Ski Boots

TIGHTLY FIT SKI BOOTS COMPROMISE SKIER BALANCE AND CONTROL

In reviewing recent articles on ski boot fitting I encountered the same perfect fit of the boot with the shape of the foot and leg and ski boots must be tightly buckled for good balance and control narrative fabricated decades ago to justify the interference with the actions of the joints of the ankle and leg created by the rigid plastic shell ski boot.

When the first rigid shell plastic ski boots were introduced, the field of biomechanics, as it exists today, was in its infancy. Even until recently, the human foot was modelled as a rigid block which was consistent with the shoe last theory and the theory that the perfect fit of ski boots with the foot and leg of the user is the best option for skiing. Further support for the support and immobilize theory came from the vilification of pronation arising out of the misapplication of Root’s Neutral theory (1.)

By the time the authoritative medical text, The Shoe in Sport, was published in 1987, the knowledge of the biomechanics of the human foot had progressed to the point where tight-fitting ski boots and loading the ankle joint were recognized as unphysiologic.

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. (2.)

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.(2.)

Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small.(2.)

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. (3.)

The articles on ski boots in the Shoe in Sport identified the objectives I was seeking in my efforts to design a ski boot based on principles of what is now referred to as neurobiomechanics. By the time I had formulated my hypothetical model of the mechanics, biomechanics and physics of skiing in 1991 I understood the need to restrain the foot in contact with the base of a ski boot and maintain the position of the foot’s key mechanical points in relation to the ski while accommodating the aspects of neurobiomechanical function of the foot and leg required for skiing. This was the underlying theme of the US patent that I wrote in February of 1992.

Existing footwear does not provide for the dynamic nature of the architecture of the foot by providing a fit system with dynamic and predictable qualities to substantially match those of the foot and lower leg. – US patent No. 5,265,350: MacPhail

On June 2, 2013 I published the post TIGHT FEET, LOOSE BOOTS – LOOSE FEET, TIGHT BOOTS (4.) in which I describe how attempts to secure the foot to a ski in a manner that interferes with the physiologic mechanisms that fascially tension and stiffen the structures of the foot that render it dynamically rigid actually reduce the integrity of the joint system of the lower limbs and hips resulting in a looser connection with the ski.

Studies done in recent years confirm the role of the active state of the architecture and physiology of the foot to postural control and balance.

These findings show that rather than serving as a rigid base of support, the foot is compliant, in an active state, and sensitive to minute deformations. In conclusion, the architecture and physiology of the foot appear to contribute to the task of bipedal postural control with great sensitivity. (5.)

The science of neurobiomechanics and the understanding of the mechanisms of balance and the role of the sensory system in human movement is accelerating. The time is long overdue for skiing to abandon it’s outdated concepts and align it’s thinking with the current state of knowledge.


  1. IS ‘SUBTALAR NEUTRAL’ SKIINGS’ HOUSE OF CARDS? – https://wp.me/p3vZhu-2mn
  2. 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
  3. Biomechanical Considerations of the Ski Boot (Alpine) – Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland
  4. https://wp.me/p3vZhu-2K
  5. Foot anatomy specialization for postural sensation and control

THE IMPORTANCE OF STRONG HEALTHY FEET IN SKIING

My work with skiers spanning more than 4 decades, in conjunction with what I have learned over the past three years and papers I have recently read, has led me to the inescapable conclusion that the best equipment available, including ski boots that constrain the foot with minimal interference to foot function, can never overcome the limitations of unhealthy, weak feet.

In working with elite skiers at both the World Cup and recreational levels, it quickly became apparent to me back in the ’70s that these skiers consistently had stronger, tighter feet than lesser skiers. They also had feet whose compact, tight physical characteristics allowed them to attain a good level of function in most ski boots of the day right out the box.

