Mechanics-Biomechanics

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

 

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.

NOTE: Since I published this post a little over a year ago I have since reduced the zeppa angle of ,my Head boots to close to zero)/

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.

 

THE FUTURE OF THE SKI BOOT – PART 2

The introduction of the rigid shell ski boot served as a foundation for the evolution of what became a science of immobilization and splinting of the joints of the foot and a leg of a skier. By creating an encasement for the foot and the portion of the leg within the rigid shell, mediums such as foam could transfer force to the ankle and leg to substantially immobilize its joints. Supporting the foot in a neutral position with a rigid footbed or orthotic in conjunction with form fitting mediums ensures maximal immobilization that is described as the Perfect Fit. The science of immobilization has evolved over the years to include thermoformable liners and even thermoformable shells.

Even though the medical textbook, The Shoe in Sport, cautioned 30 years ago that “the total immobilization by foam injection or compression by tight buckles are unphysiologic (against physiologic function)” the proponents of immobilizing the joints of the ankle continue to claim that this puts the foot in it’s strongest position for skiing.

The paper, Recent Kinematic and Kinetic Advances in Olympic Alpine Skiing: Pyeongchang and Beyond,  published on February 20, 2019, cited better transfer of the skier’s action to the skis through improved boot-fittings with individual liners and insoles. If in fact, skier performance is improved due to improvements in the science of immobilization through boot-fitting then it should be evident in studies that look at skier performance.

One such study, Challenges of talent development in alpine ski racing: a narrative review, published in March of 2019 found:

Youth and adolescent ski racers report lower injury rates compared to World Cup athletes. The knee was the most affected body part in relation to traumatic injuries. The most frequently reported overuse injuries were knee pain (youth) and low back pain (adolescent level). Athlete-related modifiable risk factors were core strength, neuromuscular control, leg extension strength and limb asymmetries.

Neuromuscular Function (NMF) affects Neuromuscular control (NMC). NMC is an unconscious trained response of a muscle to a signal associated with dynamic joint stability. This system of sensory messages (sometimes referred to as “muscle memory”) is a complex interacting system connecting different aspects of muscle actions (static, dynamic, reactive), muscle contractions, coordination, stabilization, body posture and balance. The movements of the lower extremity, including the knee joint, are controlled through this system, which needs correct sensory information for accurate sequential coordination of controlled movement.

It has been known for decades that restricting the action of a joint or joint system, especially immobilizing the joint, will cause the associated muscles to atrophy. But a study, Effect of Immobilisation on Neuromuscular Function In Vivo in Humans: A Systematic Review, published in March 2019, suggests that the effects of immobilizing joints of the body are far greater than simply causing muscles to atrophy. This is the first systematic review to consider the contribution of both muscle atrophy and deterioration in neuromuscular function (NMF) to the loss of isometric muscle strength following immobilisation. The fact that the study, Challenges of talent development in alpine ski racing: a narrative review, cited core strength and neuromuscular control as issues in the development of talent is significant. The feet are part of the core in what is called foot to core sequencing. Immobilizing the joints of the foot can affect lower limb and core strength.

Immobilisation in the study the Effect of Immobilisation on Neuromuscular Function In Vivo in Humans: A Systematic Review, was achieved by using casts, braces, slings, unilateral suspension, strapping or splints with the following locations immobilised: knee, ankle, wrist and finger. All studies measured isometric muscle strength. No studies were cited that involved bilateral immobilisation of both ankles such as occurs in form-fitting ski boots. However studies did find that multiple joint immobilisation was likely to produce the largest change in the NMF of segments consisting of both mono and biarticular muscles. Other key findings were:

  • The greatest changes in all variables occur in the earliest stages of immobilisation.
  • The loss in muscle strength during immobilisation is typically greater and occurs faster compared to the loss of muscle volume.
  • The choice of joint angle for immobilisation using the brace or cast method appears likely to play a large role in the outcomes.

