Racer posts

THE SKI BOOT FLEX INDEX INSTABILITY PROBLEM

It has been known for decades that an unbalanced moment of force or torque will be present on the outside ski when the center of pressure of the load applied to the ski by a skier is acting along the center of the transverse axis of the ski where it is offset from GRF acting along the inside edge. Ron LeMaster acknowledges the existence of an unbalanced moment of force on the ouside ski in both The Skier’s Edge and Ultimate Skiing (Edging the skis). LeMaster states in Ultimate Skiing;

The force on the snow is offset from the center of the skier’s and creates a torque on it that tries to flatten the ski.

Ron didn’t get the mechanics right. But he correctly shows the unbalanced torque acting on the ankle joint. LeMaster tries to rationalize that ice skates are easy to cut clean arcs into ice with because the blade is located under the center of the ankle. While this is correct, ice skaters and especially hockey players employ the Two Stage Heel-Forefoot Rocker to impulse load the skate for acceleration. Hockey players refer to this as kick.

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP, Robert Colborne said:

…..In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

The load or weight of COM is transferred to distal tibia that forms the ankle joint. This is the lower aspect of the central load-bearing axis that transfers the load W from COM to the foot. What happens after that depends on the biomechanics. But the force will tend to be applied on the proximate center of the stance foot. This is a significant problem in skiing, (one that LeMaster doesn’t offer a solution for) when the ski is on edge and there is air under the body of the ski. The unbalanced torques will move up the vertical column where they will manifest at the knee against a well stabilized femur.

But this unbalanced torque creates another problem, one that is described in a paper published in 2005 by two Italian engineers (1.) that describes how this load deforms the base of the boot shell.

The Italian study found large amounts of deformation at mean loads of up to 164% body weight were measured on the outer ski during turning. The paper suggests that the ski boot flex index is really a distortion index for the boot shell. The lower the flex index, the greater the distortion potential.

For the ski-boot – sole joint the main problem is not material failure, but large amounts of local deformation that can affect the efficiency of the locking system and the stiffness of the overall system.

Values of drift angle of some degree (>2-3°) cannot be accepted, even for a small period of time, because it results in a direct decrease of the incidence of the ski with the ground.

My post GS AND KNEE INJURIES – CONNECTING THE DOTS (2.) cites studies that found that knee injuries are highest in GS in the shortest radius turns where peak transient forces are highest.

As shown in Figure 2a FR (sum of centrifugal and weight forces) and F GROUND (ground reaction force) are not acting on the same axis thus generating a moment MGR that causes a deformation of the ski-boot-sole system (Figure 2b) leading to a rotation of the ground reaction force direction. The final effect is to reduce the centripetal reaction force of the ground, causing the skier to drift to the outside of the turn (R decreases, causing the drift event).

An imperfect condition of the ski slope will emphasize this problem, leading to difficulties maintaining constant turning radius and optimal trajectory. The use of SGS ski-boot in competitions requires a particular focus on this aspect due to the larger loads that can be produced during races.

I have added a sketch showing that the moment arm M R created by the offset between the F Ground and F R is in the plane of the base of the ski where it results in an Inversion-lateral rotation torque.

The importance of sole stiffness is demonstrated with a simplified skier model…..…ski boot torsional stiffness with respect to ski longitudinal axis in particular is very important as it deeply influences the performance of the skier during turning…. A passage over a bump or a hollow may generate a sudden change in ground reaction force that may lead to a rapid change in the drift angle delta. The ski boot must be as stiff as possible going from the lower part of the boot to the ski (i.e. lower shell-joint-sole system)

As explained in the method section using the simplified model, values of some degree cannot be accepted, even for a small period of time, because the skier stability and equilibrium could be seriously compromised especially when the radius of curvature is small. A non perfect condition of the ski slope will emphasize the problem, leading to big difficulties for maintaining constant turning radius and optimal trajectory.

This excellent paper by the two Italian engineers concludes with the following statements:

Authors pushed forward the integration of experiments and modeling on ski-boots that will lead to a design environment in which the optimal compromise between stiffness and comfort can be reached.

The possibility of measuring accurately the skier kinematics on the ski slope, not addressed in the presented study, could represent a further step in the understanding of skiing dynamics and thus could provide even more insightful ideas for the ski-boot design process.

I first recognized the shell deformation, boot board instability issue in 1980, at which time I started integrating rigid structural boot boots into the bases of boot shells I prepared for racers. The improvement in ski control and balance was significant. The instability of  boot boards associated with shell/sole deformation with 2 to 3 degrees of drift at modest loads of up to 164% body weight has significant implications for footbeds.


  1. AN INNOVATIVE SKI-BOOT: DESIGN, NUMERICAL SIMULATIONS AND TESTING – Stefano Corazza 􀀍 and Claudio Cobelli Department of Information Engineering – University of Padova, Italy – Published (online): 01 September 2005 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/
  2. http://wp.me/p3vZhu-zx

WHY YOUNG TALENTED SKI RACERS FAIL AND EVENTUALLY QUIT RACING

The impetus for the subject of this post came from interest in my post FEATURE POST: MIKAELA SHIFFRIN: THE POWER OF SHEAR FORCE and an article (1.) in the  February 14, 2017 edition of Ski Racing by sports psychologist, Dr. Jim Taylor.

