Footwear science posts

THE EFFECT OF LIFT PLATES

By their own (FIS) admission, boots are too complex, and plates are, too. – Black Diamond: The Deaf Ears of the FIS | Ski Racing 11/18/11.

Assuming the preceding statement is accurate, it raises more questions than answers. What does the FIS mean by too complicated? Too complicated in what respect? Is the FIS saying that boots and plates are too complicated to understand their effects?  The esoteric aspects of both issues are indeed complicated. I have already addressed the complex issues pertaining to the design and fitting of ski boots in US Patent No. 5,265,350.

So I will attempt to address some aspects associated with the introduction of lift plates between the  sole of the user’s foot and the base of the ski. The last time I checked (2013-14), FIS regulations limit the maximum stack heights from the base of the ski to the highest point on the binding interface to 50 mm (later reduced to 43 mm)  and from the sole of a ski boot to the sole of the foot to 50 mm for an aggregate maximum stack height of 100 mm (later reduced to 93 mm). I assume the FIS does not count the thickness of any socks worn by a racer. But who knows for sure?

NOTE: Check FIS regulations for current stack heights.

 

A common explanation for the noticeable effects of lift plates is that they increase the pressure that can be applied to a ski. In order to understand the effect of lift plates or any means that elevates the foot above the base of a ski one needs to understand how force acting on the CoM of a skier is transferred from the pelvis to the base of a ski.

The initial force path is by what is called the mechanical line. Since force travels in a straight line the mechanical line runs from the proximate centre of the trochanter (the ball joint of the femur with the pelvis) to the distal (lower end) tibia. This is the simple force path. Transferring force from the mechanical line to the soles of the feet gets a lot more complicated depending on the configuration of the triplanar joint system of the ankle complex and the intrinsic tension in the arches of the foot.

The sketch below shows the mechanical lines in the lower limbs. The mechanical line in each leg actually extends down as far as the talus, the bone that forms what is commonly called the ankle joint. In quiet erect standing, the force of gravity G pulls CoM down towards the center on the earth. The ball joints of the pelvis apply force to the mechanical line of each leg which extends to the distal tibia. Depending on the physiological state of the foot, force will be applied to the ground or supporting surface with a Centre of Pressure somewhere under the sole of the foot. In this graphic, the feet are in a neutral position and lie directly under the centres of the ball joints of the pelvis. Centre of Pressure will reside on a line running through the proximate centre of the heel and the centre of the head of the 2nd metatarsal (aka – ball of the 2nd toe).

ML1In the graphic below, lift plates have been inserted under each foot. According to the position of some on this issue, lift plates increase the pressure that can be applied to a ski. Seriously? How could this work? It couldn’t. Where forces are linear with no components it would make no difference whether lift plates were 1 cm high or 1 metre high. They would have no impact on pressure aside from any increase in pressure resulting from the added mass of the lift plates. Do people just make this sort of stuff up?

ML 2

In the graphic below, the feet are wider apart than the centre-to-centre dimension between the ball joints of the pelvis. The mechanical lines still run from the centre of the ball joints in the pelvis to the distal tibia. But there are now vertical and horizontal components of force Fh and Fv with a resultant force R aligned with the mechanical line. The horizontal component Fh of the resultant force R of the mechanical line is tending to rotate the feet about their outside or lateral borders. In other words, the angular relation of the mechanical line to vertical has created a moment of force or torque  that is tending to rotate the foot. What would happen if lift plates were introduced between the soles of the feet and the ground?

ML 3…………… to be continued

BIRDCAGE VS. CONVENTIONAL SKI BOOT

Once the initial Birdcage experiments had validated my hypothetical model of the mechanics, biomechanics and physics of skiing, we moved on to experiments that looked at the effects of things like second stage cuff end point resistance and total stage one/stage two cuff cuff range of motion effects. Unlike conventional ski boots that have a fixed cuff, the Birdcage had a cuff that actually rotated about an axis that was close to the proximate centre of the true axis of the ankle joint. The hinge points of a conventional ski boot cuff amount to nothing more than a means to join the cuff to the lower shell. Any forward flexion of the shin is achieved by distortion of the boot shell as a whole in what amounts to a L shaped tube bending through deformation. What appears to be a hinge is an illusion.