I started this blog six years ago for several reasons. A primary reason was to identify whether any influences existed in skiing that would serve to change the focus from immobilizing the joints of the foot and leg with the associated claims to a science-based focus. Since the future of the ski boot appears to be continued refinement of the science of immobilization this will be my final post.

I have learned a lot over the past six years that led to huge breakthroughs on skis for myself and those who I have worked with. Thank you to those who commented and contributed to The Skier’s Manifesto.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 13

The  article that follows was published on June 18, 2010 on an internet group called EPICSKI.  I have revised the article to improve clarity and consistency with the technical terms used in the THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE series of posts.

The Birdcage Experiments

 by David MacPhail

In the summer of 1991 a science team Steve Podborski and I had assembled to develop a new ski boot conducted pioneering studies on the Blackcomb summer glacier with a device we affectionately named the “Birdcage.” The purpose of the studies was to test my hypothesis of the mechanics and biomechanics of platform angle as it pertains to skier dynamic stability and the basic premise of my hypothesis that explains how  GRF acting on the inside edge of the outski is extended out under the platform of the ski. The Birdcage is shown in the photo below.

Birdcage

The Birdcage was fit with 16 sensors each with its own channel as shown in the legend below.

Specific mechanical points of the foot, in particular the ends of the eccentric torque arm, connected to specific points of the rigid structure of the Birdcage while leaving the remaining areas of the foot substantially unconstrained. The object of the experiments was to study the effects of specific forms of constraint applied to key mechanical points of the foot we had previously identified on skier balance as it pertains to steering and edge control. The experiments also included tests that studied the effect of interfering with specific joint actions. The experiments were designed in accordance with a standard scientific protocol; one that standardized conditions from test to test while varying one factor at a time.

For example, to study the effects of cuff forward lean angle on specific muscles, the range of rotation of the cuff was kept the same from test to test while the initial angle at which the cuff was set was varied from test to test. The cuff was fit tightly about the leg so as to reduce to a minimum any effects of movement of the leg within the cuff. Other aspects of the test such as position of the heel and ball of the foot in relation to the centerline and inside edge of the ski were kept the same.

By using such test protocols the firing sequence of specific muscles and their effect on dynamic stabilty could be studied. This data could then be used to determine the sequence of events and relationship steering to edge platform angle control. It was discovered that by varying the conditions that affected the firing and effectiveness of the soleus muscle, it could be played like a musical instrument. For example, if the cuff angle were set too erect the soleus muscle would make multiple attempts at the start of each loading sequence to try and get COG over the head of the first metatarsal.

Our primary tester for the experiments was Olympic bronze medallist and World Cup Downhill Champion Steve Podborski. Steve is shown in the photos below having the Birdcage adjusted to his foot and leg.

The cable coming from the rear of the device is connected to a Toshiba optical drive computer (remember, this is 1991) that Toshiba loaned us in support of our program. The biomedical engineer and the Toshiba computer are shown in the photo below.

Since telemetry was too costly and less positive we used a 1200 ft cable that linked the Birdcage to the Toshiba computer set up in a tent. Although the technician could not see the skiers being studied within a short period of time he could easily analyze their technical competence in real time by assessing the incoming flow of data from the sensors fit to the Birdcage. This was even more remarkable considering that the technician had no background in skiing, ski teaching or coaching.

The testers wore a harness to keep the cable from interfering with their movements. A chase skier ensured that the cable remained behind the testers and did not pull on the testers. Of interest is the fact that I was unable to elicit any interest in the results of the Birdcage study

As far as I know a study of this nature had never been done before and to the best of my knowledge a similar study has never been repeated since the Birdcage experiments. The Birdcage remains one of the most sophisticated analytical sports devices ever conceived even by todays’ standards. The Birdcage research vehicle is the barefoot minimum standard for the ski boot.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 12

At this point my discussion of the mechanics and biomechanics of platform angle is at what I can appropriately call the moment of truth. Moment in the context of the mechanics and biomechanics of platform angle means moment of force or torque; platform angle involves the ability of the CNS of a skier to control torques across the inside edge of the outside ski so the skier can stand and balance on the platform.

