arch height is after heel lift. It is the absence of vertical ground reaction force acting on the plantar surface of the calcaneus that enables the midtarsal joint to achieve this higher arch attitude.

If you were to manufacture your orthoses out of stiffer materials, such as polypropylene, acrylic plastics, or carbon fiber composite materials, they would not be well tolerated. Why? Because the arch must yield in order to achieve its normal, lower weightbearing functional position. Supporting the foot with the arch in a position that represents the attitude of the arch during the propulsive phase of gait is not normal and it not well tolerated. As a result, your orthoses must be more compliant and yeild more under load.

You would not need to “calibrate” your devices if they had a broader range of sufficient stiffness. I see this as a disadvantage, not an advantage to the practitioner. My exposure to your product substantiates this. I have seen mixed results with your devices, with tolerance issues being just one of them. You don’t use an extrinsic post because it would stiffen the device and hence, it would not be tolerated. I have seen some of your devices with Korex and EVA added in order to stiffen them. I have seen some that had to have the medial arch height decreased by spot heating. If calibration is so fantastic, why is this necessary?

The functional orthosis has been in use for over fifty years. Hopefully it won’t take fifty years to resolve this MASS confusion.
Respectfully,
Jeff

 

Ed, I assume you meant cyclists, not runners. What was the standard deviation of the power measurement? How many cyclists decreased their power output?

Now I remember the problem I had with the MASS theory. "the talus externally rotates on the level anterior facet which locks the talus against the calcaneus in the sagittal plane" There are several assumptions that I don't feel are valid. There is an assumption that the foot is more rigid with the talus abducted over the calcaneus. The foot can be quite rigid when its STJ is pronated. It can be rigid enough in a situation like cycling where the forces involved are less than body weight.

Although I can see the rigidity argument as plausible if you could get the STJ into that position. I question whether there is that much change in position on any orthosis or that the MASS position is supinated enough to put the talus all the way on top of the calcaneus. The simple study that I suggested that you do a couple years ago was to take a bunch of people and look at their heel bisections standing on and off MASS devices. On average, how many degrees inverted does the calcaneus become when standing on a MASS device?

This is not just a criticism or MASS theory. It is also something that we SALRE folks should be thinking about when we think that we are "moving the axis" when we add supination moment with our orthotics. How many degrees of motion are we getting? However, I am quite happy just changing the moments and reducing symptoms. On the other hand you can see quite a bit of change with forefoot valgus posting on the "oversupinated" foot that has range of motion in the direction of pronation available.

Cheers,
Eric

 

Mike,
I would like to answer you because cycling is near and dear to my heart.
1973 I spent 7 months cycling from Northern California, down the coast to LA across the Mojave desert and then up into CO and down almost to Mexico, then angled up through NM, TX, OK, KS, and MO then down the great river road along the Mississippi on the IN side and through KY, TN, GA and ended up in Orlando and Daytona FL and up the coast to NY. If only I could write like Chip Southerland, I would have stories to tell and a novel published. I think I covered 7K miles.
1979 I rode from Miami to Savanah
1981 NY to Canada.
2005 My son and I tricycled around the perimeter of Ireland. I used an Australian trike called the Greenspeed GS50, Noah on a Cattrike made in Florida. Both pulling BOB trailers with 70lbs camping gear. In the two seasons before that I put over 6500 miles on my trike and Velotechnic Grasshopper recumbent racer.

To my knowledge I am the only one with Shimano bicycling sandals with custom orthotics built in. We had to rout out the depression for the orthotic to fit right.

Personally I have always gotten my numbness on the ball of the foot. It happens less frequently in those sandals. I think that has more to do with the compressibility of the sole....and maybe there is something we can do about that.
You would think that the higher arch would cause dorsal compression problems, but it does not. We are not pushing it up to an extreme degree. Only up to where people who have normal arches live. Rarely there is a tongue problem with the shoe but a little cushion foam padding is all that is needed and maybe a cutout to accommodate the saddle bone (met. cuneiform exos).