The Birdcage experiments compared the Birdcage to a conventional Salomon boot. The tester for these trials was an intermediate skier who had been skiing in the same Salomon boot for several years. We started the experiments with several runs in the Birdcage. Keep in mind that the tester had been trained by the Salomon boot to ski a specific way. It takes time to replace learned motor patterns with new ones. Here is a data sheet from the second test trial done in the Birdcage with my notes added. The hand written notes are those of the scientist who oversaw the experiments.


John Birdcage 2

Compare this testers’ skiing to that of a World Cup champion and Olympic medalist. Note the marked difference in the pressure under the ball of the foot, ankle flexion and absence of cuff pressures.

Steve 14

Here is the first trial done by the tester in his own Salomon boots.

John Salomon

The problem with the bent tube configuration of the conventional ski boot shell  is that the glide path of the shin bone is obstructed by the throat of the shell. The throat is the point where the instep of the lower shell transitions into the cuff.

Here is a collation that compares the data fields for the 1st metatarsal (ball of the foot), front cuff and the instep-shin force plate that is pulled down and back towards the heel by a cable-buckle system. When pressure builds in the shin glide path, the contraction of the soleus muscle is interrupted resulting in a clipping of pressure under the ball of the foot.

1st MT clippingThe 2-piece tongue system with a flexible link disclosed in my US Patent No. 4,534,122 provided a partial solution to this issue (see my post OUTING THE ARCH COLLAPSE THEORY). But the configuration of the Salomon rear entry boot actually made the situation worse with an instep-shin plate that pulled the obstruction of the glide path of the shin diagonally down and back towards the heel.

Once a baseline optimal performance configuration for a skier has been captured by a Birdcage or Birdcage-like device, it is possible to conduct experiments that can accurately assess the effect of obstructing specific joint actions or comparing the effect on the user of another ski boot or a device like an insole or custom formed insole. If for example someone claims, as many are, that the foot functions best in skiing when its joints are completely immobilized we can compare this state to that of the baseline optimal performance configuration the same skier. The technology used in the Birdcage is over 20 years old. Today’s technology has the capability quantify the effect on the user of virtually any piece of ski equipment that resides between the sole of the user’s foot and the snow surface.

THE BIRDCAGE EXPERIMENTS – DATA 3

Here are two more data sheets from the Birdcage Experiments.

The first data sheet is for the same skier (Steve) shown in THE BIRDCAGE EXPERIMENTS 1. The first two data sheets shown in previous post were for world class skiers who, like Ted Ligety and Mikaela Shiffrin, ski differently than the average recreational skier and even most World Cup racers. Click on the sheet to zoom in.

Steve 14

This Trial involved aggressive GS turns with a free-hinging cuff between front and rear stops set to the skier’s functional specification. Contrary to the common wisdom in skiing, introducing significant resistance to cuff movement in the normal range of ankle flexion forces the soleus muscle to decrease its contractive force in order to overcome the resistance to forward (dorsiflexion) presented by the cuff. Diminished balance, diminished control of the ski and attenuation of the processes that dissipate energy from the interaction with the snow are the result. This was noted as far back as 1987 by Professor M. Pfeiffer of the University of Salzburg (Salzburg, Austria) in his article Kinematics of the Foot in the Ski Boot; “The shaft of the boot should provide the leg with good support, but not great resistance for about two-thirds of the possible arc, i.e., 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded.” (my emphasis added)

The data sheet below is for an intermediate skier. Part of my hypothesis that was being tested in the Birdcage experiments is that footwear especially ski boots has a training effect on the user. This was demonstrated in the early 1990s at the Human Performance Laboratory at the University of Calgary (Calgary, AB Canada).  Based on this premise, it was predicted that a ski boots act as an evolutionary filter for skiing competence in that only skiers with the right foot and leg structure will be able to use their natural intrinsic postural processes. More than any other factor the ski boot determines which skiers will rise to ranks of the elite. Those with incompatible foot and leg structures (ergo – the majority of the skier population) will be forced by the ski boot to develop compensatory mechanisms that will be be much less effective than the processes used by the elite skier. The data sheet below illustrates what happens when an intermediate skier is fit with a ski boot that accommodates the natural intrinsic postural processes. Although it will diminish over time, the ingrained (learned) motor pattern derived from using a conventional ski boot continues to regulate postural activity.

