Knee Injury posts

WHY STANCE TRAINING IS ESSENTIAL

When readers click on my blog address at skimoves.me, analytics give me a hierarchy of the countries with the most views and the most popular posts in ascending order. This helps me identify which content resonates most strongly with viewers and which content draws a blank.

As I write this post, the top five countries are the US followed by Croatia, the United Kingdom, Slovakia and France.

The most viewed post today is THE SHOCKING TRUTH ABOUT POWER STRAPS; far and away the most popular post I have published to date. But the most important posts by far that I have ever written, A DEVICE TO DETERMINE OPTIMAL PERSONAL RAMP ANGLE and STANCE MUSCLE TENSIONING SEQUENCE EXERCISE barely sputtered in comparison. This strongly suggests that far from just some small gaps in the knowledge base skiing is founded on, massive craters exist.

Arguably the most important aspect of skiing is a strong stance. Any variance in the fore-aft angle of  the plane of support under the feet and the plane of the base of the ski has significant impact on stance. Yet these subjects are barely blips on the Doppler Radar of the ski industry.

Since I started the dynamic ramp angle assessment project a few weeks ago I have found that when asked to do so, it is rare for a skier of any ability to be able to assume a strong ski stance in an off the ski hill environment. Even when a skier  skis with a relatively strong stance, they seem to lack a sense of what a strong stance feels like. Because of this, they lack the ability to consciously replicate a strong stance. If asked to do so, they would be unable to coach a skier in the sequence of events that I described in my last post

In the dynamic ramp angle assessment project, I  have also observed that skiers with with a boot/binding ramp angle greater than 2.8 degrees appear to have become accustomed to the associated unstable, dysfunctional feeling and identify with it as ‘normal’. Before I can test them, I have to spend time coaching them into the correct stance because it feels unnatural to them.

When I go back and forth between a strong functional stance on a flat, hard level surface to a stance on the dynamic ramp angle device set to an angle of 4 degrees, I can get close to the same angles of ankle, knee and hip. But when I do, I feel strong tension, stiffness and even pain in my mid to lower back which is  common in some skiers and even racers.

Based on results to date with the dynamic ramp angle device, it appears as if strong skiers ski best with ramp angles close to zero. But depending on their sense of balance and athletic ability, they may have a wide range in which they sense little difference on the effect of ramp angle until they approach the upper limit of stability. While they may be able to ski well with a ramp angle close to the maximum limit of stability, ramp angles much above 1.2 to 1.5 degrees may not offer any benefits. This can only be tested on skis where balance is tested by dynamic forces which cannot be replicated in a static setting.

Issues affecting skier stance were discussed in detail in my post, THE SHOCKING TRUTH ABOUT POWER STRAPS. Here are the excerpts I posted from the chapter on The Ski Boot in the book, The Shoe in Sport (1989), published in German in 1987 as Der Schuh Im Sport– ISNB 0-8151-7814-X

“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”. – Biomechanical Considerations of the Ski Boot (Alpine) – Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

“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., (14 degrees) 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded”.

“Previous misconceptions concerning its 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.

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

It has been over 40 years since international authorities on sports science and safety raised red flags concerning the adverse effects of ski boots design and construction on skier stance, balance and the potential to cause or contribute to injury. It is time that their concerns were taken seriously and acted on. Research on stance and the effect of such things as zeppa and delta ramp angles is urgently needed.

 

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

RX FOR SORE KNEES? LOW FAT

The recent article by Jackson Hogen, A (Slight) Swing Back to Sanity What We Learned at the SIA – http://www.realskiers.com/NEWSLETTERS/sanity.htm), 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  (https://skimoves.me/2016/01/17/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 (https://skimoves.me/2014/11/26/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.

 

THE KNEE INJURY MECHANISM PARADOX

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 – https://skimoves.me/2014/05/29/gs-and-knee-inju…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…………………..

THE TRUTH IS OUT THERE

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

 

FAT SKI SYNDROME

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.

GS AND KNEE INJURIES – CONNECTING THE DOTS

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.

dot-dot-dot-dot

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 (www.medscape.com).