Taylor’s article, aimed at U14 and younger ski racers, points out that this is the age where the foundations are laid which often determine how well a racer does and especially how long they will remain in ski racing. Taylor cites statistics that show that qualifying for Topolino or the Whistler Cup (international competitions for 13-15 year olds) isn’t highly predictive of success even five years later. Specifically, only 25% of those who qualified for those race series later earned a spot on the USST. Moreover, 35% were off the elite ski racing radar within four years; some before their 18th birthday. The problem, that is the focus of Taylor’s article, is that parents enter what he calls the “too” zone, where the parents of kids, who are 11 years old or younger, have become “too” important to the parents who have become “too” invested in how their kids do (or don’t do).

The question I have is what events preceded parents getting to the “too” zone? I have seen more than one situation where a child who started ski racing at a very young age and who was considered to be a child ski racing prodigy, had a promising racing career unravel soon after reaching their teens. Why? What, changes happened that could have caused such a tectonic shift?

Let’s go back to beginning when a racer first showed promise. Many racers demonstrate prowess when they are only 4 or 5 years old. Often, one or both parents are elite skiers and one or both may have raced. In such a situation a young racer would have had an excellent role model that would have helped them  become comfortable by following one or both of their parents down the ski hill. But there are also other important factors in a young racer’s favour:

  • They are usually light weight.
  • They are usually short in stature.
  • Their muscles and skeleton are not yet fully developed.
  • Their feet are usually small.
  • They may lack fear.

A significant factor is that young racers often learn to ski in their mother’s ski boots or boots that would be considered too big for their feet if they were older. The implications? Young racers acquire a kinesthetic sense of how to stand in their boots in what I call the SR (stretch relfex) Stance (3. to 10.). As a consequence, they acquire dynamic stability that provides superior edge and ski control while enabling the myotatic stretch reflex balance response.

The authors of a Polish study on skier balance (2.) cite three types of postural reactions to external forces that disturb equilibrium and can cause the body to lose balance can be observed.

  1. The first reaction is the myotatic stretch reflex, which appears in response to changes in the position of the ankle joints, and is recorded in the triceps surae muscles. This is the earliest mechanism, which increases the activity of the muscles surrounding a joint that is subject to destabilization. Spinal  reflex triggered by the myotatic stretch reflex response causes the muscle to contract resulting in the stiffening of the surrounding joints as a response to the stimulus that has disturbed the balance. For example, changes in the angle of the joints of the lower limbs are followed by a reflexive (fascial) tensioning of adjacent muscles. The subsequent release of the reaction prevents excessive mobility of the joints and stabilises the posture once again.
  2. The next reflex in the process of balancing is the balance-correcting response, which is evoked in response to a strongly destabilising stimulus. This reactive response has a multi-muscle range, and occurs almost simultaneously in the muscles of the lower limbs, torso and neck, while the mechanisms that initiate the reaction are centrally coordinated.
  3. The last of the three types of muscular reaction is the balance-stabilising response. In a situation of a sudden loss of balance, a myotatic stretch reflex first occurs and is then is followed by a balance correcting response, which prevents or attempts to prevent a fall.

I call these balance responses green (postural reaction 1), orange (postural reaction 2) and red (postural reaction 3).

As young racers enter their teens, a number of significant changes have occurred.

  1. They are much heavier.
  2. They have grown in height
  3. Their muscles and skeleton are more developed.
  4. Their feet have grown larger and are more defined.

It is about this time in what is appearing to be a child’s promising racing career, that parents turn to the experts in a well intended effort to maximize their child’s chances of success. One of the key things parents often do is to get race boots for their child and have them customized with footbeds, form-fit liners and increasingly, heat molded shells. The process typically involves what is called race fit wherein ski boots are downsized to the smallest possible shell that the feet can be squeezed into. Custom footbeds or orthotics are considered an essential integral component of race fit because they prevent the foot from spreading and elongating. But this actually interferes with or even prevents the fascial tensioning process that enables dynamic stability and the myotatic reflex associated with the ultra high speed spinal reflex balance response (11).

No longer able to use the myotatic reflex (Green = Normal) balance response, the CNS shifts to Level 2 (Orange = Caution) or even Level 3 (Red = DANGER).

What happens next? The young racer starts to become intimidated by courses and conditions they were previously comfortable with. When this happens, their brain senses imminent danger of serious injury or worse and resorts to what I call the Survival Technique. Survival becomes the priority of the CNS at the expense of speed. Racers start losing ground to lesser racers. Not understanding the cause, parents and coaches can start pushing the child in an effort to get results. The more the child tries, the worse things get. When this happens, frustration sets in. Eventually, the child no longer wants to race. Defeated by their boots, the child eventually quits ski racing and takes up some other sport.

Unfortunately, this story is all “too” common. This is also one of the “toos”.


