# EDGE CHANGE INERTIA + ROCKER ROTATION INERTIA

As I was in the process of writing this post, a FaceBook group on skiing posted a link to an article From PSIA: Examining Transitions. The article is based on a presentation last fall by US Ski Team Head Men’s Coach, Sasha Rearick, in which he shed new light on transitions (1.).  While Rearick did shed light on some events associated with transitions, as with previous efforts by others on this subject, Rearick failed to shed light on the mechanics and physics associated with edge change.

As I explained in my last post, transferring the weight from the outside foot and ski of a turn to the inside foot and ski in the transition phase sets in motion what I call the Eversion/Internal Rotation Cascade that rotates the base of the ski into a transient moment of full contact with the surface of the snow between changing to the new (downhill) edge.

At the start of the transition leading up to ski flat between edge change, the center of pressure (COP) of the weight of the body applied by the sole of the inside foot will be under the heel where it is aligned on the proximate center of the ski. In this configuration, the force applied to the ski by the skier is working with gravity to rotate the ski.

The post left off by showing how rotational inertia will tend to make the ski continue rotating about the uphill edge past ski flat and penetrate into the snow surface on its downhill aspect as shown in the graphic below.

Rotational inertia will tend to make the inside edge of the new outside ski automatically rotate into the turn except for the fact that the force FW applied by the skier is on the wrong side of the new edge.

The graphic below has a dashed red reference that is parallel with the snow surface.

If the force FW applied by the skier is still aligned on the transverse center of the ski, it act will act to oppose edge change as shown in the graphic below. When the axis of rotation of the body of the ski changes with a change in edges, the transverse aspect of the base of the ski and the platform under the skier’s foot will tend to accelerate into an eversion translation. But this can only happen if the associated biomechanics are not interfered with by the structures of the ski boot.

The graphic below shows the change in the mechanics of rotation associated with edge change.

At the start of the transition, movement of the mass of the skier’s upper body is in phase with the downhill rotation of the ski and the force FW applied to it. But when the ski changes pivots at edge change and the mass of the skier continues to move downhill, the force FW applied to the ski will tend to rotate it back to ski flat; i.e out of the turn, unless the point of application of force FW changes during ski flat as shown in the graphic below and COM of the skier is aligned with force FW.

………. the angle between the platform and force you apply to it, the platform angle, must be 90 degrees or smaller.  – page 19, The Ski’s Platform Angle, Ultimate skiing; Le Master

The shift in center of pressure from the heel to the ball of the foot in a turn sequence seen in pressure studies of expert skiers is well documented (2., 3., 4). What the studies are really confirming is the use by expert skiers of the Two Phase Second Rocker mechanism to rock (tip) the outside ski on edge and control the edge angle during the load phase of a turn sequence.

Since the limit of the position of the application of force by the foot in relation to the inside edge of the outside ski is the center of the ball of the foot the effect of ski width underfoot and stand height should be obvious. Both rotational inertia and torque will increase as the width of a ski underfoot (profile width) is reduced and stand height increased. When Ligey says he creates pressure, he is creating far more than just pressure.

While LeMaster appears to recognize the importance of a platform angle less than 90° for edge control and, to some degree, the effect of stand height, the explanation offered for superior edging is that this can be attributed to waist width and stand height making skis more like ice skates.In my next post, I will discuss the role of Turntable Rotation in setting up a platform under the body of the outside ski for a skier to stand and balance on while maintaining edge angle.

1. http://eliteskiing.com/2017/03/31/from-psia-examining-transitions/
2. WHAT THE TWO HIGH PRESSURE COPS IN THE UNIVERSITY OF OTTAWA STUDIES MEAN – https://wp.me/p3vZhu-1fV
3. IMPLICATIONS OF THE UNIVERSITY OF OTTAWA PRESSURE STUDIES –https://wp.me/p3vZhu-1e2
4. AN INDEPENDENT STUDY IN SUPPORT OF THE UNIVERSITY OF OTTAWA FINDINGS – https://wp.me/p3vZhu-1gR

# EDGE CHANGE INERTIA: WHY THE TRANSITION PHASE MATTERS

One of the most important events in the turn sequence is edge change. Yet, it is rarely mentioned in technical discussions. One of the few references I was able to find on edge change is in the CSIA Technical Reference which states:

Edge Change = Balance Change: Changing edges requires a change of balance.

Edge change occurs during an unbalanced, controlled fall in the transition phase that leads to the development of a balanced position on the outside ski as it crosses the fall line in the bottom of a turn. Properly executed, edge change leads to the development of a platform under the outside ski for the skier to stand and balance on.