The photos below are of the foot of a female racer who learned to ski at a young age in her mother’s ski boots when her feet were much smaller than her mothers’. The photos were taken when the racer was 20.

wedge-1

When she started racing at 5, she quickly became a phenomenom. She did not outgrow her mother’s boots until she was 11. So the critical period in the development of her feet took place under minimal constraint from her ski boots. Note the ‘natural’ (see footnote 1. below) wedge shape of her foot. There is some evidence of structural damage to her small toe. This could have occured after her she was put in tightly fit (constraining) ski boots at age 12 that were at least one size too small.

wedge-2

Here is the same foot with an outline of a typical boot liner overlaid in red.

liner-outline

Like most, until recently I reasonably assumed that the feet I have today were the feet I was born with; that good skiers were born with good  feet and there was nothing that could be done if one didn’t win the foot lottery at birth. I knew of nothing to indicate otherwise until I started to connect with the rapidly emerging barefoot/minimal shoe camp and the wealth of information on the foot damaging, often debillitating effects, of footwear, especially when one is subjected to foot damaging footwear at an early age. It was only then that I realized that the problems that prevented me from skiing as well as I thought I should didn’t start when I changed from low cut leather boots to the new higher, rigid, all plastic boots. My problems actually started when I was fit with my first pair of ‘orthopedically correct’, stiff-soled, supportive shoes when I was about two. The plastic ski boots only made the damage caused by these shoes, which persists even today, obvious.

An article in the August 9, 2010 edition of the UK newspaper, The Guardian, Why barefoot is best for children contains the following statement.

https://www.theguardian.com/lifeandstyle/2010/aug/09/barefoot-best-for-children

Tracey Byrne, podiatrist specialising in podopaediatrics, believes that wearing shoes at too young an age can hamper a child’s walking and cerebral development. “Toddlers keep their heads up more when they are walking barefoot,” she says. “The feedback they get from the ground means there is less need to look down, which is what puts them off balance and causes them to fall down.” Walking barefoot, she continues, develops the muscles and ligaments of the foot, increases the strength of the foot’s arch, improves proprioception (our awareness of where we are in relation to the space around us) and contributes to good posture.”

When I was fit with my first pair of shoes as an enfant, the big buzz phrase was ‘orthopedically correct’. This implied that orthopedic research had identified a signifcant problem, one that required intervention in the form of supportive shoes in order to ensure that an infants’ feet developed ‘properly’ and that the orthopedic community was behind this initiative. The cover story was that infants feet were weak and incapable of supporting the weight imposed on them in learning to walk. This could cause stress on bones that could lead to permanent deformation of the structures of the feet and legs. Orthopedically correct shoes with stiff soles and sidewalls that supported the foot would ensure proper and ‘normal’ (‘normal’, not ‘natural’ see footnote 1. below) development. This implied that parents who failed to put their infants into orthopedically correct shoes were guilty of child neglect.

Unfortunately for me, my mother had dated a guy in high school who opened a shoe store near our home. He was very much into orthopedically correct shoes. After he sold my mother on the idea she purchased every pair of shoes for me, all orthopedially correct, from his store right up until I was about 5 or 6 years old. The impact on my feet and my childhood was significant.

By the time I entered elementary school, my gait was so impaired that I could not walk in a straight line. Instead, I walked with a distinct, pronounced stagger that was so obvious that my school mates made fun of me. I was clumsy and unsteady on my feet. I fell a lot. My school mates started to refer to me as ‘the gimp’.  (see footnote 1. below)

As hard as I tried, I was never able to make any sports team I tried out for. By the time I reached junior high school, I had given up trying. The interesting paradox was that I could easily outpace all of my friends on a 2 wheeled pedal bicycle. I found out why when I had fitness testing  in 1988. I had a VO2 max of 66 which is phenomenal. So, it wasn’t a lack of stamina or athletic ability that was the issue. It was clearly the damage caused to my feet by the orthopedically correct shoes I was put in as a child.

The Guardian article, Why barefoot is best for children, notes that a study published in 2007 in the podiatry journal, The Foot, suggests that structural and functional changes can result from the foot having to conform to the shape and constriction of a shoe and that the younger the foot, the greater the potential for damage. Since baby feet are structurally different from adult feet, research shows that footwear can, indeed, obstruct proper foot development.

