knee injuries


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.



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.


When the FIS reduced side cut on GS skis, many were confused as to why GS was singled out over other disciplines. A new injury study (1)  sheds some light on this issue. And while the study falls short of actually identifying the injury mechanism, it provides enough clues to connect the dots. The study, Here are some key statements.

Competitive alpine skiing is considered to be a sport with a high injury risk. Injury rates per competition season and per 100 World Cup (WC) athletes were reported to be 36.7, with the knee being the most frequently affected body part.

“The injury rate was highest for giant slalom,

Associating the number of injuries per hour in WC skiing with skiers’ mechanical characteristics, injuries in super-G and downhill seem to be related to increased speed and jumps, while injuries in giant slalom may be related to high loads in turning.

 It has recently been found that many injuries occur while turning, without falling or being the result of a crash.

“Recently been found?” Seriously? Skiers have been experiencing knee injuries for years without falling and while apparently skiing in control. Without seeing any data, I can predict with confidence that, with rare exceptions, knee injuries are always associated with the outside ski of a turn. The other issue I can state with confidence is that if the moments of force (torques or twisting forces) acting on the outside ski are not tending to rotate the ski into the turn, they will be tending to rotate the ski out of the turn. In the mechanics of an outside ski on its inside edge, there can be no neutral. There are implications to moments of force that tend to rotate the outside ski out of the turn that are associated with speed and the length of any moment arm that exists between the reaction force at the inside edge and the center of the force applied at the sole of the foot of the skier. The force of gravity causes skier to accelerate in the fall line and decelerate as the skier crosses the fall line. (Force = Mass x Acceleration). The shorter the radius of the side cut of a ski, the longer the potential moment arm.

Skiers are turning for approximately 55% of the time in downhill, 80% in super-G and 93% in giant slalom. Moreover, it was shown that small turn radii might be related to an increased injury risk in giant slalom since they provoke the skiers to use their full backward and inward leaning capacities, and thus skiers have less buffer if an additional factor causes an out-of-balance situation.

Note the reference to additional factor.

Downhill had the largest mean turn radius, while giant slalom had the smallest mean turn radius.

Out-of-balance situations themselves are known to be a critical part of typical injury mechanisms, such as the ‘slipcatch’ and ‘dynamic snowplow’.”

What constitutes balance?

This is the central issue. Postural responses on a single limb involve two coordinated and interdependent balance strategies or synergies; 1) a plantarflexion strategy that resists the tendency of a disturbing force (typically gravity) that tends to topple the vertical column supporting CoM forward by causing the ankle to dorsiflex and, 2) an inversion strategy that resists the tendency of a disturbing force to topple the vertical column supporting CoM sideways (ergo, cause the foot to evert or turn away from the centre of the body). In terms of the latter strategy, if one is standing on the right foot, the disturbing force will tend to cause the foot to rotate about it’s inner or medial aspect into the ground. This is called eversion. In skiing, the external forces would tend cause the ski to rotate into the turn. The postural responses of the skier’s balance system act to control the degree of eversion. Eversion is mechanically coupled through a joint in ankle complex called the subtalar joint. When the angles at the knee are relatively small, the leg as a whole will rotate about its vertical axis on a 1:1 ratio with rotation of the foot about its long axis in eversion. Elite skiers don’t consciously cause these rotations to occur. The external forces cause them to occur. The balance system of the elite skier controls the rotations.

 The ‘balance problem’ is that there is only a very short window when the outside ski of the new turn is flat on the snow between edge changes in which to set up the biomechanics that engage the external forces that drive the moments of force into the turn. Among other things, this requires that the skier be able to rapidly dorsiflex their ankle so they can move CoM forward; ergo so the foot can pronate. It is relatively easy to prevent the foot from pronating, but extremely difficult to stop the foot from supinating especially under the influence of a high instantaneous peak force. The configuration that engages the external forces must be established before the outside ski acquires a significant edge angle and especially before the external forces start to increase.

The ‘slipcatch’ technique is the worst possible way to engage the inside edge of the outside ski. The outside ski is slipping sideways as the angle of inclination of the skier and associated edge angle are increasing until a point is reached where the inside edge catches or locks up. Should the slipping ski encounter a frozen ice formation the ski could suddenly decelerate. Should this happen, the offset or moment arm between the inside edge and the centre of the force applied at the sole of the foot will tend to rapidly rotate the foot about it’s long axis out of the turn. This will also rotate the tibia on its vertical axis against a well-stabilized femur.