The edge change sequence starts in the transition phase when a skier begins to transfer weight from the outside (downhill) ski to the inside (uphill ski). At the start of the transition, the edges of the inside ski are uphill and on the lateral (little toe) side of the foot. From a perspective of the gait cycle, the base of the ski is inverted (turned inward towards the center of the body). This is the normal configuration when the foot is unweighted in the gait cycle. The foot strikes the ground on the lateral (little toe) side and rotates about it’s long axis in the direction of eversion to bring the three points of the tripod of the foot into contact with the ground. As the foot everts, the leg rotates internally through torque coupling in the subtalar joint. The normal kinetic flow from foot strike to the support phase in mid to late stance is one of inversion of the foot/external rotation of the leg to eversion of the foot/internal rotation of the leg. Put another way, the human lower limbs will naturally rotate into a turn so long as the biomechanics are not interfered with.

At the start of the transition leading up to ski flat between edge change, the center of pressure (COP) of the weight of the body applied by the sole of the inside foot will be under the heel where it is aligned on the proximate center of the ski.

Transferring the weight from the outside foot and ski to the inside foot and ski in the transition phase sets in motion what I call the  Eversion/Internal Rotation Cascade. When the cascade starts, the force F W applied to the ski by the foot  by the weight of the body will impart rotational inertia as the ski rotates about the pivot point formed by its inside edge.

For the sake of simplicity, the stack of equipment between the sole of the skier’s foot and the snow is represented by a rectangle in a 3:2 ratio where the stand height is 50% higher than the width (FIS maximum stand height = 93 mm – maximum profile width = 63 mm). Sidecut is also not shown.

The following graphics show the sequence of the Eversion Cascade. Note: Internal rotation of the leg is not shown in this sequence.

The first graphic below shows the moment or torque arm ma that is set up by the offset that exists between GRF from the firm piste acting at the inside edge and the point where the center of pressure of the weight of the body acts in the plane of the base of the ski. The large red arc shows the radius of rotation. The small red arc shows the radius of the moment of force. In this sequence, the ski is rotating downhill away from the pivot at the uphill edge.

When the base of the ski comes into full contact with the surface of the snow, rotational inertia, will make it want to continue rotating about the uphill edge and penetrate into the snow surface on the downhill aspect. If the force FW applied by the weight of the body is still aligned on the transverse center of the ski, it will oppose edge change.

In my next post I will discuss how the Second Rocker affects the mechanics of edge change at ski flat.

# THE ORIGINS OF KNEE ANGULATION

A recent post on the Foot Collective Facebook page titled, Are you stable on 1 leg?, advises that if  you stand on one leg and look like the top row of pictures in the graphic below (red X), you have a foot & hip that are dysfunctional. This test is best done barefoot on a hard, flat, level surface.

Graphic with permission of Correct Toes

The lower photo (green checkmark) shows the alignment of a leg that is torsionally balanced (stiffened) in the ankle and knee joints. The foot and knee cap align straight ahead and square with the pelvis while the alignment of the knee with the foot, leg and thigh is substantially linear. If you can move to single limb support from two feet, easily achieve this alignment with minimal effort, sustain it for 30 seconds or more, and achieve similar alignment on both left and right legs, you probably have good stability in single limb support.

If you look like the upper photo (red x), it indicates dysfunction and especially a lack of torsional stability in the support limb. The problem is usually caused by constrictive, supportive, cushioned footwear and/or arch supports that, over time, deform feet and weaken the arches. Ski boots are one of the worst offenders in this regard.

If you and when you can achieve good stability in single limb support, you are ready to test the effect of footwear, especially your ski boots. Start by putting on your day to day footwear. Then do the same test on the same surface with each pair of shoes. Work your way up to your ski boots. Adjust the closures of your ski boots to the tension you normally set for skiing. If you are not able to quickly and easily assume the stable position shown in the lower photo (green checkmark), then you know that cause  is the footwear. You can then test the effects of insoles, including ski boot footbeds by removing them from the footwear, placing them on the test surface and moving to single leg support. While not perfect, these tests will help determine the cause of single support limb instability.