Tracey Byrne: “The human foot at birth is not a miniature version of an adult foot. In fact, it contains no bones at all and consists of a mass of cartilage, which, over a period of years, ossifies to become the 28 bones that exist in the adult human foot. This process is not complete until the late teens, so it is crucial that footwear – when worn – is well chosen.”

In the same article, Mike O’Neill, a consultant podiatrist and spokesperson for the Society of Chiropodists and Podiatrists, said that he believes that too many parents treat their children as fashion accessories and choose shoes on their attractiveness or coolness, rather than their ergonomics. Byrne agrees, but points out that it’s not just parents but manufacturers who have a responsibility. “People see particular shoe styles on sale in the shops – whether it’s a high heel for toddlers, a ‘Crawler’ (a shoe for babies not yet walking) or a cute Havaiana flip flop, with no more than an elastic band at the back … And they think ‘Well, if it’s on the shelf, it must be OK,'” she says.

“As more and more evidence comes to light regarding the importance of going barefoot and the potential dangers of bad footwear, the ‘barefoot model’ will have to become more widely adopted by shoe manufacturers,” says Byrne.

The Bottom Line

“……… the bottom line is the more we use our feet and toes, the stronger they will become. By wearing less of a shoe, we will use our toes to stabilize the foot against the ground and by activating these muscles more often, they become stronger. Simple concept, yet we’ve been missing it for over 40 years by focusing on building the perfect shoe. We already had the perfect shoe, our own foot. We just needed to wake it up and use it. By feeling the ground, our foot can tell the brain which muscles to activate and the foot responds by absorbing shock and working more naturally- the way it was intended to work.  (see footnote 2. below)

“We’ve come to regard the way we dwell permanently in shoes as normal and natural [but it is] anything but,” explained John Woodward, an Alexander Technique teacher who has allegedly been barefoot for 25 years.” ((see footnote 3. below)

All of the preceding applies to ski boots.

In my next post I will explain why going barefoot as much as possible will strengthen the feet but barefoot alone is unlikely to correct the damage done, especially if it was done when feet were developing.


  1. Why Shoes Make “Normal” Gait Impossible: How flaws in footwear affect this complex human function By William A. Rossi, D.P.M. – http://www.unshod.org/pfbc/pfrossi2.htm
  2. STUDY DEMONSTRATES VIBRAM FIVEFINGERS WILL STRENGTHEN THE FOOT. http://www.drnicksrunningblog.com/study-demonstrates-vibram-fivefingers-will-strengthen-the-foot/
  3. Why barefoot is best for children  – https://www.theguardian.com/lifeandstyle/2010/aug/09/barefoot-best-for-children

DETERMINING OPTIMAL BOOT SHAFT/BOOT BOARD RAMP ANGLE

A follower of skimoves posed the following;

“I’m trying to determine my optimal boot shaft angle and ramp angle given my physiology – i.e. what works best for me. I’ve done some of this work on my own by adjusting binding ramp angle (last season). What is interesting is the shaft angle of my newer Head vs. Lange boots”.


As discussed in recent posts, the importance of the cumulative effect of boot board ramp (zeppa) and binding ramp (delta) angles on stance is becoming increasingly recognized. Although binding ramp angle (delta) typically varies widely from one binding to another in recreational bindings, boot board ramp angle seems to be coming into line with functional reality in race boots. Reliable sources in Europe tell me that the boot boot board ramp angle in World Cup boots is in the order of 2.6 degrees. After I eliminated the arch profile in boot boards for a 23.5 Head race boot, I calculated the ramp angle at 2.35 degrees, a far cry from the 5 degrees claimed for the boot boards. I calculated the boot board ramp angle of an Atomic race boot of a local ski pro at a little over 2 degrees. I have also been told that shim kits are available for all race bindings that allow the delta angle to be zeroed.