Most of the benefit to the Knee comes when the foot supinates and the talus externally rotates on the near level anterior facet. This external rotation changes the tracking direction of the patella. Patella tracking disorder common to cyclists.

Alyson got to cast the Cannondale team, both off road and track. They gave her a Cannondale Razor (Mountain Bike with single prong front fork), they were so pleased.

BTW, If you are really interested in cycling you should check out the Rotor cranks that are made in Spain. You never have a dead spot during max. extension. I have them on my Greenspeed trike and it gives you the feeling that the bike wants to move foreward.

Thanks,
Ed

 

Produce it and market it. See if it flys.

Early on we used something like the Chatillon. We made it ourselves out of an arbor press. We wanted consistent vertical force so that we could compare apples and apples. We had considerable trouble getting repeatable measurements. We experimented with many different interfaces but the surface area in contact with the device varied as the curvatures of the orthoses varied. This seemed to affect measurements. That is why we went to a full contact measurement device. You can see from the videos on our Sole Supports TV channel on youtube basically how the machine is constructed. We are working on a much more advanced machine with full electronic sensing to measure medial curvature with more parameters. This will all be for naught if FEA works.

I explained how we determine the resistive force. It seems to work quite well. That is one ambitious research project on measuring shoe stiffness. I don’t think it will get done because of the enormous expense and the constant development in the shoe industry which will make the study obsolete too fast….not to mention manufacturing inconsistencies.


The CNS will do its job and adjust the leg stiffness accordingly. I really don’t know how or why you would stiffen the heel. Stiffen it from what deforming force? The heel is a rocker, a round object at heel strike that rolls forward as the Calcaneal inclination angle decreases…..carrying the STJ axis along for the ride. I can’t imagine what you would stiffen; the walls of the heel cup or the plantar surface? Ours is too thin for that. To allow for better shoe fit we have opted for a 1.15mm callipered shell thickness in the thinnest part of the heel.

I disagree. During heel strike I like the orthotic to apply more resistive force under the sustentaculum tali. This will invert the calcaneus while increasing its inclination angle. If we can influence the posture of the foot inside the shoe before heel strike, maybe we can prepare the foot for heel strike and delay the onset and decrease the extent of pronation. We are in a very big research project right now and should know the answers soon.

CNS moderated leg stiffness is a very good argument we should give all our patients to break their devices in slowly. This allow muscle learning or training to occur.

You are so right that there is very little time. In the high speed videos we’ve done for 3mph walking (approximately), there is only 0.05-0.08 sec. from heel strike to flatfoot. That is another good argument for full contact in the MASS posture. No lost time for the foot to pronate into the orthotic…..which dramatically reduces impact forces. Not enough time is a criticism I have of posting in general.

I encourage you to continue. Truth will come with numbers….a larger “n”. Now if we could control the shoe, that would be a whole other world. I don’t think ZOOLS is a constant at all. We are just experimenting with various force curve slopes on several individuals whom you know their weight, forefoot flexibility(subjectively) and level of activity are known, and see what works best. Then plot your calibration units on a scatter graph and you have a great first approximation that will give you a range that works in most athletes. We work with several division 1 and NFL teams through their DPM, PT’s and ATc’s and these guys are picky with their athletes. They are demanding of performance and injury reduction. Orthotics don’t determine sport outcomes otherwise I would move to Las Vegas but they should reduce injuries. To WOW these guys injury reduction must be significant. Orthotics are not a magic carpet though and some injuries will occur regardless. Now if you wanted to get real fancy and offer the most elite and rich athletes orthotics that were actually calibrated to individual force measurements on their feet during training and competition….that would work. Most people can’t afford that.

Sounds like a cumbersome approach right now. You would have to get the cycle time to less than 1 minute to make it a practical tool for a lab in orthotic manufacturing. Sometimes we can do cool stuff in the lab once or many times but the trick is to make it usable for every doc and every patient and still remain cost competitive in the marketplace. A solution is not a solution until it is practical.

Our philosophy is to encourage returns (50% no questions asked 6 mos return polity). Valuable data. We also encourage complaints. We consider it an opportunity to problem solve with our doc, and show him the kind of WOW service that would cause him to refer his colleagues

Just since 2002 our “n” is over 400,000.

