biomechanics

FOOTBEDS: THE UNKNOWN COST OF SUPPORTING THE ARCH OF A SKIER’S FOOT

Two recent studies (1.), (2.) question the merits of supporting the arch of a skier’s foot and especially any claims made that supporting the arch in neutral is the strongest position for skiing. 

It is well established in the scientific literature that the plantar aponeurosis (aka plantar fascia or PA) is one of the major arch-supporting structures of the human foot.  A positive correlation between Achilles tendon loading (ATF) and plantar fascia tension (PAF) has been reported. A study (3.) found that plantar aponeurosis forces (PAF) gradually increased during mid stance and peaked in late mid stance. The study found a good correlation between plantar aponeurosis tension (APF) and Achilles tendon force (ATF). The study concluded:

The plantar aponeurosis transmits large forces between the hindfoot and forefoot during the (mid) stance  phase of gait. The varying pattern of plantar aponeurosis force (PAF) and its relationship to Achilles tendon force (ATF) demonstrates the importance of analyzing the function of the plantar aponeurosis throughout the stance phase of the gait cycle rather than in a static standing position. – (my emphasis added in bold)

I discussed this in my post TRANSITIONING TO A HIGHER LEVEL OF SKIER PERFORMANCE.

The graphic below from Kevin Kirby’s Foot and Lower Extremity Biomechanics II:  Precision Intricast Newsletters, 1997-2002 illustrates how the position of COM in relation to the foot tensions the GS (gastroc-Soleus) compressing the arch which tensions the plantar aponeurosis ligament. I have added arrows to indicate PA strain Force F and Shear Force as well as Arch Compression Force.

The graphic below also from Kevin Kirby’s Foot and Lower Extremity Biomechanics II:  Precision Intricast Newsletters, 1997-2002 illustrates how the anterior (forward) advance of CoM in relation to the foot decreases rear foot loading (GRF-RF) and increases fore foot loading (GRF-FF). I have added a red dashed vertical line and a red triangle to show the approximate location of what would be what I term the tipping or pivot point where the foot would rock rearward and forward with a corresponding shift in CoM.

The two recent studies I referred to (1.), (2.) that question the merits of supporting the arch of a skier’s foot were actually done with subjects walking and running on flat and inclined surfaces. But the effect on arch compression is applicable to the effect of arch supports used in ski boots.

New Balance Minimus road MR00 shoes were provided to all participants to wear for testing (approx. weight 180 grams, zero heel-toe drop, no medial arch support and a uniform EVA midsole). Pockets filled with lead weights were affixed to the laces of both shoes in order to standardize foot weight across all shoe and insole conditions. The minimal shoe was chosen as a control condition in order to standardize non-insole effects as much as possible.

Two separate custom insoles were designed for each participant and fabricated by orthotic laboratory. The first insole was designed to restrict arch compression near-maximally compared to that during shod (barefoot) running (Full Arch Insole; FAI). The second insole was designed to restrict compression by approximately 50% during stance (Half Arch Insole; HAI). TO qualify for the study participants could not wear orthotics on a regular basis.

The study found:

The insert restricted maximum arch compression by approximately 70% when compared to unrestricted shod running and consequentially resulted in lower strain values throughout the entire stance phase. It should be noted that the PLF length only surpasses the estimated resting length between ~25%-80% of the stance phase in the insert condition (Fig 3). The negative strain values should be regarded as a slack PLF length, not as the PLF shortening beyond the resting length. 

The graphic below from the paper The Foot’s Arch and the Energetics of Human Locomotion shows the maximum arch compression of subjects shod barefoot (Shoe-only), with the Half Insole that restricted arch compression to 50% of the maximum amount and with the Full Insole that maximally restricted arch compression. The Full Insole is typical of insoles used to support the arch of a skiers’ foot.

 

The insoles had no effect on the metabolic cost of walking despite restricting ~80% of arch compression. 

In a personal communication with Sarah Stearne she advised me that the study didn’t measure muscle EMG activation with and without the insole but they did know that the ankle performed less positive (-8%) and negative (-10%) mechanical work when the insole was worn and that the ankle peak dorsiflexion moment was reduced (-7%). Based on the ankle moment and Achilles tendon moment arm data they calculated that there was ~6% less force in the Achilles tendon when the insole was worn.

