Knee Injury posts


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 –


In The Shoe in Sport, the detached, objective assessment of the conventional ski boot articulated in Part 6, The Ski Boot, by preeminent authorities on biomechanics and safety served to support my conclusions of the past 10 years that little of what forms the basis of knowledge in skiing can be defended. This made me acutely aware of the dangers of going forward with a concept for a new ski boot that could not defended with a validated hypothesis based on principles of applied science and especially data captured during actual ski maneuvers that could be readily and consistently reproduced by others.

Key critical comments made in Part 6 of The Shoe in Sport, (first published in 1987 in Germany as Der Schuh im Sport – ISNB 0-8151-7814-X ) follow below as a prelude to the discussion of the Birdcage format as a basis for flex options for the FreeMotion ski boot and ski boots in general.

Biomechanical Considerations of the Ski Boot (Alpine)

Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

  • Correct flexibility (of the boot shaft) is not the whole answer. The functional anatomy of the ankle joint must also be an important consideration. If the axis of flexion of the ankle joint is not precisely in line with the axis of flexion of the shaft, an unfavorable lever ratio will ensue, and the advantages of flexibility will be lost.
  • If flexion resistance stays the same over the entire range of flexion of the ski boot, the resulting flexion on the tibia will be decreased. With respect to the safety of the knee, however, this is a very poor solution. The increasing stiffness of the flexion joint of the boot decreases the ability of the ankle to compensate for the load and places the entire load on the knee.
  • Improvements in the load acting on the ankle make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.
  • From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions. These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier.
  • The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).
  • It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).
  • It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

Kinematics of the Foot in the Ski Boot – Professor Dr. M. Pfeiffer – Institute for the Athletic Science, University of Salzburg, Salzburg, Austria

  • The shaft of the boot should provide the leg with good support, but not with great resistance for about two-thirds of the possible arc, i.e., (12 degrees) 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded.
  • Because of its effects on the foot, the arc described by the shaft is divided into a “lead segment” and into a later “lever segment”.
  • Forward sliding of the foot should not be possible. There should also be no loss of contact of the sole and no decrease in the “feel”.
  • Previous misconceptions concerning its (the boot shafts) role in absorbing energy must be replaced by the realization that shaft pressure generates impulses affecting the motion patterns of the upper body, which in turn profoundly affect acceleration and balance.
  • Correct positioning of the foot is more important than forced constraint and “squeezing” the foot. This will prevent the misuse of the ligaments and weakening, particularly of the fibular musculature and ligaments. (This will also explain why even competitive skiers can suffer ankle sprains while engaged in light athletics or even just in everyday activities.)
  • When the lateral stability of the shaft (the leg) is properly maintained, the forces acting in the sagittal direction should not be merely passive but should be the result of active muscle participation and tonic muscular tension. If muscular function is inhibited in the ankle area, greater loads will be placed on the knee.
  • 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.
  • The medical requirements with respect to sports should not be construed as criticism of the boot industry. It is hoped that they are contribution to the development of a ski boot designed along anatomical principles. This goal has not yet been achieved.

Ski-Specific Injuries and Overload Problems – Orthopedic Design of the Ski Boot –  Dr. med. H.W. Bar, Orthopedics-Sportsmedicine, member of GOTS, Murnau, West Germany

  • Few forms of athletics place as high demands on the footwear used in their performance as alpine skiing. It (the ski boot) functions as a connecting link between the binding and the body and performs a series of difficult complex tasks.
  • Investigations by Pfeiffer have shown that the foot maintains some spontaneous mobility in the ski boot. Thus the total immobilization by foam injection or compression by tight buckles are unphysiologic.
  • The boot must assure freedom of mobility to the toes. This is accomplished by having a large enough inner shoe. Only in the case of major congenital or post traumatic deformities should foam injection with elastic plastic materials be used to provide a satisfactory fixation of the foot in the boot.