All true except you are ignoring the Postural effect of putting the foot in more Supination is that it allows the anterior facet of the talus to approach level which in turn allow more transverse plane rotation of the head of the talus which inhibits marital plane rotation between the talus and calcaneus making propulsion more efficient.

Simon, great discussion I am enjoying the exchange of ideas...it is constructive.

Respectfully,
Ed

 

That is a question for Stu, who was just handed the raw data or the researchers in CO. Cyclists…yes. There were very few who decreased. I also can’t remember how long they wore the orthotics before testing. This data is in our things to look at later file. We have no time right now to crunch the numbers on this one.

It’s just closed pack and loose pack positions. In Supination the bones nest more tightly, ligaments are pulled to the ends of their ROM tightening joints of the midfoot. It is a very observable phenomena. Just twist anyone’s foot into Supination and feel its rigidity. I thought Root did a great job describing this. There’s some great graphics of this in Root’s book.

Would there be an advantage to doing it in static stance over walking?
We are doing that with walking and measuring kinematic changes. I do not get to see all the raw data until the paper is published. We won’t know for sure until the data is in. I guess this could be done with radiographs.

I believe Kinematic changes are changes in function and kinetic changes are a simple redistribution of plantar forces to move tissue stresses around the bottom of the foot, off of inflamed areas. I don’t buy the see-saw theory. There are too many joints out of alignment by the end of pronation which are often too floppy to only consider the one axis. Funny, Robert Isaacs acts like ….of course we knew all along that posture controls function and you think posture and function are independent. Look at the postural changes that occur after subtalar arthrorhesis procedures. I see a major change in function. Why don’t you? Help me understand why the positive gait changes following this procedure which certainly rotates the talus externally on the anterior facet to open the sinus tarsi, are less desirable than allowing the foot to over pronate while tilting the heel. Didn’t Kogler teach us something about rearfoot posts.

This is all theoretical modeling. We are each trying to understand and describe the same thing in different ways. What you call medially deviated axis I call a flat foot, RCSP, end ROM, full pronation, poor posture.

Great post as usual Eric.
Thanks,
Ed

 

Posture and function could never be independant. If they were one could walk on scrunched up toes or on tip toes and not change function.

Insoles have, since the dawn of time, tried to put the foot in a different position. Ironically we now tend to think more about kinetics but if you go back to some of the more retro or commercial brands they all talk about correcting over pronation to cure pathology. What is this if not claiming to improve function by changing position?

Any time you want to answer my point about fit I'm waiting patiently by the way. What do you think of the MASS + position? I'm quite pumped about it. I think it's better than MASS.

Call me pedestrian but as fascinating as the calibration is I don't think it helps unless we agree what position / range is best to aim for with our calibrated orthotic.

Perhaps that wants it's own thread...

 

All depends on the type of tip you use on your force gauge, it is easy to overcome the problem of variable contact area with the right tip. You will still need to test the validity of your FEA models against physical tests.

But you didn't say what that stiffness should be, in your opinion.

Indeed, but you cannot ignore the additive effect of the shoe sole in your "calibration". If stiffness is significant, and you maintain that it is, how on earth can stiffness be "calibrated" when you do not know the stiffness of the orthosis interface with the shoe and the shoe with the ground?

You miss the point. Leg stiffness (kleg) is significant in running related injuries, Kleg is in a large part determined by the surface stiffness (Ksurf). Kleg is moderated by the CNS at the time of heel strike. Viz. in order to influence Kleg, and therefore influence running related injuries, one approach may be to alter the Ksurf beneath the heel because it is at this time that Kleg is determined, by the time the arch begins to compress against an orthosis it may be too late (it may not be). Hence we may need to influence the Ksurf at the point of heel strike since it may be the heel strike transient that is the cause of pathology.