  1. What Young Ski Racers Need – http://www.drjimtaylor.com/4.0/young-ski-racers-need-dont-need/
  2. Influence of a nine-day alpine ski training programme on the postural stability of people with different levels of skills  (April 2016, Biomedical Human Kinetics (DOI: 10.1515/bhk-2016-0004) – Michał Staniszewski, Przemysław Zybko and  Ida Wiszomirska,  Józef Piłsudski University, Warsaw, Poland.
  3. THE SR STANCE: SURFACE EFFECTS,
  4. THE SR STANCE AND TOTAL BODY CORE INTEGRATION
  5. SR STANCE: ROUNDING THE BACK AND SHOULDERS
  6. THE SR STANCE: AFFECT OF JOINT ANGLES ON COM
  7. LEARN THE SR STANCE IN 3 EASY STEPS
  8. SR: ACHILLES-ARCH TENSION
  9. SR STANCE BASICS: ECCENTRIC MUSCLE POWER AND THE STRETCH REFLEX
  10. I-C-E: SR
  11. INNATE FLOW BALANCE

SKI RACING: AN UNLEVEL PLAYING FIELD

Ski racing is a lot like a lottery. With rare exceptions, equipment, but especially ski boots, can create what amounts to an unlevel playing field, one that can prevent racers from performing to their full potential. Because of the often significant physical differences in the feet and lower limbs from one competitor to another, the degree to which racers can be compromised by equipment varies greatly. This can be especially true if a racer happens to stumble upon the right combination. In ski racing, although luck is a factor, having the right feet with ski boots that allow them to function the way they were intended to can be everything.

Back in 1977 Lange was the only boot I had found of that allowed me to literally build a pair of race boots from the ground up. Like  Alan Trimble (boot tech for Lange USA), I was starting the process with a shell bottom and adjusting cuff side cant and forward lean angle of the rear spoiler to conform to the racer’s legs. In other words, I was building every pair of boots to each racer’s individual functional specification. But I was adjusting two other things that no one even seemed to even be aware were issues; net ramp angle and position of the ball of the foot and big toe in relation to the ski edge underfoot.

Net ramp angle is the inclination of the sole of the foot in relation to the surface of the snow. It affects the muscles a racer can use and especially the ability to apply force to the fore body of a ski and the ability to activate what I call the auto processes of edge control and turning forces. Two factors contribute to forward inclination; 1) the ramp angle of the boot board that the sole of the foot is supported on and , 2) the angle created by the heel and toe plates of the ski binding. I don’t know where they are today, but back in 1977 ramp angles were all over the map in both boot and binding brands and even models. Because of this, both the base angle of the boot sole plate and binding had to be considered as a unit in determining net ramp angle. Although few if any were aware of this issue, to me it explained why a racer’s skiing and results sometimes went downhill when they changed to a different ski binding or boot or worse, changed both. The generally accepted assumption, one that persists even today, is that a good athlete can ‘adjust’ to their equipment. Because of this, boots are not considered a factor. Fast skis are everything. While it is true that a talented racer can ski in just about any equipment, adaptation comes at a cost. The cost comes in the form of a reduced ability to perform.

If a NASCAR team were to announce that they intended to enter a stock sedan right off the showroom floor with no modifications in the Daytona 500 and that they would be competitive because their driver would ‘adapt’ to the lack of power and handling of the car they would be laughed off the circuit. If a world class sprinter announced that he or she that was going to compete wearing stiff leather dress shoes and that they would ‘adapt’ to the considerable limitations of the footwear no one would take them seriously. Yet most coaches, and even some racers, tend to minimize the role of the ski boot. When a racer dominates his or her competitors it is attributed to ‘exceptional talent’ and they are elevated to the status of a god with mystical powers.

In a November 13, 1990 Globe & Mail article appropriately titled, Boyd putting best foot forward,  Whistler’s Rob Boyd describes how the boots I had built for him changed his skiing and renewed his enthusiasm for racing. Said Boyd, “In Chile, I skied easily. It was fun again. It rekindled my love of skiing. Everything was so smooth…….” Boyd went on to say how he used to only worry about a solid fit in his boots, how he skied from the ankle up and that after skiing in the boots I had made for him he realized, “how much the foot can be used and should be used.”  Earlier that year my coaching counterpart, Glen (Meister) Wurtele (head coach of the men’s team), had summoned me to action in the World Cup Wars to work on Rob’s boots. After preparing his boots in my Whistler shop I flew to Portillo, Chile to hook up with the Canadian Ski Team who were training on the summer glacier.

The next day I was standing on a knoll about half way down a 45 second downhill course behind the lodge with the team coaches when Boyd took his first run in his new boots. When he came into view Boyd was skiing so differently that the coaches standing with the Meister and I didn’t recognize him. They were sure it was another racer. As he drew near, it became obvious that it really was Rob. When the coaches began to speculate as to what to what had caused such a dramatic change in Boyd’s technical skiing Wurtele said, “It’s his new boots”. The coaches were emphatic, ski boots could not possibly affect a racer’s skiing to that degree. Yes, they really can. In future posts I will explain why.