Sports Medical Criteria of the Alpine Ski Boot – W Hauser, P. Schaff, Technical Surveillance Association, Munich, West Germany

  • Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small. This results in a static, stiff run. It does not correspond at all to the ideal of a wide range of mobility in the area of the knee and ankle, which was proposed and taught during early alpine ski lessons. Even the best diadactic (patronizing) methodology is not always successful in imparting to the student the full range of motion. The lack of proper technique seem so often is not due to a lack of ability, but to an unsatisfactory functional configuration of the shaft in so many ski boots. This is particularly true in models designed for children, adolescent and women.
  • In the future, ski boots will be designed rationally and according to the increasing requirements of the ski performance target groups.

The comment Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland 30 years ago has turned out to be prescient:

  • Improvements in the load acting on the ankle make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.

Unfortunately, Stussi’s warning does not appear to have been heeded.

In an article called Getting Serious About Skier’s Knee (Ski Tech, October 1993), Andy Bigford states, “Horror stories about ACL injuries are a dime a dozen” and “Victims tell of the cost involved (the whole package can run to $50,00 per injury), the difficulties of rehab, the lost work time and the fear of never being able to ski again”. Bigford goes on to point out that not everyone is convinced that the stiff boot-aggressive ski combination is totally at fault; that ACL injuries in the late ’80s and early ’90s came when softer (flex) and ‘more forgiving’ rear entry boots were popular.

Bigford ends his article by stating that the (knee) problem needs to be solved without scarring away skiers, but “If something isn’t done, they (skiers) will have plenty of reason to be scared”.

More than 20 years later, the cover heading What’s New With Knees (December 2016 Ski Canada magazine) is the lead-in to a feature article called Tear and Repair. The article header  states, “With knee injuries so commonplace, especially among skiers, the medical world is constantly updating procedures and surgical techniques for the big fix. The take away message from Tear and Repair  appears to be that the solution to knee injuries is not addressing the cause, it is repairing the resulting damage.

In the same magazine, a comment under Short Turns (Save Your Knees) states: “If you haven’t had a serious knee injury, you likely know someone who has”.  The take away message appears to be that knee injuries are part of skiing. Get used to it!

If my next post, I will discuss how the knowledge gleaned from The Shoe In Sport influenced my thinking on the design criteria for a new ski boot, in particular, shaft configuration and resistance to the rotation while creating an awareness of the need to be able to defend my design criteria with principles of applied science data acquired during actual ski maneuvers.



The recent article by Jackson Hogen, A (Slight) Swing Back to Sanity What We Learned at the SIA –, has implications for where I am going with Whistler Ski Pro, Matt. Hogen’s article prompted me to delay my post on the problems with ski boots and add my views that resonate with his.

In his article,  Hogen relates what he saw at the annual ski show in Denver, Colorado as a welcome trend; new ski models for next season will be less than 85 mm underfoot. After reaching widths that could best be described as grossly obese, skis have gone on a low fat diet. Next season, slim is in. Fat is no longer where it’s at, if it ever really was. Why is this trend important and why does it matter?

Like Hogen, I have witnessed the effect of wide skis on technique and it is not good. Hogen expresses the view that once skiers are on boards so broad they can’t comfortably tip them to a high edge angle, the chances of them ever acquiring this foundational skill are virtually nil. I agree. But the issue is more than just high edge angles. It is the ability to apply forces with the foot at turn initiation that rotate or tip the new outside ski onto its inside edge, period. It’s not just skis, boots are equally to blame for the demise of technical skills. In my post, FAILURE TO CARVE  (, I reported on the feature article, To Bend a Ski, in January 2016 Ski Magazine. In the article, PSIA instructors estimated that 9 out of 10 skiers don’t carve their turns. From what I have witnessed on the slopes of Whistler-Blackcomb, these past few seasons, that number is probably more like 1 in 100 and includes a lot of ski pros.

Hogen also commented that most skiers remain unaware that a wide ski on hard snow poses inherently higher risks of knee discomfort and increases the odds of a serious knee injury. In the fall of 2014, Kim Hewson, MD, and I coined the term Fat Ski Syndrome. When Dr. Hewson warned about the stress on the knee caused by fat skis in the November 2014 post, FAT SKI SYNDROME (, he and I were lone voices calling out in the ski world about the potential dangers of fat skis on hard pistes. Now Hogen has chimed in.