You are correct in that because your orthoses have a very small thickness their ability to moderate Kleg at strike will be minimal in terms of their ability to reduce stiffness; you could however increase stiffness significantly with 1.5mm of titanium. A device which is thicker beneath the heel may be able to deform more under the same load, so we could reduce the stiffness this way. The point being that whatever the thickness and stiffness of the material it's influence on Ksurf will not be zero; whatever the stiffness of that 1.15mm is, it will be added in to the net Ksurf. Published research suggests that we need a Ksurf within a specific range to increase performance and reduce injury rate, therefore by measuring the stiffness of the shoe and manipulating the stiffness of the orthosis beneath the heel, theoretically it is possible to create a Ksurf within the correct stiffness range.


What the orthosis may be able to moderate is foot stiffness (kfoot) and we may not be able to ignore the net effect of Kfoot+ Kleg. So when we have some values for kfoot which links it to specific injury, we may be able to moderate the stiffness of the orthosis to influence this in a positive way through manipulating the orthosis stiffness in the midfoot into the the correct range. You say you have found that correct range- so what is the optimal stiffness for a foot orthosis?

You really need a bone pin study to validate the contention that your devices invert the calcaneus and increase its inclination angle during gait. Then you need to show that the time of "onset" and "extent" of pronation are predictors of pathology- are they?

Maybe. The body will respond immediately to a change in surface stiffness by moderating Kleg, this will occur via change in kinematics at predominately the hip and knee and to a lesser extent at the ankle, the muscles recruitment to achieve this may require more or less muscle activity- depends on where within it's ZOOLS the orthosis causes the limb to function.

Ed, you misunderstand. An extrinsic rearfoot post on the orthoses changes a number of variables, not least the stiffness of the device; not just at the rearfoot, but along the medial and lateral arch section of the shell too. So it's really just another way of calibrating the stiffness of the orthosis. As I said yesterday, full contact can be achieved without MASS position, manipulation of orthosis stiffness can be achieved without mass position, so what you need to show is that a mass position device will put the foot into a different position to another casted device during dynamic function, and that this position is more efficacious than any other position. This is Robert's point, I believe.

I didn't say it was.

I think I'm missing your point: force curve slopes of what? What exactly are you plotting against what? Please define the variables in the model Y = ?, X1 = ?... Xn =? Then give the equation for the line: Y = m x1 +... xn +c, where: Y is the dependent variable; X1... Xn are the independents; m is the slope and C is the intercept constant.

That's the point I was making, it's impracticable.

Its a kind of data Ed and I'm sure you could do something with it, but I would not directly measure success or failure from it and assume that it is valid.

And it is that which you need to demonstrate.

What you may show is that MASS position devices increase Kfoot, This is fine if someone has a foot which is too compliant and functioning below its zone of optimal foot stiffness (ZOOFS), but it will be problematic if you increase Kfoot and push the foot to function above the upper limit of its ZOOFS.

 

BTW, in terms of Kfoot we need to determine not only the vertical stiffness, i.e. through dynamic navicular "drop", but also the horizontal stiffness, i.e. through dynamic navicular drift. Look at the measures in "normals" and in various pathologies; that way we may find our zones of optimal foot stiffness (ZOOFS) and then "calibrate" devices to attempt to place the pathological subjects , functioning with their feet outside of the the ZOOFS, back within their ZOOFS.