The default barefoot ramp angle for humans is zero. It has been unequivocally established that anything more than a small amount of ‘drop’ (heel higher than forefoot) in footwear will have a detrimental effect on stance, balance and movement patterns. This especially true for balance on one foot, something that is fundamental to sound ski technique.

Elevating the heel relative to the forefoot will cause the muscles in the back of the lower leg to contract. Over time, these muscles will become chronically shortened. The key muscles affected are the calf muscles; the gastrocnemius and soleus. But the small muscles that stabilize the knee and pelvis are also adversely affected, not a good thing.

If I want to find the optimal boot shaft angle and compare the shaft angle of two or more boots, I start by making the boot boards perfectly flat with the transverse aspect horizontal with the base of the ski. I set the boot board ramp angles for both boots at 2.5 or 2.6 degrees. Since it can take a long time for the body to adapt to even small changes in ramp angle underfoot, the angle is not hypercritical.   I have settled on 2.5 to 2.6 degrees of total ramp (zeppa + delta) as an arbitrary starting point. Although there appears to be a positive effect of a small delta binding angle in SL and GS, I prefer to work with a zero delta angle initially since a positive or negative delta affects the shaft angle of a ski boot.

When moving from one boot model to a different model or to another boot brand, the first thing I do is remove the boot boards and calculate the ramp angles with the top surface monplanar. If the boot boards are not flat, I plane or grind them flat. If a new boot is to be be compared to a current boot with a boot board angle of 2.5 to 2.6 degrees, I modify the boot board of the new boot so it has the same angle as the current boot.

Next, I compare the shells and the angles of the spine at the back of the shaft of each boot. Even if the angles of the spines of the boot shells appear similar, there is no guarantee that what I call the static preload shank angle (more on this in a future post) will be the same.

A quick check of how the structure of the shell of the new boot is affecting the functional configuration of the foot and leg compared to the current boot, is to put the current boot on one foot then put the new boot shell with the liner from the current boot on the other foot. If a significant difference is perceived, the source is the new shell.

At this point, it may be apparent that there is a difference in the shank angles of the left and right legs when comparing the current boot to the new boot. But whether one boot is better than the other or even if one boot eanables the optimal static preload shank angle would not be known. I will explain how I identify this angle in my next post. For now, study this recent video of Lindsey Vonn starting off by skiing in what appears to be a strange ski stance. In fact, the exercise Vonn is doing is a familiar routine to me, one that I do before I start skiing – https://www.facebook.com/LindseyVonnUSA/videos/10154672700589728/

Why is Vonn skiing this way? What is she trying to do?

Also, check out this screen shot of Anna Fenninger. Note her compact, forward in the hips stance.

fenninger-1

Finally, watch this video in which Brandon Dyksterhouse compares Shiffrin and Fenninger – Shiffrin GS Analysis – https://youtu.be/phchHWwDhdY

What do Vonn and Fenninger have in common? Why?

 

BOOT BOARD (ZEPPA) RAMP ANGLE VS. BOOT SIZE

It is becoming clear, the angle the boot board (zeppa) establishes for the skier’s foot relative to the ground, is vitally important to the ability to balance and function on skis. Therefore, knowing boot board angle (ramp angle) and skier preferences should become part of every boot setup and purchase. Yet there appears to be a fundamental error in the understanding of ramp angle in boots. This is evident when someone states, for example: “The head Raptor has a ramp angle of 4.5 degrees”. The statement may only true if the angle is linked to the boot size.

There are production controls applied to boots just as controls and standards are applied to all other things mass produced. In boots, it means the first prototypes are designed to a specific size (generally Mondo 26). All other sizes are scaled up or down from it. Each Mondo size is a change of one centimeter. Zeppas are fixed in both rear foot and forefoot height in the prototype standard. Only the zeppa length changes as boot size changes.