The big question is why are most skiers unaware of the inherently higher risks of knee discomfort and increased odds of serious knee injury associated with the use wide skis on hard snow? Perhaps, because no one is telling them? It is hard to find even innocuous products today without encountering warnings on the package about the potential for the product to cause injury, no matter how slight. This is true of even toothpicks and cotton swabs. Skiing and especially ski equipment, seems to be operating in a parallel universe or perhaps a product liability vacuum where the more radical a product, the more ski magazines and ski enthusiasts welcome it as cutting-edge innovation. The only apparent upper limit to the width of fats seemed to be the width between a skiers feet. Risk? What’s that?

The problem is that when those who endorse or promote a product, don’t disclose what should be, or at the very least, what ought to be, an obvious risk like potentially injurious torque on the foot and leg, from skis over a certain width underfoot, the consumer reasonably assumes there are no risks. For the same reason, when those who endorse or promote products like ski boots that have been criticized by independent scientists such as Dr. Stussi, Dr. Pfieffer, Dr Schaff and others, as not founded on principles of anatomy and that loading the ankle sends the stress of skiing up the leg to the rather delicate knee, and those who endorse or promote products, fail to respond to, let alone address these issues,  the consumer reasonably assumes that such products are supported by appropriate science and that there are no significant risks associated with their use.

Hogen ends his article by stating that the reinvestment in the Frontside and Technical categories inspires hope that Americans will rediscover the joys of riding a narrower ski, such as speed control, trajectory management, balance and timing, more succinctly summarized as “skill.” Unfortunately, I tend to agree with Hogen’s statement that once skiers learn a coping mechanism; one that does an end run around sound foundational technical skills, the odds of a skier acquiring such skills in the future is virtually nil.

In working with skiers like Matt, Morgan and racer X, they often have video documentation, notes and records of equipment dating back years. In the case of racer X, I have been provided with copies of hundreds of videos dating back more than 10 years. These have enabled me to study the effect of problematic equipment that precipitated problematic coaching. I was even able to clearly see evidence of a back injury on the racer’s technique. A series of adverse changes over a number of years derailed a promising career to the point where the racer wanted to quit. As Matt recognized, talented athletes are able to fake it and look like the real deal. Race results tell a different story. They can’t be faked.

As I will illustrate with Matt, fixing a skier’s boots is one thing. Erasing the skiing hard drive in the brain and reprogramming and rebooting it with a technique of not just sound, but superior technical skills, is a long, slow process; one that is seldom a straight line. It is unlikely that the average skier has the motivation to pursue such an arduous journey. This being the case, ski technique will continue its downhill descent in spite of any low fat ski diet.



In response to a feature article, ‘Busted knees and broken legs’, that ran recently in one of Whistler’s weekly newsmagazines, I wrote the letter to the editor that follows below.

As seems typical of articles on the subject of knee injuries, the take home message to the reader is that most knee injuries are caused by skier error.  Skiers fall improperly or try to pull themselves forward from a sitting back position. This creates what is called a drawer shift mechanism that pulls the femur back against the tibia at the knee joint rupturing the ACL. While this mechanism is definitely a factor, as Tone Bere of the Oslo Sports Trauma Research Center noted, knee injuries do occur before a fall or without a racer falling after a knee injury is sustained. When a knee injury does occur before a fall, the knee injury invariably causes the fall. This was also noted in the paper I cited in my post GS AND KNEE INJURIES – CONNECTING THE DOTS –…necting-the-dots/

Articles such as the one that appeared in the Whistler newsmagazine, also infer that progress has been made in equipment and that skiing is much safer than it was decades ago. This position is contradicted by studies such as Bere’s that found that 1 in 3 WC racers are injured each year. What I have never seen mentioned in any consumer article on ski injuries, is the fact that not only has the modern plastic ski boot never undergone any product safety testing intended to determine the effect on the consumer that I am aware of, but that plastic boot was cited by a number of preeminent authorities in the Shoe in Sport, published in 1987, as being unphysiologic and not designed along anatomical principles.