Comparing the mean and minimal turn radii between discipline, it is evident that giant slalom has substantially smaller turn radii than super-G and downhill. Additional analysis of the data showed that the radial component is the main contributor to the increased FGRF in giant slalom. Thus, the combination of small turn radii and speed leads to larger mean and maximum FGRF in giant slalom compared with super-G and downhill. Furthermore, in giant slalom, skiers’ balance might be challenged simultaneously by small turn radii and high forces.

Injuries in giant slalom were linked to high loads in turning;

It gets worse.

First, the model for the computation of FGRF does not capture the high frequency force components and, therefore, might underestimate the work load (impulse), in particular for giant slalom.

In other words, the actual impulse forces in a GS turn could be much higher than the model predicted.

Furthermore, giant slalom includes a larger number of turns (52.0±3.5) compared with super-G (40.0±3.5) and downhill. Hence, skiers have to find balance in turning more frequently in a run and thus might be more often susceptible to balance-related mistakes in turn initiations.

 The implications are that the skier needs to be able to set up the processes responsible for dynamic equilibrium in the outside leg at the initiation of every turn.


sidecut radius + length of moment arm + instantaneous peak moment of unbalanced force out of the turn on the outside foot + high GRF

  1. Mechanics of Turning and Jumping and Skier Speed Are Associated With Injury Risk in Men’s World Cup Alpine Skiing – A Comparison Between the Competition Disciplines: Matthias Gilgien, Jörg Spörri, Josef Kröll, Philip Crivelli, Erich Müller: Br J Sports Med. 2014;48(9):742-747′, is available for free at Medscape (






When talking about ski boots, the big buzz word in the industry is energy transfer. According to the experts, the better the boot, the more energy it will transfer inferring that transferring energy is both desirable and beneficial. But is it? And what is meant by energy transfer? It is not clear what the objective of energy transfer is or how it relates to effecting control of a ski.  As far as I know, no one has offered an explanation. Yet it is consistently stated as the broad objective of a ski boot. In addition, there seems to be consensus among the experts that the foot functions best when it’s joints are immobilized in the ski boot. Presumably this would serve to maximize energy transfer.


  • energy |ˈenərjē|noun ( pl. energies ) – Physics, the property of matter and radiation that is manifest as a capacity to perform work (such as causing motion or the interaction of molecules).
  • transfer |noun |ˈtransfər|  – an act of moving something or someone to another place.

It follows from the above definitions that energy is a general capacity to perform work with no particular sense or control over the process. Therefor, energy transfer is the act of moving energy from one place to another. If energy transfer is the primary function of the ski boot as the experts seem to agree on, then the boot will act to transfer energy from the skier to the snow surface. In accordance with Newton’s Third Law, the snow will transfer an equal and opposite amount of energy back to the boot. Where does this energy go? Up the vertical column of the body.

What follows is an excerpt from a letter of support offered by G.Robert Colborne for my nomination for the Gold Medal in Applied Science & Engineering in the 1995 British Columbia Science & Engineering Awards. Dr. Colborne’s area of expertise is the human lower limbs in particular quantifying mechanical moments of force (ergo; torques) around the joints of the lower limb, and the mechanical power generated or absorbed by the muscle group crossing these joints. At the time he wrote his letter, Dr. Colborne was the assistant Professor of Anatomy at the University of Saskatchewan, Canada

“Recent considerations of safety in skiing highlight the importance of dissipating (NB; preventing the transfer of energy) ground  reaction forces (i.e.; snow reaction forces) through the joints of the foot and ankle, which are multi-axial and able to absorb significant energy without sustaining injury. The next more proximal joint (ergo; closer to the torso) is the knee, and this structure is implicated all too often in skiing injuries, where forces are transmitted (ergo; energy is transferred) by rigid boots that restrict ankle and foot movement”.

  • dissipate |ˈdisəˌpāt|verb1 [ no obj. ] disperse or scatter: the cloud of smoke dissipated.• (with reference to a feeling or other intangible thing) disappear or cause to disappear:

Dr. Colborne went on to state:

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

In summary, the lower limbs protect the knee by dissipating ground reaction forces through the joints of the foot and ankle. The stated objective of the ski boot is to immobilize the joints of the foot and ankle and transfer energy. Where doe the transferred energy manifest itself? in the knee joint. There are other implications of restricting movement of the joints of the foot and ankle that I will discuss in future posts.