Alex 2Suddenly we see the pattern of pressure on the front and inner aspects of the boot cuff, high pressure under and behind the heel and minimal pressure under the ball of the foot associated with an unbalanced base of support that necessitates the use of the leg as a compensatory lever to control the edge angle of the ski.

THE BIRDCAGE EXPERIMENTS – DATA 2

The Birdcage was more than just a research vehicle. It was a device that allowed us to fine tune the interface that connects the user to a ski and capture what amounts to a Performance DNA or Performance Signature that represents the optimal performance prescription for a specific skier. As in any properly done science experiment, I had formulated a hypothetical model in advance that predicated specific outcomes. This is very different from the process of subjective observation where gross movement patterns are observed and noted without any attempt to understand the underlying process responsible for the movement pattern. In the Birdcage experiments, specific constraints were placed on a skier while all other factors were kept constant. This ensured that any change in a specific constraint was actually responsible for any change in performance where a change was seen in the data. Hard data from the instrumentation package was correlated with tester comments (the testers were mic’d and recorded) and video analysis.

One of the outcomes that I did not predict, and did not believe possible when several testers suggested it, was doing experiments with zero resistance in the cuff and no forward stop. In other words a free hinging cuff. Early in the experiments we had determined, based on theoretical and practical considerations, that a rear cuff stop with the cuff set at a functional angle was essential as a rearward reference. And contrary to conventional wisdom, my model had predicted that a specific amount of zero resistance cuff movement was essential to maximize the contraction of the soleus muscle. The soleus is part of a chain of muscles in the back of the leg that is responsible for postural responses in skiing and especially the loading of the ball of the foot when the foot is pronated. The graphic below shows the soleus muscle.

SoleusIn order to understand the pressure readings on the four aspects of the cuff of the Birdcage (front, back, inside, outside) it is necessary to know how the inside ankle bone moves when the foot is pronated. The graphic below shows a neutral foot (left) and a pronated foot (right). There is a mechanical line from the ball or head of the femur that runs to the base of the tibia where it forms the ankle joint with the talus. No matter how curved the leg is the force from the pelvis is transferred to the foot on the mechanical line. In order for the foot to pronate the inside or medial ankle bone must move inward, towards the shell wall of the boot. The top of the boot cuff and the sides of the heel of the boot act as axes or pivots for the inward movement of the tibia. The graphic below shows this movement. The Figures are from US Patent No. 5,265,350 – MacPhail with notes added.

Leg Movement

The Birdcage was intentionally configured with sufficient clearance to ensure that no testers’ inside ankle bone could contact the arm of the cuff. The photo below shows the Birdcage in the process of being fit to the foot and leg of a tester.

Birdcage 3

Here is another sheet of Birdcage experiment data. The data was originally in digital form which made it much easier to expand and overlay. This process was used to arrive at the notes on the hard copies. At this point I only have the hard data sheets. So the data is not as easy to expand and overlay to compare different data fields. In the first data set there was almost no pressure on any aspect of the cuff. The reason for this is that the cross-sectional area of the testers’ shin was smaller than the cuff enclosure. In the data field below, the cross-sectional area of the testers shin is a little larger than the cuff enclosure. Hence we see a constant pressure line. This is not force being applied to the cuff for the purpose of putting the outside ski of a turn on its inside edge. In fact, when the feet of the elite skiers were allowed to pronate, there was little or no pressure applied to the sides of the cuff at any time.

Here is the data sheet for an experiment done with a free hinging cuff. Much to my amazement testers skiing with a free hinging cuff were able to ski through mogul fields like they were skiing on groomed runs while the muscles in their feet and legs absorbed the energy that would normally have disrupted balance and thrown them all over the place. Click on the data sheet to enlarge it.

Birdcage Data 3

THE BIRDCAGE EXPERIMENTS – DATA 1

In this post, I am going to discuss some of the Birdcage data from the experiments done in 1991 on Whistler Mountain’s summer glacier. After we did the experiments, we decided not publish the data. The intent was to share the information with the ski industry. But there was no interest. So this is the first time this data has ever been shown in public forum. In order to correlate the data it needs to be referenced to the legend below.