In the Shoe in Sport, Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland, commented,

The increasing stiffness of the flexion joint of the boot decreases the ability of the ankle to compensate for the load and places the entire load on the knee. Improvements in the load acting on the ankle make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.

Dr. med. H.W. Bar, Orthopedics-Sportsmedicine, member of GOTS, Murnau, West Germany, commented,

Investigations by Pfeiffer have shown that the foot maintains some spontaneous mobility in the ski boot. Thus the total immobilization by foam injection or compression by tight buckles is unphysiologic. Only in the case of major congenital or post traumatic deformities should foam injection with elastic plastic materials be used to provide a satisfactory fixation of the foot in the boot.

In effect, the comments of experts in the field such as Dr. E. Stussi, Dr. med. H.W. Bar, Professor  Dr. M. Pfeiffer, were putting the industry on notice that design of the plastic ski boot was problematic.

Seth Masia provides an excellent overview of the ‘anything goes so long as it sells’, marketing approach of the ’70s in his article,  Almost Hits, Mostly Misses, Skiing Heritage 2005 – Google books

A copy of The Shoe in Sport was given to me in 1988 by two German radiologists who were both keen skiers. They were aware of the deficiencies of the conventional boot and had witnessed the injuries. They heard that I was working on a new ski boot design and offered their assistance in designing a boot based on principles of functional anatomy. As the University of Ottawa papers noted, technologies did not exist prior to 1997 that enabled the study of the effects of a ski boot on a user during actual ski maneuvers.  In going forward with the ski boot project in 1991, I was aware of the issues raised in Shoe in Sport. Given that I was starting from a clean sheet of paper so to speak,  if I could not overcome the problems and produce a design based on principles of functional anatomy, one that would be supported and even endorsed by accredited scientists, I could not go forward. I had to get it right. Otherwise, there was no point in proceeding.

The problem for the industry is that I not only succeeded, but the Birdcage studies identified key markers of ski control and balance that can easily be put into algorithm that can assess sound ski technique and be applied to teaching softwares that guide skier learning. In the last 5 years, the introduction of small motion and force sensors has exploded. Used in conjunction with a smartphone app, technologies are now possible that will not only capture and interpret the exchange of forces between the foot of a user and the snow, but act in the capacity of a black box or flight recorder in acquiring a digital record of all events associated with an injury. I foresaw this possibility back in 1991.

As my letter to the editor says…………………..


In response to the cover feature Pique ran Jan. 21 I wanted to share that in 2013, PhD candidate, Tone Bere of the Oslo Sports Trauma Research Center, asked in Mechanisms of injuries in World Cup alpine skiing” (March 2013), why one out of every three elite alpine skiers is injured during the five month ski season.

Bere’s research found that ACL injuries that occur before a fall, or without falling, develop rapidly due to high skiing speeds and that there is no single solution that will prevent them.

This is due to the presence of phantom torques on the outside leg of a turn that can bend and rotate the leg and shear off the ACL far faster than current binding technologies can detect, let alone react to.

The predisposition of a skier to knee sprains is exacerbated by the fact that tightly-fitted rigid plastic ski boots transfer the forces of skiing up the leg to the knee. The tighter, more precise the fit, the greater the transfer of forces from the ski to the knee.

Prior to the widespread acceptance of the new rigid, plastic ski boot in the early ’70s, knee injuries in skiing were rare. Broken legs occupied centre stage. The introduction of the safety release binding changed that. But the jubilation from the dramatic decline in broken legs had barely subsided when a worse problem began to emerge — severe knee sprains, especially to the ACL.

Contrary to what many believe, there has never been support in sound principles of science for the idea of clamping the foot and leg in what amounts to an orthopedic splint and then attaching a large lever to the boot and applying stress to it.

When the authoritative Shoe In Sport was published in 1987, preeminent experts raised red flags about the effect of the new ski boots on knee injuries. Dr. M. Pfeiffer of Institute for the Athletic Science spoke from the literal epicenter of the ski world at the University of Salzburg when he said, “The ski boot and its 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.”