In LESS REALLY IS MORE I talked about how I gone in a direction opposite from that of the industry after my perfect fit experience with Mur. I was now removing material from ski boots instead of adding material and expanding shells where necessary to make room for the structures of the foot. While this seemed to generally have a positive effect on skier balance and the ability to control skis, especially edging, removing material from the sides of the boot liner  exacerbated the fact that in the majority of cases I was encountering the shell wasn’t loading the instep of the foot. The reason for this turned out to be  that there was a void between the top of the tongue of the liner and the inner surface of the shell over the forefoot. This was allowing the foot to move upward into the void space or unload from contact with the sole plate (aka boot board) in response to changes or perturbations in ground reaction force. I coined the effect Separation Anxiety because of the alarm bells it was setting off in the skier’s balance system.

After I became aware of this effect, I started doing experiments to try and understand how it was affecting a skier’s balance and ability to control their skis. While riding ski lifts with foot rests (the old slow chairlifts) I would let one of my feet drop off the foot rest and try and feel what was happening with my foot and leg inside the boot when the foot unloaded from the boot board. At that time, I wasn’t thinking in terms of trying find a solution for knee injuries. I saw this as an issue that would be addressed by refinements in bindings which at that time were rapidly evolving. Through my experiments I had come to the realization that the unloading and reloading of the sole of the foot with the boot board, such as occurs when a skier is moving over irregular terrain, was setting off a chain-reaction of physiological events that were creating balance issues. Although I didn’t know exactly how, this unload/load cycle  seemed to be placing stress on the knee. But my focus was trying to find a way to reduce the effect on skier balance. In effect, I was trying to achieve a net improvement in skier balance by reducing negative balance artifacts.

The standard solution in those days was to attempt wedge the heel with heel or L-pads inserted in the liner. The objective was to keep the foot from lifting. I tried this approach. But I  found it didn’t work as advertised. The pads invariably caused problems with the Achilles tendon or they prevented the heel from seating in the back of the shell, or both. The latter had the effect of making the liner shorter and the boot hell to put on. I was looking for a better solution. But it wasn’t until 1980, while working on Podborski’s boots, that I came up with a device that eventually led to my being granted US Patent Number 4,534,122.


When I started skiing in 1970, the buzz was all about the new safety bindings. Debates raged in magazines and ski shops over which binding was the best as in the safest. After years of skiing being perceived as dangerous because of the incidence of broken legs, a new era had arrived with the introduction of a generation of sophisticated bindings. This created the perception that it was finally safe to go out play on the ski hills. But as the sound of snapping leg bones faded into the background it was replaced by an even grimmer sound; the popping of knee ligaments, in particular, torn ACLs. Before the introduction of the rigid plastic ski boot, few skiers had ever heard of an ACL. That was about to change.

It was about the time that I started working with National Ski Team members in 1977 that I began to hear of racers suffering knee injuries. Knee injuries seemed to start with a trickle. I can’t even recall hearing of a recreational skier suffering one. Like most skiers, I believed that the new bindings had addressed the injury issue. Even after knee injuries started to increase in frequency I thought it only a matter of time before refinements would be made to ski bindings and that this would be the end of them. As the popping of ligaments got more frequent, panic seemed to set in in the industry. Skiing had entered a period of vigorous growth. The last thing it needed was a good news, bad news story as in, “The good news is that the rigid plastic boot has made skiing easier. Now for the bad news…..”. As best I can recall, it was around 1980 that a team of spanish orthopaedic surgeons published a study linking the introduction of the rigid plastic boot to knee injuries noting that the incidence appeared to be rising in lock-step with sales of the boot. A classic problem-solving strategy is to go back to the time when a problem first emerged and look for anything that changed. In this case, the most significant change was in the boot. Meantime, those with expertise in biomechanics were pointing out that by stiffening the ankle the boot was sending the forces of skiing upward to the relatively weak knee.

In retrospect, it seemed like it should have been obvious that encasing the foot within what amounts to an orthopedic splint would act to transfer force up the leg. It’s ironic, if not erroneous, that the industry, even today, talks about the boot transferring energy to the ski as if this were the end game of skiing. The reality is that unless the ski industry has repealed Newton’s Third Law (which is doubtful), if a skier were to transfer energy to anything through the boot it would be through the stack of equipment between the sole of the boot to the source of Ground (or Snow) Reaction Force at the snow. This being the case, according Newton’s Third Law which states; “For every action there is an equal and opposite reaction”, the snow will transfer an equal amount of energy through the stack of equipment back up the skier’s leg to the knee. The issues are way more complex than a simple transfer of energy. But I will start with the simple and obvious then build from here.

The question is, “Given the established reputation of skiing as being a dangerous sport prior to the introduction of the rigid plastic ski boot and the fact that skis attached to the foot and leg act as force multipliers, did anyone consider the implications of trying to immobilize the foot and leg within a rigid plastic ski boot?”