Whilst several studies have acknowledged the elastic energy storage potential of the PLF, this ligament is primarily regarded for its role in providing integrity to the bony arch structure, and in supplying the rigidity required for the foot to function as a lever during propulsion (or skiing, my comment) 

This study confirms what I experienced in 1973 after I had full support custom orthotics made by a well known sports podiatrist. The orthotics felt comfortable standing on them and even walking. I experienced some discomfort when attempting to run with the orthotics in my jogging shoes. But when I tried skiing with them in my ski boots I felt as if my foot were floating on the top of the orthotic with little or no sensation of any force under my first MPJ.

Based on the results of two cited studies I believe there is no basis to assume that supporting the arch of a skier’s feet will have positive benefits or is without adverse consequences without first conducting comparative studies using standardized controlls (no insole, flat boot board) and established scientific protocols.


  1. The Foot’s Arch and the Energetics of Human Locomotion – Sarah M. Stearne1, Kirsty A. McDonald1, Jacqueline A. Alderson1, Ian North2, Charles E. Oxnard3 & Jonas Rubenson1,4 – (January 19, 2016)
  2. The Role of Arch Compression and Metatarsophalangeal Joint Dynamics in Modulating Plantar Fascia Strain in Running – Kirsty A. McDonald1, Sarah M. Stearne1, Jacqueline A. Alderson1, Ian North2, Neville J. Pires1, Jonas Rubenson1,3* – (April 7, 2016)
  3. Dynamic loading of the plantar aponeurosis in walking – Erdemir A, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA

THE ULTIMATE LOOSE FOOT TEST OF METAL

The human foot is a masterpiece of engineering and a work of art.

                                                                                                                  Leonardo da Vinci

Despite what da Vinci said, skiers seem to have an inherent distrust in the structural capacity and integrity of the human foot.

In skiing demonstrations with ski boot prototypes based on the Birdcage it didn’t matter how hard I tried to explain to testers how the dorsal loading system worked and how little force was needed to secure their foot, it didn’t stop them from attempting to crush their foot by tightening down the dorsal plate until their noses bled. They were so conditioned by the persistent, ‘the tighter the boot, the better the ski control’ message that they just didn’t want to believe how little force it takes to activate the auto stiffening mechanism of the longitudinal arch (FIT VS. FUNCTION) and retain the foot in solid contact with the base of the boot.

In order to try and convince testers how little force was required to make their foot dynamically rigid one of our team members had a device we called the Logan Chassis designed and fabricated. The photo below is of the Logan Chassis aka The Convincer.

If it’s not obvious from the photo  the Logan Chassis was very heavy. The components were milled from solid blocks of aluminum. The heel counter and a few other components are missing. But the photo should give you a good enough idea. This thing was a tank. This device was not intended for skiing. It was a pre-ski boot skiing test conditioner.

To demonstrate how little force it takes to make the foot so rigid it is like steel I would get the test subject to put their foot in the Logan Chassis. Then I would try to get them to adjust the knob on the screw to the point where it applied firm but gentle pressure on the dorsum of their foot making sure there was no discomfort. Then I would ask them to stand up and lift the foot in Logan Chassis off the floor and tell me what they felt. They were shocked. Hell, I was shocked when I tried this.

The Logan Chassis feels incredibly light and the foot feels glued to the base with no sensation of pressure or discomfort. It defies logic. But I doubt I would have to convince da Vinci.

The truth is whatever people are willing to believe.

The problem is that most skiers have been convinced to believe that tight is not just right, tight is might.

TRANSITIONING TO A HIGHER LEVEL OF SKIER PERFORMANCE

The transition to a higher level of skier performance for my spouse and I started in the 2012-13 ski season. After a ten-year hiatus from skiing we were returning to the ski hills with renewed enthusiasm coupled with a desire to reach a higher level of performance. I purchased new narrow waisted skis for both of us. I intended to purchase new ski boots as well. But I quickly backed off from even considering this after assessing a number of new boots as too difficult to work with.

I started The Skier’s Manifesto in the spring of 2013 for a number of reasons. The primary reason was that the forum provided me with an opportunity to acquire new information and increase my knowledge so I could learn how to transition my spouse and I to a higher level of skier performance. The process of attempting to explain complex technical issues by writing articles and posts serves as the impetus for me to think deeply, thoroughly and analytically. As the process unfolded, I discovered issues I had overlooked in the past or not fully explored.