His comments were intended to spur the development of a ski boot designed along anatomical principles, a goal that remains to be achieved even today.

Meantime, Dr. E. Stussi, member of GOTS, chief of Biomechanical Laboratory ETH stated, “Improvements in the load acting on the ankle (meaning a tighter fitting boot) make it very likely that the problems arising in the rather delicate knee joint will increase.” In other words, Dr. Stussi stated that if the industry kept improving the fit of ski boots, knee injuries would increase.

But the industry kept right on improving the fit of the ski boot and knee injuries increased exactly as Dr. Stussi had predicted. Rather than heed the warnings of experts like Dr Stussi and Dr. Pfeiffer, the industry represented the perfect fit, as the Holy Grail with the apparent end objective of transferring 100 per cent of the potentially injurious forces of skiing to the knee,

In 1991, at the request of Canada’s most successful alpine skier, Steve Podborski, I agreed to try and develop a new ski boot. But I agreed on the condition that we would do the prudent and responsible thing, engage scientists with the appropriate expertise to provide oversight and guidance.

My mission was to develop a ski boot that made skiing easier, but more important, made skiing safer. Podborski had competed and won in 1980 on some of the world’s most difficult downhill courses mere months after reconstructive knee surgery using an innovative in-boot technology I had invented. It reduced the stress on his knee to the point where he could compete and win whereas that was impossible with a conventional boot. I knew I was headed in the right direction.

But I wanted accredited experts to confirm that I was. Our company, spent close to $140,000 on studies intended to prove or disprove my theory. The studies proved my theory to be correct.

In 1995, I was nominated for the gold medal in the categories of applied science and engineering in the B.C. Science and Engineering Awards by the industrial technology advisor to the National Research Council of Canada. In order to go forward, a nomination must garner support from a candidate’s peers in the field. In his letter of support, Dr. Robert Colborne, assistant Professor of Anatomy at the University of Saskatchewan, an expert in the human lower limbs, said the following.

“Recent considerations of safety in skiing highlight the importance of dissipating ground reaction forces through the joints of the foot and ankle, which are multi-axial and able to absorb significant energy without sustaining injury.

“Mr. MacPhail’s design enables the musculature of the lower limb to absorb these forces before they are directed into the ligaments of the knee, thus protecting this relatively stiff tissues from injury.”

In his letter of support, Alex Sochaniwsky, P. Eng., the biomedical engineer who designed the research vehicle, wrote the software (where none existed) and conducted the studies said,

The design and development strategies used by David MacPhail are very holistic in nature, placing the human system as the central and most critical component in the biomechanical system. His intent is to maximize human performance and efficiency, while foremost preserving the well-being and safety of the users and minimizing biomechanical compromises.

That is where I drew a line in the snow in 1991, “foremost preserving the well-being and safety of the users.”

I am still waiting for others to join me.

David MacPhail



Today marks an anniversary for me, albeit a bitter sweet one. It was 20 years ago to the day that a patent application I filed on November 29, 1993  was published world wide. The material in the application represents what I believe to be the first disclosure of the mechanism that Kim Hewson and I describe as a state of inversion stress. The application resulted in the awarding of US Patent No. 5,459,949 on October 24, 1995 and subsequent international patents. US Patent No. 5,459,949 formalized and made public the validation of the findings of my hypothetical model of the mechanics, biomechanics and physics of skiing as confirmed with data from the Birdcage research vehicle and my first 2 turns on rock hard piste on a steep slope of Whistler’s summer glacier.


The reason this achievement is bittersweet for me is that it confirmed my perception that much of what is held sacred in skiing is based on eminence, not evidence and with little or no supporting principles in science. The concern that immediately arose out of the Birdcage studies  was the emerging trend away from conventional ski geometries with their standardized width underfoot in the order of 7o mm. Marketing is about statements or stories that resonate with the consumer. Light (aka Lite) resonates. Long (as in ski length) doesn’t. Width as in wider does. It was obvious to me that the odds were great that marketing would influence a trend towards wider skis, especially wider skis underfoot and that this direction would find enthusiastic, albeit unquestioning, support. In recent years, width underfoot has exceeded 100 mm and has now passed 150 mm.