 

 

 

 

THE EFFECT OF LIFT PLATES – CONTINUED

In skiing,  a myriad of complex issues are associated with riser plates that elevate the foot above the base of s ski. FIS regulations for 2013-2014 permit a maximum stack height of 100 mm total between the base of a ski and the sole of a racer’s foot.  Because of the complex nature of the issues, I am going to use a simplistic model to explain the primary effects of riser plates.

NOTE: Check FIS regulations for current stack heights.

Platform shoes, high heels and similar footwear that elevate the sole of the foot above the supporting surface, tend to be make the wearer susceptible to ankle sprains when a lateral thrust or cutting move is made off the stance foot. The high centre of the ankle joint in relation to the supporting surface makes the ankle susceptible to lateral rotation and twisting above the contact surface of the sole when angular forces are applied. As the ankle progressively rolls over, the twisting force dramatically increases due to the resulting over-centre mechanism. This is called an inversion sprain because the sole of the foot turns inward towards the centre of the body. In the days when skiers used low-cut leather boots, the experts, who could make their edges hold on hard pistes, would call it ‘falling off the edge’ when their outside foot and ski rolled downhill, away from an edge set.

The model I am going use for my explanation assumes that the feet and legs have been aligned and fixed in the magical neutral position. Technically, a neutral position means that the joint that underlies the subtalar joint (what most people think of as the ankle joint) has been effectively rendered non-functional.

In a neutral position it doesn’t really matter whether the foot is a block of wood or a marvel of anatomy, the force applied by the weight of the body resulting from the force of gravity applied on the mechanical line will impress a portion of the weight to the proximate transverse centre line of the foot. For the following explanation the foot will be considered as solid entity like a block of wood. In a neutral position, both feet would be under the femoral heads. For the sake of simplicity I am showing one foot with the mechanical line vertical to gravity. Static references are shown with dashed red lines.  In skiing the force are more complicated.

The sketch below shows a schematic model of a right leg. As shown in my previous post on this subject, the mechanical line has a ball joint at the top (pelvis) with the subtalar joint below the ankle at the bottom. Both joints allow for rotation in the plane facing the reader. The subtalar joint acts in two coupled planes. For the following explanation only the effects associated with rotation of the foot about an axis below the edge of its inner aspect are considered. Fa is the force applied to the base of foot from the force of gravity acting on the mechanical line. Ma is the length of the moment or torque arm resulting from a line from the pivot axis that is perpendicular to the force vector of Fa.

1In the sketch below, the foot model has rotated counterclockwise 10 degrees about the pivot from the original configuration. The original configuration prior to rotation is superimposed in light grey over the new rotated configuration. Note that the mechanical line as a whole and the centres of the ball and subtalar joints have dropped in relation to the pivot point. The vector of force Fa has shifted to the left and is now angled towards the left hand side of the base of the foot model. The moment arm, Ma, that drives the rotation, has grown longer.  This is called an over-centre mechanism because reversing or unwinding the rotated configuration requires that the load that created it be overcome. As the rotation progresses the mechanics associated with reversing direction become increasingly unfavourable.

2In the sketch below a lift plate has been added to the bottom of the foot model. The previous rotated configuration is superimposed in light grey over the new configuration. Note that the mechanical line as a whole and the centre of the subtalar joint have not changed significantly in relation to the position in the previous sketch. But the vector of force Fa has shifted significantly further to the left and is now almost at the left hand edge of the base of the foot model. In addition moment arm Ma, that drives the rotation, has grown significantly longer.

Screen Shot 2014-03-18 at 1.36.12 PM

The sketch below compares the original unrotated configuration (light grey)  to the rotated configuration with the lift.

4Preventing the outside foot of a turn from pronating by fixing the foot in neutral or otherwise obstructing pronation with arch supporting insoles and/or injected and heat formable liners ensures that any force applied by the balls of the feet will be on the outside turn aspect of the inside edge of the outside ski and that the tibia cannot rotate into the turn. The mechanics described above would be similar to that of a situation where a skier gets inside, ends up losing contact with the snow of their outside ski then re-establishes contact. Although the constraints imposed on the foot and leg by the structures of a rigid ski boot would probably make 10 degrees of rotation unlikely, having the applied force on the outside turn aspect of the inside edge of the outside ski will almost guarantee mechanics similar to those described above.  Clearly lift plates can have a positive effect but only if the moments forces acting on the ski are going into the turn and can be dynamically balanced by muscular effort mediated by the skier.