And this is where it gets interesting. If we have a foot which has too much compliance, i.e. functioning below it's ZOOFS, lets say, we measure this as too much dynamic navicular drop / unit load, then we need a device which will effectively reduce the navicular drop per unit load (but not too much). So it would seem to me a fairly simple task of calibrating the orthosis within the shoe to allow the navicular to drop, but to maintain it within its optimal range of load/ deformation, i.e. stiffness. However, in a foot functioning above it's ZOOFS, i.e. too stiff = too little dynamic navicular drop / unit load, we should need a device which should increase the dynamic navicular drop/ unit load, i.e. decrease Kfoot. In a high arched (over supinating foot) we might achieve this by increasing the plantarflexory moments acting on the navicular, i.e. through a "pronatory foot orthosis"- valgus rearfoot posting, stiff lateral longitudinal arch etc. I do not believe a MASS position cast / device would be suitable here. But also remember we could have a foot which is too stiff because it is functioning maximally pronated. N.B. what we need is an optimal amount of deformation /unit load, so if we have a foot which is too stiff because the subtalar joint is maximally pronated and the the navicular is on the "floor", we are not going to induce more navicular drop/ unit load by increasing the plantarflexion moment acting on the navicular via a "pronatory" foot orthoses, i.e. lateral rearfoot skive. Rather, we need to first lift the navicular off the floor and then allow it to displace vertically downward by the right amount during dynamic function. Viz. we need to provide it with an orthosis + shoe of optimal stiffness to allow dynamic stiffness to be within normal limits. In other words, the device has got to be stiff enough to overcome the supination resistance, but compliant enough to allow the right amount of downward displacement of the navicular/ unit load during gait. A MASS position cast may be suitable here to achieve this aim, but so might other casting positions too.

SO, we must first decide whether the foot is too stiff or too compliant and decide whether this is due to too much pronation at the rearfoot or too much supination at the rearfoot, i.e. where within its Zone Of Optimal Stress (ZOOS) the subtalar joint is functioning. We must also consider the forefoot dorsiflexory stiffness, too much dorsiflexion at the forefoot or too little dorsiflexion at the forefoot. Then we need to manufacture a suitable orthoses which allows the foot to function within its zone of optimal foot stiffness (ZOOFS). There will be "more than one way to skin the cat". In other words there will be a range of casting techniques and orthotic prescription variables which will result in suitable foot orthoses to achieve the desired aim of maintaining the foot within its ZOOFS.

I believe that using stiffness the theories of foot biomechanics can be "unified". Like I said in a previous thread, "stiffness" appears to be the new black this season. I'll be presenting a lot of this stuff at Biomechanics Summer School in July, here in the UK.

 

Especially the last bit that mass is more efficatious yes.

We can argue the toss about the insole being precisely what we want it to be, and that's great, but it's not terribly relevant to the relative virtue of MASS if there is a fundamental dispute over the position itself!

It would be like me turning up on a sniper training course boasting that my rifle can hit seagulls (and only seagulls) with pinpoint accuracy every time. No one would bother asking to check my sights calibration (though they may be interested in whether I really have set them as I claim) they'd ask why the hell I was shooting seagulls.

 

A question: how does leg stiffness modulation influence foot stiffness?

So lets say we decrease leg stiffness (Kleg) in response to a stiffer surface (ksurf) by increasing knee flexion, what influence will this have on foot stiffness (kfoot)?

It's all about the Achilles tendon tension and the relationship between this and plantar fascia tension- right, Mike?

 

I think so yes Simon.

Those papers in the ZOOLS thread that we discussed last week indicates that. With knee flexion-extension moments controlling the tension of the plantar fascia level to a point. There is an inverted U type result to this I believe. Your Navicular on the floor example or STJ pronation moments leading to too much tension in the Plantar fascia puts a "spanner in the works" with the bodies ability to regulate leg stiffness if reduced leg stiffness is what is required and it´s effect on foot stiffness.

But I´m still mulling this inverted U idea over at the moment.

 

Some further thoughts on this. If we need to change the vertical height of the navicular, we must first overcome the supination resistance. If we change the position of the navicular by pushing directly underneath the navicular to raise it off the ground, then the stiffness of the orthosis beneath the navicular may be too high to then allow downward displacement of the navicular during function such that the foots stiffness is optimised. From the time of forefoot loading through to heel lift the navicular height should probably decrease and then start to increase- sinusoidal(ish) pathway- right, Dave? So we may well need to elevate the navicular by some other means than pushing up directly beneath the navicular. How is this achievable? Any thoughts, Kevin? ;-)

 

Sorry not Kevin heres my thoughts.
By giving the muscles which attach directly or near the Navicular a greater mechancial advantage through an increase in distance in their lever arms in relation to the MTJ (?) and STJ axis and get the Navicular to rise thru the windlass mechanism.