Bird Cage Sensors

Screen Shot 2014-02-15 at 6.22.32 AM

Here is one data sheet. The pressure under the ball of the foot is in the second column, third row down. Every peak is moving the Centre of Pressure to the ball of the; ergo the foot is pronating. You can see the corresponding ‘coupling’ of the horizontal torque into the turn with the lateral heel or outer corner of the heel. The medial or inside corner of the heel is loading because the foot is pronating. The reference to ‘LIGHT SPRING BEGINNER’ means that the end point forward travel resistance of the cuff was provided by a light spring. Note that there is zero pressure on the front of the cuff and very little pressure on the rear of the cuff.

Most telling is the almost complete absence of force on any aspect of the cuff of the Birdcage. According to conventional wisdom. the best skiers use the inside or medial aspect of the boot cuff to hold the outside ski on edge. The data in the pink highlighted zone clearly shows that the best skiers do not use the cuff in this manner. Instead, they use the cuff as an alignment device to direct the application of force to the ski through the ball of the foot in combination with force under the heel. Also note the consistent forward and back flexion (dorsi-plantarflexion) of the ankle joint (2nd column, last row). In future posts I will present more data from the Birdcage experiments.

Birdcage Data 5

PRONATION AND THE BIOMECHANICS OF EDGE CONTROL

The stick man sketch below are Figures 23 A and 23 B from US Patent No. 5,265,350 (expired) awarded to the writer.  The stick man in FIG. 23 A is engaged in quiet standing with the weight equally distributed between the left and right feet. This is called Bipedal (two-footed) Stance. The force vector W emanating from the Centre of Mass or CoM  is the ‘disturbing force’ of gravity.  W is called a disturbing force because it is tending to disturb the equilibrium of the stick man and cause him to topple.

Bi-Monopedal

Gravity is an ‘attractive force’ like magnetism. CoM is where you are in relation to the supporting surface. In this case, ground. But W is not the force applied to ground by the stick man. The applied force occurs at the contact points of the foot or feet with ground.  In FIG 23 A,  W lies equidistant between the two feet in the transverse plane.

In normal Bipedal Stance, each foot supports equal proportions of the bodyʼs weight W, assuming equal leg lengths. Approximately 50 percent of the load is borne by the heel. The remaining 50 percent is borne by the heads of the long metatarsal bones. The load on the head of the first metatarsal (aka the ‘ball of the foot’) is twice that of each of the heads of the other four metatarsals. The anteroposterior (ergo, front to back) distribution  of the load through the foot is due to the position of the CoM of the body above.  The point on the foot where the centre of the applied force appears to act is called the Centre of Pressure or CoP. I say ‘appears to act’ because CoP could lie somewhere in the vault of the arches of the foot. In Bipedal Stance CoP lies on an axis that runs through the proximate centre of the heel and the head (ball) of the 2nd metatarsal. In ice skates, this is the ‘balance point’ where the ice blade is mounted. The forces shown as w2 are the centres of the ground reaction force or  that opposes CoP.

The footwear industry’s dirty little secret is that shoes are made on lasts that approximate the shape of the human foot in Bipedal Stance; standing on two feet and not moving. When you start to walk in a shoe, the structures deform and distort to accommodate changes in the architecture of the foot. Ski boots are worse. Not only are they built on lasts that are the approximate the shape of your feet and legs in quiet Bipedal Stance, the structures impede or even prevent the user from attaining a dynamically balanced base of support on one foot. Claims in relation to skiing are made that the human foot functions best in skiing when its joints are immobilized, preferably in a neutral position. In a neutral position, joint actions of the foot and knee and hip are limited to flexion and extension with transverse and orbital movement of the leg in hip joint within its normal range of motion.

A whole industry has been established on methods of immobilizing the foot and stabilizing it in a neutral position with custom formed boot shells, custom formed liners and custom formed footbeds and orthotics that significantly restrict or prevent pronation. The indirect effect of preventing pronation is that the position of CoP on the axis running through the proximate centre of the heel and head of the 2nd metatarsal becomes fixed.  For reasons that will be explained in future posts, this can have the effect of preventing the user from being able to establish the over-centre edge control mechanics that the best skiers use and especially an inability to establish a dynamically stable base of support on which to move from ski to ski.

TIGHT FEET, LOOSE BOOTS – LOOSE FEET, TIGHT BOOTS

In order to get the best connection of the foot with the ski the boot must fit the foot as closely and as tightly as possible. Race boots need to be narrow in forefoot, significantly narrower than recreational boots, so that the boot will grip the forefoot tightly for ‘optimal steering’ control. We know these things to be true because that’s what the experts preach, or at least that’s the official story. If everyone agrees on an issue then consensus equals truth. In my case, I knew this was true because I couldn’t seem to get a tight fit of my feet with my ski boots. Other skiers must have been having the same problem because in the ’70s improving fit was a common theme of ski magazine articles on boot fitting. And there was an array of ankle pads and other aids available to help tighten the fit.