It means; if the prototype size is twenty six, the zeppa of a twenty three is three centimeters shorter with the same toe and heel heights. Therefore, the ramp angle of the zeppa of a twenty three is steeper than the ramp angle of the zeppa of a twenty six. Since many women’s boots are scaled from the twenty-six Mondo standard, boot set-up problems can be more difficult to solve for women than for men. This is the reason women are more adversely affected by boot configuration than men. The graphic below compares the boot board (zeppa) ramp angles of larger and smaller boots to the standard Mondo 26 boot.

Zeppas Mondo 26

 

Bindings obviously confer the same effect, since with most models heel height is greater than toe height. As the heel and toe change distances from each other according to boot size, binding angle (delta) changes and its angle is additive with the boot ramp angle to determine gross equipment angle as shown in the graphic below. Binding delta has a double effect, since as delta increases boot cuff angle relative the ground also increases.

Zeppas Mondo 26 bindings

When talking about boot boad ramp, we should include the boot size or always use the ramp of the Mondo 26 as a known reference.


Lou Rosenfeld has an MSc. in Mechanical Engineering with Specialization in Biomechanics earned at the University of Calgary Human Performance Laboratory. His research was titled, “Are Foot Orthotic Caused Gait Changes Permanent”.

While at HPL, he assisted with research on the effects of binding position for Atomic, and later conducted research for Nordica that compared Campbell Balancer established binding position to the Nordica factory recommended binding position.

Lou is one of the invited boot-fitters on the EpicSki forum “Ask the Boot Guys” and has authored articles on boot fit, balance, alignment and binding position for Ski Canada, Ski Press, Super G, Calgary Herald, and Ski Racing, USA. He is a CSIA Level 2 instructor and CSCF Level 1 coach. He currently resides in Calgary where he owns and operates Lou’s Performance Centre. A selection of his articles may be found at www.Lous.ca.

RACE BOOT SETUP: BODY ALIGNMENT CHECK

Since this is the time of year when racers tend to either make changes to their boots or change to a new boot brand, I will describe the initial steps in the process (and it is a lengthy process) that I follow in setting up ski boots for a racer. Although the process is similar for any skier, it may be less structured and less intensive depending on the desired end result.

As a general rule, the closer a racer’s boots are to creating an optimal functional environment for the feet and legs (lower limbs), the more critical any changes become. Optimal is a moving target in that ski boots have such a significant effect on racer/skier function that the body is constantly making small changes in an effort to maximize performance. In my experience, that the farther a racer/skier’s boots are from optimal, the more unlikely that any changes, even in the wrong direction, will create a noticeable impact on performance. But when the boot/binding/skis system is close to optimal, even small changes can have a large impact. In this situation, changes in boot board ramp angle of a tenth of a degree or changes in the thickness of an insole of a mm are usually readily perceived by an elite racer/skier.

Where to Start? The body

The process starts with a quick visual assessment of the racer’s posture to see if any obvious issue such as significant duck feet (toed out) of one of both feet are present. The ideal Plumb Line Alignment of the major body segments and joints is shown in reference books such as Muscles Function and Testing, Third Edition by Kendell and McCreary. The most mechanically efficient alignment occurs when the gravity line of a plumb bob as viewed from the side falls through the back of the ear lobe and passes through the center of the shoulder joint, just behind the center of the hip joint and just in front of the centers of the knee and ankle joints.

If any structural issues are obvious, I recommend that the racer/skier have alignment and kinesologic assessments done by certified medical professionals. This is especially important if a skier or racer has been injured. Often, full function has not been completely restored.

I am not talking about the static alignment usually done in ski or boot fit shops. I am talking about an assessment process that evaluates and corrects the processes responsible for the maintenance of dynamic alignment, generally referred to as balance. It is superior balance that gives elite racers and skiers the edge over others.

One of the several resources in Whistler that I personally use is Dr. Andrea Bologna, DC, CACCP of the Village Centre Chiropractic & Massage Centre. Dr. Bologna wrote the following as an overview of the process that she uses to assess Body Alignment (Structural).