One issue I had not fully explored, let alone addressed, is a way of identifying the optimal ramp angle specific to each skier. Ramp angle is the angle of the ramp of the plantar plane under a skier’s foot with the base plane of the ski. Finding a method of identifying optimal ramp angle proved far more difficult than I had anticipated. But when I succeeded in identifying and then implementing the optimal ramp angles for my spouse and I last ski season this proved to be the gateway to a higher level of skier performance than I could ever have envisioned. After identifying and then confirming my optimal ramp angle as 1.2 degrees (bindings zero) I finally understood after almost 45 years how and why changing from the leather ski boots I learned to ski in to the new plastic boots had such a devastating impact on my skiing. It was the change in ramp angle. The ramp angle in my leather boots was much less than the ramp angle in my plastic boots.

By 1978 I had subjectively found that a ramp angle greater than 3 degrees adversely affects skier performance with some skiers affected more than others. I knew there was no one size fits all, only that more than 3 degrees seemed to cause problems. From 1978 onward I was improving skier performance by ensuring the total ramp angle of the combined boot board/binding (zeppa + delta) was about 3 degrees. For females with small feet this required grinding the boot board in Lange boots flat or even negative (heel down) to compensate for binding ramp angle which increased as the toe and heel pieces moved closer together for small boots. I wasn’t always able to get the ramp angle set at 3 degrees. But getting it in the 3 degree range consistently resulted in significant improvement in skier performance.

It was becoming increasingly apparent to me that finding the optimal individual ramp was critical.

Critical Ramp Angle

In 2018 I identified the critical ramp angle as the angle of the plantar plane in relation to the base plane of the ski that enables a skier to apply maximum vertical force to the ball of the outside foot when the COM in the pelvis is stacked vertically over the head of the first metatarsal.

The vertical force is applied passively by force transfered to the plantar aponeurosis ligament (PA) by Achilles tendon (AT) tension.  As COM moves forward towards the head of the first metatarsal in the support phase where skier resists the force of gravity, AT-PA tension applies an increasingly greater down force to the head of the first metatarsal. Ramp angle is optimal when the vertical force peaks just prior to the end of the support phase in what is called Mid Stance in the Gait Cycle of walking.  I qualified this mechanism as enabling a skier to apply maximum vertical force to the head of the first metatarsal. Studies have shown in the skiing the position of the pelvis in relation to its vertical position with foot is the most reliable indicator of the position of COM. A skier is able to control the vertical force applied to the head of the first metatarsal by controlling the position of the pelvis.

The photos below show Marcel Hirscher and Tesa Worley applying maximum force to the head of the first metatarsal of their outside foot by stacking their pelvis over it.

The Problem with Adapting

The primary determinant of the critical ramp angle is the length of skier’s Achilles tendon (AT).

The length of the AT can and does vary significantly among the general and skier populations. The type of everyday footwear worn and especially what is called drop (heel elevated above the forefoot) can affect the length of Achilles tendon.

Drop affects the timing of the process that stiffens the foot transforming it into a rigid lever for propulsion. Over time, the predominate wearing of footwear with significant drop can cause the AT to shorten as a way for the body to adjust the timing of the stiffening process. In activities such as walking and standing, a shortened Achilles tendon may not have a noticeable affect on performance. But in skiing, the timing of the AT-PA tensioning process is critical. Those who learned to ski in boots with ramp angles close to optimal for the length of their Achilles tendon typically excel at skiing regardless of athletic prowess while gifted athletes who learned to ski in boots with sub optimal ramp angle can struggle in spite of innate athletic ability. For a racer whose equipment is close to their critical ramp angle a change in equipment that significantly changes ramp angle can be fatal to a promising career.

Most skiers would assume that they can just adapt to a sub optimal ramp angle. But adaptation is precisely the reason why skiers and racers with a sub optimal ramp angle reach a threshold from which they cannot advance. When their brain makes repeated attempts to apply force to the head of the first metatarsal without success it starts to make adjustments in what are called synaptic connections to create a new movement pattern to adapt to sub optimal ramp angle. The more the equipment with a sub optimal ramp angle is used the more the associated synaptic connections are strengthened and reinforced. Once the movement pattern associated with sub optimal ramp angle is hardened,  optimal ramp angle is likely to be perceived by the brain as wrong. Telling a racer with sub optimal ramp angle to get forward or get over it (what that means) will only make matters worse because a sub optimal ramp angle makes it impossible. Correcting the ramp angle and/or the length of the AT will not help because neither will change the hard-wired movement pattern in the brain. Deleting a bad movement program can be done. But it usually takes a structured program and a protracted effort.