If we can laterally deviate the STJ axis and/or (if the STJ axis is medial to the 1st MTP joint ) reduce the 1st MTP joint dorsiflexion stiffness to allow for greater plantarflexion moment of the head of the 1st metatarsal, which all things being equal should mean the Navicular begins to rise.

 

No apology necessary, Mike. I was thinking more about specific orthoses modifications. Specifically by pushing up under the heel to increase the supinated position of the STJ joint via something like a medial heel skive- hence my rhetoric to Kevin. If we decrease the 1st met dorsiflexion stiffness the first met head should displace upward by more per until load under GRF- will this raise or lower the navicular?

 

I was thinking this would happen

 

You are thinking about dorsiflexion stiffness of the hallux, not the first met, which is what you said. And, I was talking about navicular displacement from forefoot loading up until the point of heel lift.

Draw you diagram again, with the first met head relatively more dorsiflexed (this would be the result of decreased dorsiflexion stiffness of the 1st met), does this higher or lower the arch; increase or decrease the dorsiflexion stiffness at the hallux?

 

Sorry missed this bit

, and Hullux yes..... Homer Simpson moments * 2.

So I´ll stay with the lateral deviation of the STJ axis to affect the length of the lever arm of the muscles which attach near or directly to the Navicular.

 

Anything that creates closed chain supination of the STJ should help to dorsiflex the navicular. It is interesting when we look at certain pathology, we can get a better appreciation of how orthoses should function.

Take for example, adult acquired flatfoot. This is arguable the most pronated foot we regularly treat. The most common orthotic complaint and need for shell or prescription modification occurs due to excessive pressure under the navicular and to a lesser extent, the medial cuneiform. So we slightly accommodate and offload either or both bones, yet we get a vast improvement in foot position and symptom responses. Why? Because the support occurs across the entire surface of the device, including the heel cup and the lateral column of the foot in addition to the medial arch. The relative change in GRF between the unsupported foot as compared to the orthotic supported foot is remarkable. This is what Dr. Root meant when he said that a functional orthosis is not an arch support, a point that appears to have escaped the MASS confusion proponents.

The question again becomes, how much pronation does one need to resist to get a positive outcome? Pronation is a necessary and normal motion. Without pronation motion, there can be no re-supination of the foot!

Respectfully,
Jeff

 

It should result in decreased arch height, therefore lower Navicular and an increase in the dorsiflexion stiffness of the Hallux.

 

Thanks, Jeff. Your final paragraph above sums up my point regarding zones of optimal foot stiffness (ZOOFS), we need our orthoses to allow the foot to function such that it has the correct spring stiffness, too much stiffness = not enough pronation, too little stiffness = too much pronation, both will probably result in pathology. Thus, our orthoses need to be neither too stiff, nor too compliant. As I said at PFOLA in Montreal, your father was probably right, but maybe with the wrong explanation. I'll update that to say he probably gave the best explanation he could within the knowledge-base at the time. Someone once described the subtalar joint as the "great benefactor", I'll describe it as the "great modulator". The STJ, along with ankle joint and plantar intrinsics, modulates the foot's spring stiffness. When the STJ is functioning near it's midrange, i.e. within it's zone of optimal stress (ZOOS); viz. when the axis is neither too medially deviated, nor too laterally deviated, then it is able to modulate the foots spring stiffness to maintain the foot within its zone of optimal stiffness (ZOOFS), but when it is functioning towards its end of range (in either direction) it will fail to do this. In other words its "compensation" will be insufficient. I am not the resurrection, nor am I the light, but I do think that through "stiffness" we will become one. :drinks

 

Yep. So we don't want to decrease the dorsiflexion stiffness of the metatarsal in this situation-right? :drinks

 

Simon, Mike and Jeff:

Just arrived in Rome this morning and we have an apartment overlooking the city close to the Spanish Steps. The conference I'm lecturing at isn't starting for 11 days so it's vacation time now.