Body alignment (structural) assessment gives a skier a baseline to determine any deviations from “normal” in terms of positioning and alignment of the structure of the body.  Correcting misalignments will give a skier the edge on not only skiing or any other activity pursued by taking stress off of joints and muscles, improving posture and allowing the body to move freely with the correct biomechanics.

The following components make up the Body Alignment (Structural) Assessment

Step 1: A complete history is taken that includes past injuries, activities, etc.

Step 2: Body posture is assessed to determine how the body lives in space.

Assess main postural alterations and compensatory changes.

Anterior-Posterior Posture:

  • The pelvis may show a high ilium on one side and/or rotational component to the sacrum which may stem from changes in the spinal structure or in ankle or knee alignment and biomechanics.
  • One shoulder may be elevated and/or a rotational component observed to the rib cage.
  • Head tilt and/or a rotational component may be observed.

Lateral Posture:

  • An increase or decrease in the lumbar lordosis and/or thoracic kyphosis may be observed.
  • Knees may be hyper-extended.
  • One or both shoulders may be rolled forwarded.
  • Head forward position may be observed.

Step 3: Two computerized spinal scans are performed (thermal and EMG or electromyography) to determine which areas of the spine have nerve irritation or interference and which muscles are working harder or pulled tighter due to physical stress.

Step 4: A 3D digital foot scan is performed to determine changes in the arches of the feet, compensating posture affecting the knees and pelvis, and weight imbalance between the right and left sides of the body.

Step 5: A palpatory spinal assessment will determine spinal misalignments causing altered structure and resulting aberrant biomechanics.

The body evaluation process is key to determine what changes need to be made to correct the body structurally to allow for ideal biomechanics during ski training and racing.  The evaluation will determine the most specific way to adjust the spine and related joints for lasting results in the shortest time possible.

Dr. Andrea graduated from Parker University in Dallas Texas with a doctorate of chiropractic in 2005. She completed a 180 hour certification in Chiropractic Pediatrics from The Academy of Chiropractic Family Practice and the Council on Chiropractic Pediatrics. She is Webster Certified through the International Chiropractic Pediatric Association.

Dr. Andrea specializes in pediatrics and pregnancy, and sees a variety of world class athletes as well as weekend warriors. She moved back to BC to work together with her brother Dr. Michael Bologna after living in Texas for 10 years, resides in Whistler, and enjoys downhill and cross country biking.

In addition to body alignment, it is also important to assess foot function. There are many excellent resources that I will discuss in future posts.

In my next post, I will discuss where I start the process of racer boot setup.

 

 

 

THE BOOT BOARD FACTOR

While the Ottawa researchers did not explore this aspect, they correctly identified that equipment, including custom insoles, technical skills and ski technique might explain why the pressures recorded under the heel and the head of the first metatarsal of some instructors were much higher than the pressures seen in the same locations in other instructors.  The University of Ottawa studies are the only ones I am aware where the researchers considered the effect  of what is known in research as uncontrolled variables on their findings. Poor technique and interference with the function of the foot and leg caused by the ski boot can ensure that COP remains under the heel.

Although boot board ramp angle and shape have an undeniable impact on the function of the feet and lower limbs, as evidenced by the photographs below of a sampling of boot boards, there does not appear to be any continuity, let alone any standard for boot board ramp angle and the form of the surface that interfaces with the sole of the foot.

1

 

2

IMG_5560

3

IMG_6187

4

IMG_0144

When the effect of  binding ramp angle, which appears to have even more variation than boot board ramp angle, is added to ramp angle equation to arrive at Net Ramp Angle, the possible combinations that make up Net Ramp Angles becomes unlimited and can range from as little as two to as much as ten degrees.

As if the lack of any apparent standard for boot board and binding ramp angles were not causing enough of an impact on skier/racer performance, there is a factor that appears to be compounding the issue by introducing a layer of inconsistency; boot base shell deformation under loads typical of recreational skiing.

I will discuss boot base shell  deformation in a future post. In my next post I will propose a starting point for a boot board standard.