What is Balance?

That balance is the single most important factor in human movement, especially movement associated with athletic performance, is undisputed. In complex activities like skiing that involve movement in 3 dimensional space in a dynamic physical environment, optimal balance is critical. But what constitutes balance? In order to know if a skier is has optimal balance or is even in balance one has to know what balance is and what factors enable or compromise balance (i.e. postural) responses and  especially the factors that enable optimal balance.

The Balance Zone

A skier is in balance when the CNS is able to maintain the position of a skiers’ COG within the limits of a narrow band close to the inside edge of the outside ski during the load phase of a turn. The load phase of a turn occurs in the bottom of a turn when the force exerted on the platform by the COM of a skier must be balanced against the external resultant force of gravity and centrifugal force. In the load phase, the CNS must maintain COG within the forward limit of the Balance Zone within close proximity to the ball of the foot. When balance is challenged COG must not exceed the rearmost limit of the Balance Zone that lies just in front of the ankle joint. The Balance Zone and its limits are shown in the graphic below. If COG exceeds the limits of the Balance Zone shown in pink, the skier will lose their state of balance and with it dynamic control of the platform underfoot.  They will also suffer a lose of dynamic stability in the joint system of the lower limb.

The Balance Plane

In the ski system platform the plantar plane under the plantar aspect (sole) of the foot is the interface of CNS mediated balance activity. When the coordinated, concurrent forces are applied at the main force transfer point of the foot that I call the Center of Control, shown in the preceding graphic, the applied forces will manifest in more than one plane as shown in the graphic below.Force Fa applied under the head of the first metatarsal will be distributed over an area around its center.  When the force applied in the plantar plane is transferred through the structure of the platform to the base plane the center of force will maintain its position. But when the force area of distribution will increase as shown in the pink zones under the head of the first metataral and the base plane. In free rotation of the ski, resistance from the force of friction Ff will be minimal as will any force applied in the torque arm plane by the eccentric torque arm. Rotational force will be largely confined to the base plane.

The Missing Force Factor: Sidecut

In the free rotation, the effect of the sidecut of a ski is not a significant factor in terms of a source of resistance. But as the transverse aspect of the base plane of the ski acquires an angular relation with surface of the snow the resistance created by GRF acting at the  limit of sidecut at the shovel sets up an interaction between the rotational force applied to the inner wall of the boot shell adjacent the medial aspect of the head of the first metatarsal with the resistance created by GRF at the limit of sidecut at the shovel. In the graphic below I have connected the  2 dots of the platform ground effect problem with a line drawn between the two points.The graphic below shows a schematic of the mechanical aspects of the opposing moment or torque arms between the two dots that I connected in the preceding graphic. The inside edge below the head of the first metatarsal acts as a pivot in conjunction with the Center of Force applied 90 degrees to the transverse aspect of the base plane for the plaform to rotate about as the ski goes on edge.

As the base plane of a ski acquires an angular relationship with the snow the torque arm rotating the ski goes into what cane best be described as turbo torque boost. Whole leg rotational force continues to rotate the whole ski but the eccentric torque arm engages and applies a high torsional load that winds the body of the platform about the shovel. This mechanism has to be considered in the perspective of the of the inertia from the movement of the skier driving the cutting action of the shovel.  The graphic below shows the opposing how opposing torsional forces at the limit of sidecut and applied by the application of for by eccentic torque arm to the vertical shell wall by the medial aspect of the head of the first metarasal act to apply a upward force that extends to the outboard end of the plantar plane of the platform.  This is the mechanism that enables elite skiers to balance on their outside ski and initiate precise movement from from a dynamically stable platform.I first solved basic mechanics and biomechanics of the outside ski balance problem 30 years ago. The degree of difficulty was not great. Solving the problem took diligence and persistence in researching all the relevant aspects and identifying all significant forces and associated planes.

I’ll let the readers ponder the informaton in this for a while after which I will be happy to respond to questions and comments.

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.