Mid Stance Misinformation

A factor that I believe may have contributed to the critical ramp angle issue being overlooked is misinformation about mid stance. The story used to sell footbeds and even some orthotics is that skiing is a Mid Stance activity and in Mid Stance the foot is pronated and weak necessitating a foundation under the arch to support it. While it is true that the load phase of skiing occurs in Mid Stance the statement that the foot is weak is only partially true because it doesn’t encompass the whole picture.

The Stance or Support Phase of what is called the Gait Cycle of walking consists of four phases:

  1. Loading Response
  2. Mid Stance
  3. Terminal Stance
  4. Pre-Swing

All four phases happen in a ski turn sequence. The support phase, where one foot is flat on the ground and the leg is supporting the weight of COM, is called Mid Stance. The position of COM in relation to the head of the first metatarsal in Mid Stance and how fast COM can move forward over the head of the first metatarsal (center of the ski) of the outside foot in the load phase is a major factor in dynamic control and the ability of a skier to apply maximum force to head of the first metatarsal. But Mid Stance is a range and a sequential stiffening process, not a fixed point as has been misrepresented for decades by many in the ski industry.

The graphic below shows the relationship of 1. Achilles Tendon Force with 2. Plantar Aponeurosis Force with 3. Vertical GRF and how the tensioning process and transfer of force to the head of the first metatarsal occurs as COM progress forward in the Mid Stance cycle. The timing of the forward advance of COM/Pelvis to sync with peak AT-PA force transfer to the head of the first metatarsal is shown with a red circle and vertical arrow.

If I had only shown the segment of Mid Stance in the grey rectangle at the beginning of Mid Stance on the left I could have made a case that the arch is weak and in need of support since Achilles Tension is zero and Plantar Aponeurosis Force (called strain) is very low. But this would be misinformation because it does not show the whole picture. If the foot were weak as is alleged it would be impossible for it to act in the capacity of a lever in propelling the weight of the body forward in locomotion.

In my next post I will explain how I used NABOSO surface science technology to confirm my optimal ramp angle.

 

TRANSITIONING FROM FIT TO HIGH PERFORMANCE FUNCTION


That footwear can negatively impact the physiologic function of the user has been known for many decades. But the issue of the effect of footwear on athletic performance came into sharp focus in 1987 with the publication of the medical textbook, The Shoe in Sport (published German in 1987 as Der Schu im Sport). The Shoe in Sport brought together the collective expertise of 44 international authorities on orthopedics and biomechanics to focus their attention on the SHOE PROBLEM in the context of problems shoes can cause for athletes in terms of compromising performance and contributing to injury. The Shoe in Sport focusses on the medical orthopedic criteria in offering guidelines for the design of shoes for specific athletic activities including skiing and ice skating.

In the Introduction to the Shoe in Sport, Dr. med. B. Segesser and Prof. Dr. med. W, Pforringer state that the findings in the textbook should enable the interested reader to distinguish between hucksterism and humbug on the one side and the scientifically sound improvements in the athletic shoe on the other. The Shoe in Sport made it abundantly clear that it is not a question of if structures of footwear will affect the physiologic function of the user, it is a question of how structures of footwear will affect the physiologic function of the user and especially whether they will compromise athletic performance and/or contribute to injury.

With regard to guidelines for ski boots, the international authorities on orthopedics and biomechanics who contributed their expertise and knowledge to Part IV The Ski Boot took the position that, among a number of other things:

  • ………. the total immobilization by foam injection or compression by tight buckles are unphysiologic.
  • The ski boot and it’s shaft must be adapted to the technical skill of the skier, and the technical skills of the skier must be adapted to the preexisting biomechanical functions of the leg and the foot.
  • It (the design) should not make compromises at the expense of other joints ………
  • It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

The position of international authorities on orthopedics and biomechanics on the medical and biomechanical criteria for ski boots was succinct, concise and unequivocal:

…….total immobilization by foam injection (implying by any means) or compression (of the foot) by tight buckles are (both) unphysiologic.