THE SHOCKING TRUTH ABOUT POWER STRAPS

When used appropriately, power straps can be very effective in decelerating forward movement of the shank when transient perturbations in snow reaction force exceed the limits of the balance system. But Power straps are typically used to provide a very snug fit of the leg with the rear spine of the boot shaft by reducing space between the calf muscle and the rear spine. As shaft buckles are increasingly tensioned, volume and fore-aft space within the confines of the shaft is proportionally reduced. But by acting directly on the leading edge and wrapping around the sides of the shank, a securely tightened power strap can severely limit ankle dorsiflexion by fixing the forward most position of the shank and eliminating any free space between the calf muscle and the spine of the boot shaft. By binding the shank to the structurally stiffest element of the shaft, the spine acts to rigidly splint the shank while impinging on the soft tissue that is normally effective in absorbing energy from transient shock loads from perturbations in snow reaction force.  The unavoidable consequence of a securely tightened  power strap is that flexion of the ankle joint is greatly constricted or substantially eliminated.

The two photos below use a skeleton leg to graphically simulate the effect of a single lap power strap on shank position without the shaft buckles being operated. In the left photo, neither the shaft buckles or the power strap are operated. In the right photo, only the power strap is operated. Operating the power strap with moderately light force had the effect of reducing the angle dorsiflexion of the shaft by 11 degrees.

Pwr Diff

Rigidly connecting the leg to the ski has its origins in the widely held view that the leg should be used as a lever with which to apply force to the ski. Power straps became ubiquitous in race boots when self-turning, fixed-radius skis spawned the technique of skiing on two skis and holding the skis on edge with the legs and later, the short-lived slip-catch technique that placed high loads on the lower limbs. But interfering with ankle flexion and especially shank position in relation to the proximate center of the head of the first metatarsal, can have serious implications.

A skier in motion across the surface of the snow is standing on a moving platform that is simultaneously being perturbed in two planes (saggital and frontal). The situation is similar to those that exist in balance studies conducted in laboratories where a subject is standing on a platform that is suddenly tilted without warning, perturbing the subject’s balance. The difference is that in skiing the COM of a skier has momentum that tends to smooth gross perturbations of COM. In the management of perturbations in skiing, the ankle is the primary joint at which perturbations in GRF are modulated by dorsiflexion/plantarflexion primarily through changes in the magnitude of contraction of the soleus muscle.  This is the balance strategy used to maintain upright postures. The pull of gravity on COM disturbs balance by causing the ankle to dorsiflex. The CNS modulates forward sway by regulating contraction of the triceps surae in an ankle plantarflexion  strategy that maintains balance by opposing ankle dorsiflexion. Shaft resistance to the shank movement associated with ankle dorsiflexion can greatly diminish muscle contraction and degrade the mechanism that maintains balance.

In a similar manner, perturbing forces travelling along the length of the outside ski of a turn are modulated primarily by the soleus muscle. But this is only possible when the ankle joint is in the Resistive Shank Angle and has a range of motion sufficient to allow the soleus to modulate perturbations in GRF. Without the ability to move, the shank becomes a vertical shock transmitter. In addition to modulating perturbing forces, the soleus acts as a powerful shock absorber in dissipating perturbations in GRF.

Shock Absorbers

Securing the shank of the user to the rear spine by drawing it rearward suppresses the 3 degrees of freedom in the ankle/foot complex. Depending on how linear the alignment of the shank with the femur is, transient shocks from peak perturbations in GRF may bypass knee and go straight to pelvis and lower back where they can cause gross disturbances in skier equilibrium compromising pressure control of the skis. Limiting flexion of the ankle joint limits the suspension travel from coordinated ankle, knee and hip flexion that maintains contact of the skis with the snow over terrain changes and also the control of pressure exerted on the snow by the skis.