By the way, I really appreciate Jeff finding that article that his father wrote when I was a junior podiatry student at CCPM. Jeff, we should have at least some excerpts of that article reprinted in Podiatry Management or Podiatry Today magazine since your father's words ring so true even today. These thoughts need to be shared with the rest of the podiatric profession. If they won't publish it, then I will include it my short "Forum" article in Podiatry Today in a few months. I don't remember ever reading that article and I have gladly added it to my collection of Mert's papers. Thanks for sharing, Jeff.

As far as medial longitudinal arch (MLA) mechanics is concerned, the concept of MLA stiffness is a fascinating subject. Certainly, we can assume that MLA orthosis reaction force (ORF) will tend to be increased with orthoses with increased MLA height or in orthoses with increased MLA stiffness. Over the past 20 years, in the many patients I have treated with pronation-related pathologies with custom foot orthoses, I have combined the external subtalar joint (STJ) moment effects of an increased MLA height along with varying thicknesses of medial heel skives in order to help increase the external STJ supination moments acting on their foot.

As many of you who have read my newsletters and attended my lectures over the past two decades know, one of the points I make is that the medial heel skive seems to work better when the MLA height is increased. I realized early on that when the medial heel skive is used alone without increasing the MLA height of the orthoses, that there was more chance of medial heel irritation and less observable pronation control. However, if the patient was given an orthosis with increased MLA height without the varus wedging effects in the heel cup of the orthosis from a medial heel skive, then there is far more chance of MLA irritation developing in the patient. Therefore, both the increased MLA height and the medial heel skive should be used together to increase the medial shifting of the ORF in both the rearfoot and midfoot area of the plantar foot to achieve optimum increases in magnitude of external STJ supination moment from the orthosis. Either an increase in MLA height or stiffness by itself, or a medial heel skive by itself, simply doesn't work as well in the orthoses for patients with pronation-related mechanical pathologies of the foot and lower extremity in my clinical experience.

As you know, Simon, I do like the idea of using "stiffness" to describe the load-deformation characteristics of the foot and lower extremity. However, when stiffness terminology is being used in these discussions, the body segment that we are interested in should also be carefully described since this will further aid our communication and understanding. As an example, if one makes a foot orthosis that has a stiffer MLA (i.e. less MLA deformation per given MLA loading force from the foot) and puts it underneath a foot with functional hallux limitus, then the dorsiflexion stiffness of the 1st metatarsophalangeal should decrease (i.e. more hallux dorsiflexion with less increase in plantar hallux load) during static and dynamic stance.

One last thing, orthosis stiffness is only one of the many mechanical characteristics of the orthosis that may affect foot function. We must remember that the unloaded shape of the orthosis is very important since the unloaded shape of the orthosis will form the basis for the load-deformation curve and therefore will significantly influence the MLA stiffness of the orthosis device. For example, we may have an orthosis with a high MLA shape that has low MLA stiffness (e.g. thin shell material with high MLA) that better corrects STJ pronation than an orthosis with low MLA height that has very high MLA stiffness (e.g. thick, rigid shell material). By taking care to precisely define these shape/stiffness characteristics of the orthosis in our discussions and our written and verbal communications with our colleagues, then our common goal of an improved understanding of orthosis function will better be achieved.

Now, time for some gelato. Ciao.

 

That is what I said . Mike suggested that to raise the navicular we might decrease the 1st metatarsal dorsiflexion stiffness, I pointed out to him that this should decrease navicular height and increase plantar fascial tension resulting in an increase in hallux dorsiflexion stiffness. What you are suggesting above is that by increasing the medial longitudinal arch stiffness, hallux dorsiflexion stiffness should be reduced, which is in agreement with the point I made to Michael, but looking at it from the other direction.

 

Arch stiffness measurement here:
http://www.asbweb.org/conferences/2008/abstracts/494.pdf

 

Simon:

I read through your posts to Mike and Jeff, but that message didn't come through clearly. When you said:

I assume you mean increasing the medial longitudinal arch stiffness of the orthosis, not increasing the medial longitudinal arch stiffness of the foot?