Wide skis or Fats as they are now affectionately referred to, make perfect sense on untracked snows which are closer to the physical qualities of liquids or even gases. But on groomed conditions, and especially hard pistes, skis that are wide under foot will unavoidably and unequivocally cause Fat Ski Syndrome because of the long unbalanced inversion lever. Unfortunately, fats are being widely promoted as all mountain or big mountain skis implying that they are suitable for all conditions. This is not the case. Fats have a place in skiing. But that place is not on groomed conditions and especially not on hard pistes. When I included a description of the mechanism of inversion stress as a source of trauma to the fragile knee in the patent application, I did so with the fervent hope that it would raise a red flag in what should have been an obvious issue, one that if ignored, would eventually place the design of ski equipment under a microscope. It appears as if my hopes were in vain. Fat Ski Syndrome has drawn attention to width of the ski underfoot as an obvious and undeniable source of lower limb  imbalance and, with it, the erosion of stable edging technique leading to compromised skier performance and  knee trauma.

Cols 59-60

Inversion stress in 549

The patent figures below represent a best case scenario, one in which W is transferred to the ball of the foot and from there to the ski. Over a certain width threshold this is no longer possible. When skiing on fats, W will be transferred to the proximate transverse centre of the ski. This will result in an unbalanced inversion moment on the outside ski and foot in the order of 50 mm for a ski of 100 mm width underfoot and 75 mm for a ski of 150 mm width underfoot: almost 5 times the length of the potential central axis load transfer moment arm for a typical GS ski.


Figs 63-66

Figs 67-69


Figs 70-72


Figs 70-72


Here is a video clip that shows what Inversion Stress looks like. Inversion Stress with its co-star, Varus Thrust, makes a regular appearance at all levels of skiing. It is a star performer in World Cup Alpine events and even in the Olympics. The sweeping in-out pumping arc of the knee of the outside leg is the classic signature of Inversion Stress.


by Kim Hewson, MD

In medicine, a syndrome is a group of signs and symptoms that are consistently observed that are characteristic of a single condition. As a retired orthopaedic surgeon and a ski instructor, I am often sought out for consultation on injuries suffered by friends, clients and fellow instructors. Over the past two seasons, a symptom of medial (inside) knee pain has emerged in some skiers with a common pattern of fat ski use on groomed terrain.

The common sign observed has been tenderness over and below the joint line of the knee associated with mild soft tissue swelling and tenderness. The skier symptoms are complaints of progressive soreness and difficulty initiating turns on the inside edge of the outside ski. After skiing, walking and nighttime discomfort are common. Some report temporary pain relief with ice and anti-inflammatory agents. The skier often will not return to skiing for several days and upon return uses a narrow waisted ski.

This syndrome with fat ski use occurs only when using fats on groomed or rough terrain, not in powder terrain. We define fat skis as skis >100mm underfoot.

Biomechanics of Inversion stress and varus thrust: a two-phase oscillating micro-trauma to the knee.

Phase I – Compression: The leveraged outside foot is forced into inversion stress. As a result, the outside ski flattens on the snow and the leg rotates slightly externally. The medial boot cuff adds subtle pressure [varus thrust] to the inside lower leg creating a varus stress or bowing compression force at the knee joint.

Phase II – Strain: In repeated attempts to recover the flattening ski, the skier corrects ski inversion by active foot eversion and internal leg rotation. As a result, medial hamstring muscle insertions just below knee are repeatedly put under strain resulting in tendonitis and bursitis.

Kim Hewson is an Orthopaedic Surgeon and former Director of Orthopaedic Sports Medicine and Team Physician at the University of Arizona. He is currently a veteran Telluride Ski School Alpine Instructor in his 15th season and Staff Trainer in the Biomechanics of Alpine Skiing.