Dr. E. Stussi,  Member of GOTS and Chief of the Biomechanical Laboratory ETH, Zurich, Switzerland made a prescient statement with implications for the future of knee injuries in skiing:

Improvements in the load acting on the ankle (implying load from improved fit) make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.

While the international authorities on orthopedics and biomechanics who contributed to The Shoe in Sport provided valuable guidelines for the design of the ski boot they did not offer a specification that would assist designers and those who work with ski boots in meeting the medical and biomechanical criteria in the guidelines. My hope and intent was that the Birdcage studies and the content of my US Patent 5,365,350 (issued on 11-30-1993, expired on 12-28-2005) would serve as a foundation on which to build a specification that would enable the structures of ski boots to be adjusted to accommodate the personal functional requirements of the skier.

The steps in my transition from Fit to High Performance Function

After the unprecedented success of my dorsal loading invention with Crazy Canuck, Steve Podborski, I used the same system with similar success in the boots of a small number of other racers. I also incorporated this system into my own and my spouses’ ski boots in conjunction with suitable liner modifications and a reduction of the ramp angle of the boot boards to just under 3 degrees which I had identified in about 1978 as the maximum angle for skier performance.

I can’t recall exactly when, but about 20 years ago I decided to move away from Lange ski boots. I purchased a pair Head World Cup 335 mm ski boots for myself and a pair of Head X-80 295 mm ski boots for my spouse. I say built because to me ski boots are raw material.

I had to completely disassemble the Head X-80s and drastically modify and reconfigure the components to adapt them to the morphology of my spouses’ feet and legs. The process took me about 35 hours. I was able to modify my Head World Cup liners to make them work without the same degree of modification. I made a dorsal loading system for my spouse similar to the one I made for Steve Podborski’s Lange ski boots.  But I was able to modify the existing Head tongue so it would adequately load the dorsum of my foot. The reason I went this route is that the shell of my Head World Cup boot is very stiff. This makes inserting my size 12 US men’s foot and a dorsal system, like I fabricated for my spouse, challenging. In the order of things the dorsal system is inserted after inserting the foot in the shell.

The photo below shows my Head liner after initial modifications.

The photo below shows the Lange tricot liner I used in my spouses’ Head boots on the left with no modification other than removing the Lange flow fit pads in the side pockets. I was unable sufficiently modify the liner that came with her Head X-80 boot. The version on the right in the photo below is the same liner after modifications i made for it work with the dorsal system shown in the photo underneath. The dorsal system in itself took many hours of painstaking effort to fabricate and fine tune.

With our modified Head boots fit with my dorsal loading technology my spouse and I would easily be classified as expert skiers. As recreational skiers with skiing limited to 10-15 days a season, most skiers would have no incentive to question the adequacy of their boots or especially devote time and effort towards finding ways to reach a higher level of performance. To the contrary, I found it disturbing that the ability to ski better than the majority of skiers fostered an intoxicating sense of superiority. But I knew what I didn’t know and I knew that I still had a lot to learn. In my mind, the transition required to realise our full performance potential was not yet complete.  I knew that the potential for improvement has no boundaries.

The transition to High Performance Function continues In my next post……….

FIT VS. FUNCTION

With rare exceptions, the consistently stated objective of boot-fitting systems and modification efforts is to create a perfect fit of the foot and leg of a skier with the rigid shell of a ski boot by applying uniform force to the entire surface of the foot and the portion of the leg in the boot in what pits Fit against Function. The end objective of the Perfect Fit is to achieve a secure connection of the leg of the skier with the ski. In the name of achieving a secure connection of the foot with the ski, the function of the skiers’ foot has become unitended collateral damage.

But boot design and boot fitting effors didn’t start off with the intent of compromising the physiologic function of the foot. It just sort of happened as a consequence of the limited ability to change the shape of the rigid plastic ski boots to address issues of user discomfort when plastic boots were first introduced. The new plastic boots worked well for some skiers. But for most, myself included, my foot moved around inside the shell when I tried to ski. The feeling of insecurity created by the looseness made skiing with any semblance of balance or control impossible. The fix seemed to be a simple matter of trying to figure out where to place a pad or pads between the foot and shell to stop the foot from moving.