For reasons I will explain in a future post called, STANCE BASICS 101: RESISTIVE SHANK ANGLE, the boot shaft angle should allow the shank angle that occurs in late stance. This shank angle allows the load from the central load-bearing axis to be transferred to the heads of the first and second metatarsals. Power straps can be used to advantage by adjusting them so they help decelerate forward movement of the shank beyond the limits of eccentric gastrocnemius-soleus muscle contraction. But the margin for error is narrow.

Long before the introduction of power straps, the importance of ankle flexion was stressed in 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

“Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small. 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”. – 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

 

BIO-MEDICAL PERSPECTIVE

Biomechanics in sports, can be stated as the muscular, joint and skeletal actions of the body during the execution of a given task, skill and/or technique.  Athletic performance is governed by these actions. Coordinated, uninhibited, fluid execution of these actions leads to efficient superior performance. Interference or inhibition of these synchronized body mechanics leads to poor performance and injury. These interferences may be caused by inherent structural limitations in our own bodies, injury induced, training deficiencies or equipment related.

In an attempt to enhance your understanding of David’s piece on power straps, let’s review the functional anatomy and mechanics in the skier’s foot/ankle complex in a ski turn.  Ankle dorsiflexion is critical to stance and balance on a stable turning foot in a ski boot. Dorsiflexion enhances pronation and leg rotation. This combination of forces controls the edge angle. Edge angle is increased by increasing pressure on the inside (medial) aspect of the foot by pronation.  As pronation increases, an obligatory 1:1 internal rotation of the lower leg (tibia) occurs. Whole leg internal rotation with hip joint stabilization completes the rotary response.

3 degrees of freedom r1

We can see that interfering with ankle flexion and especially lower leg (shank) position in relation to the center of the head of the first metatarsal of the foot can have serious implications. Compression of the foot in normal pronation stretches the plantar aponeurosis (plantar fascia) which is a primary source of sensory feedback.  The ankle joint is also a source of sensory feedback modulated by dorsiflexion/plantarflexion through stretch receptors in the soleus muscle of the calf.  As the soleus muscle contracts or relaxes, its combined Achilles tendon insertion to the calcaneus (heel bone) lowers or elevates the rear foot in association with ankle dorsiflexion or plantarflexion. The other calf muscle, the gastrocnemius, reflexly flexes the knee joint during ankle dorsiflexion, since it crosses the knee joint. TRY FLEXING YOUR ANKLE WITHOUT FLEXING YOUR KNEE!

Active muscle contraction does not flex a ski boot,  Leg pressure from COM driven tibial flexion is used to create ankle dorsiflexion in a ski boot. Here are some variables that interfere with this mechanism:

  • The power strap is a 5th buckle. It increases the height of the boot cuff anteriorly in some boots by as much as 45mm. In short-legged individuals, the lever of the high cuff is most instrumental in preventing ankle flexion. This is exacerbated in the vertical cuffs of more contemporary boots. This makes lower leg (shank) length a major factor in overcoming a higher boot cuff. In women with shorter legs and larger lower calf diameters fitted in a higher volume boot shell, the lever of the higher cuff inhibits ankle flexion.
  • Power straps inhibit knee flexion. As a result lower leg flexion, lower leg rotation and ankle    flexion are restricted and pronation is impaired.
  • Boot flex (stiffness) is another ankle motion modifying factor that varies greatly from each boot manufacturer and model.  All boot flex indices should be standardized to accurately inform boot fitters and boot buyers.

The natural reflex interaction among the foot, ankle and knee joint muscles should expose the misconception of adding structural supports that interfere with normal anatomic function.  When a power strap inhibits knee flexion, lower leg flexion, lower leg rotation and ankle flexion are restricted and pronation is impaired.

CONCLUSION:  Synergistic reflex responses and muscle co-contractions cannot occur when their sources of neural sensory input such as stretch or positional proprioception are blocked by mechanical interference.  This is especially true in the foot, the body’s base of support. Interference in sensory input leads to poor skeletal alignment and loss of balance. Good balance minimizes the stress on the body while maximizing the efficiency of movement. In skiing, loss of alignment and balance leads to poor performance and at times, severe falls and injury.

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