 

I was following your lead:

I'm feeling :dizzy:

So, I was talking about the orthosis. In terms of the medial longitudinal arch stiffness of the foot, if the orthosis raises the arch and decreases tension in the plantar fascia/ plantar intrinsics, theoretically the foots medial longitudinal arch stiffness should be reduced. However, if we measure the foots stiffness as dynamic vertical displacement of the navicular / load with the orthosis in situ, would the device effectively stiffen the foot at medial longitudinal arch? This is what I was talking about before when I said:

"If we need to change the vertical height of the navicular, we must first overcome the supination resistance. If we change the position of the navicular by pushing directly underneath the navicular to raise it off the ground, then the stiffness of the orthosis beneath the navicular may be too high to then allow downward displacement of the navicular during function such that the foots stiffness is optimised"

I think it depends on the change in position from rest that the orthosis initially produces and on the stiffness of the device and movement of the foot on top of it during function. If we have a joint that is functioning near end of range it will be relatively stiff in the direction towards that end of range and more compliant away from that end of range of motion, i.e. a STJ which is near maximally pronated will have a high degree of pronation stiffness, but will have lower supination stiffness (assuming it's not fused). So if we re-supinate the foot with our orthosis, we reduce some of that pronation stiffness which can then allow greater deformation per unit load in the direction of pronation during dynamic function.

 

So, we come back to whether the foot "drives" the leg or whether the leg "drives" the foot? If I recall correctly Craig's contention in the biomechanical pearls of wisdom thread was that the leg "drives" the foot. If you recall our little experiment of stiff knee walking versus flexed knee walking, this seemed to me to add weight to this contention. If this hypothesis is correct, we should be able to control the foot stiffness through controlling leg stiffness: target the leg because this is driving the foot- right?.

So, if leg stiffness is determined at the time of heel strike , we should be able to modulate foot stiffness, i.e. arch lowering, by controlling leg stiffness at the time of heel strike through variation of the surface stiffness beneath the point of strike, i.e. the posterior lateral heel. This being the case do we need a traditional shaped foot orthosis, or just a "strike plate" of the correct stiffness material beneath the heel? In other words, do we need something pushing up into the arch at all?

Indeed, I should be able to treat plantar fasciitis by manipulating the range of knee extension during gait- right? Perhaps a study with a "Donjoy" type knee brace and plantar fasciitis outcomes is in order? Or a study with a soft layer of foam beneath the strike area (which should result in an increase in leg stiffness at strike via less knee flexion at strike; less knee flexion at strike = less pronation- right? = less stress on the plantar fascia.) Maybe a layer of plastazoate like "sham orthoses" are sometimes made out of would sufficiently lower the surface stiffness to alter knee kinematics? Who knows?

Here's the issue, how is leg stiffness measured....?

 

Depends on the researcher and the paper. This is what I was talking about in one of the earlier threads. The term "leg stiffness" means different things to different researchers. This makes it all very confusing for those of us trying to make sense out of all this research.

However, I do love the concept of load versus deformation mechanics (i.e. stiffness) of the joint segments of the foot and lower extremity. In addition, I am happy to see that you, Dr. Spooner, are moving forward in your understanding of this complex subject so that you can, hopefully some day, educate us as to how all this knowledge of leg stiffness in walking and running will allow us to better treat our patients.:drinks

 

and thats the big question which leads to many more. Once leg stiffness can be mesured what are the Zones of optimial leg stiffness for Pat A for said activity opposed to Pat B for same activity.

The other major question revolves around leg stiffness, spring mass model in walking.

The strike plate idea is intersting, but maybe as we discussed we can effect leg stiffness through adjustment in stiffness at the foot- surface interface and through mechanical changes through orthotics etc.

And an idea I´ve been mulling over is the use of knee braces with diffferent fixed angles to control the bodies ability in leg stiffness regulation. A bit mad science but the mental wheels are in motion.

 

Kevin, the point I'm trying to make is that leg stiffness is determined by looking at the load versus change in CoM position( deformation), so in running analysis it's probably from strike to midstance- right? SO, is the leg stiffness really determined at the time of strike or is it just the foot's stiffness that is determined at the time of strike, i.e. the GRF vector and the moments it produce about the STJ axis will determine everything else that proceeds?