In 1973 when I first started tinkering with my own ski boots the craft of boot fitting barely existed. Like myself, those who were trying to solve the problem of a loose fit were doing proceeding by trial mostly with alot of errors. After what seemed like unending frustration from many failed attempts at trying to find and then solve the source of my loose fit, a consensus began to emerge within the ranks of the ski industry that the easiest and quickest solution was a process that would create a tight fit of the foot everywhere with the boot instead of wasting time trying to find the elusive right place to add pads. The Perfect Fit was born.

Injected foam fit was first off the mark as a Perfect Fit solution. But injected foam fit wasn’t tight or precise enough for my standards. So I tried to take the Perfect Fit to the next level with Crazy Canuck, Dave Murray. I started the process by carefully trimming and laminating together pieces of sheet vinyl to form a matrix of solid material that I inserted into the liners of Mur’s boots. The process took about 2 weeks of painstaking effort. Finally, I satisfied that Mur’s feet were securely locked and loaded; ready for the best turns of his life. The result? One of the world’s best racers was instantly reduced to a struggling beginner, the exact opposite of what I had expected! This experience served as a wakeup call for me; one that caused me to rethink what I thought I knew and question whether the Perfect Fit was the best approach or even the right approach.

I started looking for alternate ways to restrain the foot so it was secure in the shell of a ski boot without compromising foot function. In 1980 when I was building a pair of race boots for Crazy Canuck, Steve Podborski I literally put my finger on the solution when I pressed firmly, but not forcefully, on the instep of his foot just in front of the ankle and asked if he thought we should try holding his foot like this in his new race boots. Without the slightest hesitation he said, “That feels amazing. Let’s do it!”

It took me more several few days to fabricate a system to secure Pod’s foot in his boots by loading the area of the instep that I had pressed my finger on. The problem we faced when the system was finished was that the liner made it impossible to use the system without modifying it. So a decision was made to eliminate the liner except for the cuff portion around the sides and back of his leg which I riveted to shell. At the time I wasn’t sure the system would even work. So I made a pair of boots with fined tuned conventional fit as backup. A boot with no liner seemed like an insane idea. But Podborski was not only able to immediately dominate his competition on the most difficult downhill courses on the World Cup circuit but go on to become the first non-European to win the World Cup Downhill title. Even more remarkable is that in his first season on the new system he was able to compete and win less than 4 months after reconstructive ACL surgery.

What I discovered set me off in a whole new direction. Pressing on the instep of Podborski’s foot activated what I later found out is called the Longitudinal Arch Auto-Stiffening Mechanism of the Foot. This system is normally activated as the mid stance (support) phase of walking approaches late mid stance where the foot is transformed into a rigid structure so it can apply the forces required for propulsion. As I learned about the processes that transform the foot into a rigid lever I began to understand how interfering with the function of the foot can compromise or even prevent the Longitudinal Arch Auto-Stiffening Mechanism from activating and, in doing so, cause the structures of the foot to remain ‘loose’ regardless of any efforts made to secure it.  A rigid foot is necessary to effectively apply force to a ski.

The graphic below shows a sketch on the left from Kevin Kirby, DPM’s 2017 paper, Longitudinal Arch Load-Sharing System of the Foot (1.) Figure 44 A on the right is from my 1993 US Patent 5,265,350.

The above graphics clarify the details of the arch loading system I first disclosed in my US Patent 4,534,122. This system challenges the current Perfect Fit paradigm in which the physiologic function of the foot is compromised in an effort to try and achieve a secure connection of a skier’s foot with the ski.

Figure 44A above shows the principle components of the arch loading system which is comprised of a number of complimentary elements. I will discuss these elements in my next post which will focus on solutions.


  1.  Kirby KA. Longitudinal arch load-sharing system of the foot. Rev Esp Podol. 2017 – http://dx.doi.org/10.1016/j.repod.2017.03.003

 

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 11

In my preceding post I said that after a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that rotational (steering) force should be applied to an isolated area of the inner shell wall of the ski boot by the medial aspect of the head of the first metatarsal. The reason I conducted a thorough investigation and analysis of the forces is that as a problem solver this is my standard protocol. Protocol aside, the need for a thorough investigation of every aspect affecting athletic (skier) performance was known as far back as 1983.