If one had a loose fit of their foot in their ski boots the answer was simple; improve the fit with pads or foam injected liners that precisely conform to the shape of the foot. Today heat formable liners and even heat formable boot shells, considered by some to be the Holy Grail of skiing, are available as are boots formed to lasts made from 3 D scans of the user’s foot. Perfection draws closer. That all boot fit technology has gravitated towards the perfect fit serves to prove the soundness of the concept………or maybe not. Maybe there is something else going on.

First, let’s be clear. The foot must be constrained in some fashion to achieve an intimate connection with the ski. But what I started to notice is that the feet of elite skiers were different in a fundamental way from those of lesser skiers whose feet were in turn different from the feet of those (of which I was one) who were struggling to ski in rigid plastic ski boots. Specifically, the feet of the elite skiers were compact and seemed tighter than the feet of lesser skiers. What do I mean by tighter? The foot has 28 bones that shift in relation to each other in three-dimensional space. The movement of the bones is constrained by soft tissue, in particular  ligaments that bind the bones together. The movement of the bones of the feet of elite skiers seemed to be more constrained by ligaments than the bones of the feet of lesser skiers. Put another way, the feet of elite skiers appeared to be able to function reasonably well within the constraints of the rigid plastic ski boot.

An excellent animation showing the movement of the bones of the foot called, ‘Ankle & Subtalar Joint Motion Function Explained Biomechanics of the Foot – Pronation & Supination by Dr Glass DPM’ can be viewed at – http://www.youtube.com/watch?v=0R4zRSE_-40

The problem is that in an industry where the foot is represented by an inanimate one-piece last, the tendency is to view all but the most deformed feet as equal. Clearly this is not the case. Research on foot characteristics has classified the human foot into three categories; 1) tightly bound, 2) moderately bound and, 3) loosely bound. If your feet fall into category 1), tightly bound, and your foot is compact with moderate or less width, the odds are that you will find skiing easy. Category 2 is less certain while category 3), of which, I was one, means that attempts to tighten the fit of the ski boot could, and usually do, actually make the bones of the foot looser because the structures of the boot interfere with the processes that tighten the bones of the foot. This results in a perceived looseness of the foot, which can lead to subsequent attempts to tighten the fit of the boot resulting in a vicious circle.

Recognizing that foot structure can affect the ability to ski adds a layer of complexity, one that marketers would probably rather ignore. The essence of effective marketing is simplicity. “Watch what I do. Listen to what I tell you to do and you will ski like me”. Easy! Except for the fact that for the majority it doesn’t work this way. They may try their best to ski like the best. But the simple fact of the matter is…….they can’t because of their type of foot type. Like the earth is flat story, once people buy into concept they tend to stick with with it even in the face of overwhelming evidence that it is flawed. The brain subconsciously filters out any information that disagrees with the official position. So those with loosely bound feet “don’t get no respect”.

But it gets worse. Skiers with tightly bound feet tend to ski with their boots loosely buckled. Cases have been documented of racers winning races who had forgotten to close their boot buckles. Skiers with tightly bound feet usually ski best with loosely buckled ski boots, something those with loosely bound feet would find unthinkable. The resulting Paradox flies in the face of the common sense. So it is conveniently ignored. Skiers with tightly bound feet usually end up being the ski pros by a process of elimination. The best feet rise to the top. Because the best skiers tend to assume that skiing is a simple matter of teaching someone to do what they can easily do, they don’t appreciate, let alone understand, why others can’t seem to get it.

In summary, loosely bound feet require much more three-dimensional space in which to acquire tightness than tightly bound feet. Tightly bound feet function best in minimally tensioned ski boots. But maximal constraint can never make loosely bound feet tight because a tight fitting boot inhibits the physiologic processes that tighten the joints of the foot. Knowing this, I knew that the answer had to be to find a way to constrain the foot in a manner that allowed the tightening processes of all types of feet to engage. If this were possible all feet would become equal. In effect, this would serve to level the playing field or perhaps better stated, make the slope more consistent for everyone.