….. quality teaching – coaching of neuromuscular skills in physical education should always be preceded by an analytical process where the professional physical educator synthesizes observations and theory from scientific and technical perspectives……

There are many sports skills which require that sports objects, implements, equipments, and apparatus be utilized. (implements such as ski boots and skis)

All factors must be studied in terms of the skill objective. If problems are noted in the performance of the skill, where did they originate? Within the performer? Within the sport object? Both? What precise changes must be made to obtain the skill objectives?

The directions for improvement given to the performer must be based on scientific and technical analysis of the total skill.

Analysis of Sport Motion (May 1, 1983): John W. Northrip

Planes of Forces

The ability to conduct a thorough investigation and analysis of the forces associated with platform angle mechanics and biomechanics requires as a minimum, a basic understanding of the engineering aspects of the associated forces. In the case of platform mechanics and biomechanics, knowing the plane or planes in which a force or combination of forces are acting is essential.

The Force Plane in the Perfect Fit

The objective of achieving a perfect fit of the foot and leg of a skier is create a perfect envelopment of the foot and leg of a skier with the rigid shell wall of a ski boot so that force is applied evenly to the entire surface of the foot and leg to create a unified mass with the ski so that the slightest movement of the leg will produce edging and steering forces. In this format force(s) applied to the base plane by the leg will be distributed to a broad area with limited ability to apply coordinated forces to a specific area of the ski. Sensory input is also limited by the uniform force applied to all apects of the foot by the perfect fit format creating what amounts to the skiing equivalent of the Bird Box.

Platform Planes

In the mechanics and biomechanics of platform angle there are potentially three horizontal planes in which forces can be applied as shown in the graphic below. The left hand image shows the rotational force applied to a torque arm plane elevated about the base and plantar planes. In the perfect fit format in the right hand image the leg is shown as a rigid strut extending to the base plane where rotational force is applied.When the foot and leg of a skier are perfectly fit within to the rigid shell of a ski boot any force applied by the leg can only applied to the base plane of the ski where the force is distributed over a broad area. Steering and edging forces applied to the ski by the leg lack precision because they cannot be applied to specific areas or applied in a coordinated manner.

In the above graphic the whole leg rotational effort applied to the base plane by foot in the two examples is shown with no resistance. In my next post I will discuss what happens when resistance is added that opposes the rotational force applied to the base plane.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 10

In THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 8,  I stated that after a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that rotational (steering) force should be applied to an isolated area of the inner shell wall of the ski boot by the medial aspect of the head of the first metatarsal as shown in the graphic below.Applying rotational or steering force to the medial (inner) aspect of the head of the first metatarsal requires the application of an effort by the skier that attempts to rotate the foot inside the confines of the ski boot. The application of rotational effort to the inner aspect of the vertical wall of the boot shell opposite the head of the first metatarsal will result in a reaction force that pushes the lateral (outside) aspect of the heel bone against the outer corner of the vertical shell wall as shown in the graphic below. The robust structure of the bones of the first metatarsal, midfoot and heel bone serve as a structural element in transferring rotational force to opposing aspects of the shell walls in an eccentric torque couple.The outline of the boot shell in the above graphic was generated from a vertical plane photo of an actual ski boot. The interference created by the inner wall with the localized application of rotational force to the shell wall by the medial aspect of the head of the first metatarsal should be obvious.

The radius of the moment arm acting on the outer aspect of the heel area of the shell is much smaller than the radius of the moment arm acting on the inner aspect of the shell opposite the head of the first metatarsal and many times shorter than the length of the moment arm acting at the shovel of the ski. The result is that rotational force applied to the eccentric torque arm couple by rotation applied to the ankle will attempt to rotate the torque arm and the axis of rotation at the ankle joint about an axis of rotation at the lateral aspect of the heel as shown in the graphic below. This mechanism enables a skier to  apply much greater rotational force into a turn at the center of the ski than can be applied at the shovel. This has signficant implications for platform angle mechanics. In addition to the above, the plane of the rotational force applied by the medial aspect of the head of the first metatarsal and lateral aspect of the heel bone to the shell wall is elevated above the plane of the rotational force at base of the ski below.

In my next post I will discuss what happens when the reaction force from the snow that opposes the 180 degree force applied to the base plane of the ski becomes sufficient to arrest rotation of the ski about its axis of rotation at the ankle joint.