In skiing, an unstable outside support leg is characteristics of most skiers and even racers at the World Cup level. It is typically caused by ski boots interfering with the physiological processes that fascially tension the arches and forefoot that create the triplanar torsional stability of the ankle and knee joints of the biokinetic chain necessary to set up a platform under the outside ski to stand and balance on. But instead of addressing the underlying cause, the ski industry invented the term, knee angulation. Knee angulation is indicative of unbalanced torques acting about the uphill edges of the skis, especially the outside ski. When unbalanced torques are present about the edges of a skis or skis, unbalanced torques will also be present across the joints of the lower limb; not a good thing.

The alignment of the knee illustrated in the lower image (green checkmark) is seem as skier or racer enters the fall or rise line with outside leg extended, confirms the existence of a platform under the outside foot on which the skier or racer is balancing on with dynamic balance of torques across the joints of the ankle foot complex and knee. See my post MIKAELA SHIFFRIN AND THE SIDECUT FACTOR – http://wp.me/p3vZhu-1Uu

There is an abundance of information on programs to correct foot deformities,  muscle weakness and imbalances on web sites, YouTube and FaceBook groups such as The Foot Collective, Correct Toes, Feet Freex and the Evidence Based Fitness Academy – EBFA (Dr. Emily Splichal).

The Foot Collective web site has a series of posts on An Introduction to Feet and Footwear (1.) as well as a series of Foot-Casts (2.)

Meantime, a post on a web site called Rewire Me (3.) has an interview with Dr. Emily Splichal called No Shoes Allowed in which she discusses the importance of sensory information entering the body and the need to be able to process this information and handle the load and impact. Dr. Splichal suggests starting the process by getting the body and foot accustomed to sensory information without shoes acting as a barrier.

An excellent free paper with great graphics is The foot core system: a new paradigm for understanding intrinsic foot muscle function (4.)

# CARVE, MEET BIRDCAGE – BIRDCAGE, MEET CARV

## July 1991: Birdcage Research Vehicle – Cost approximately \$140,000

### Secret  Toshiba Prototype Portable Computer used for Birdcage studies – Value? Priceless!

#### Birdcage Co-Designer and Team Science Leader, Alex Sochaniwskyj, P. Eng.

After interviewing a number of candidates in the spring of 1991 for the science component of the MACPOD project to develop a ski boot based on anatomical principles, I chose Alex Sochaniwskyj, P. Eng. as the most qualified candidate and one of the most intelligent and creative persons I have ever had the privilege of meeting.

Alex provided the CV that follows in his letter in support of my nomination for the Gold Medal in the categories of Applied Science and Engineering in the 1995 British Columbia Science & Engineering Awards.

Alex Sochaniwskyj, P. Eng.

Alex is a professional engineer with 12 years of biomedical and rehabilitation engineering research experience in the Human Movement and Motor Functions Research Programmes at the Hugh MacMillan Rehabilitation Centre in Toronto, Ontario, Canada. The principle aim of these labs is to provide detailed information and objective analysis of movement, dynamics and motor function of persons with various physical disabilities. The information is used to objectively assess the effects of a variety of therapeutic and surgical interventions.

Alex holds a Bachelor of Science degree from the University of Toronto in Human Physiology and a Bachelor of Applied Science from the University of Toronto. Most recently, Alex has worked with several companies including ADCOM ELectronics Limited in Toronto, where he was responsible for the design and development of video conferencing and multi-media communication systems, and the Arnott Design Group, where he focused on physiological human factors in product system design, prototyping and testing.

Currently, as a principal at designfarm inc., he consults to design and manufacturing firms on the development of programs to evaluate human physiological, biomechanical, ergonomic and environmental response for product and interface design, and the planning of comprehensive technology implementation strategies for the integration of computing, telecommunication and telepresence technologies. Alex is also a Certified Alias Instructor in the Information Technology Design Centre in the School of Architecture and Landscape Architecture at the University of Toronto, where he teaches courses in computer literacy, three-dimensional design, modelling, simulation and animation.

Alex is a member of the Association of Professional Engineers of Ontario (APEO), the Institute of Electrical and Electronics Engineers (IEEE), the Association of Computing Machinery (ACM) and the University of Toronto, Department of Rehabilitation Medicine Ethics Review Committee. He is co-author of numerous publications in refereed medical and engineering journals and has produced several video productions regarding biomedical and rehabilitation engineering.

– March 24, 1995

### 2017 – CARV: Cost? Approximately \$300 US – See footnote re special price

Birdcage to CARV: “Where have you been? I’ve been waiting 26 years for you. Welcome! The future of skiing has arrived.”

# DIGITAL SALVATION FOR THE SOLE [BACK TO THE FUTURE]

“Any sufficiently advanced technology is indistinguishable from magic.”  – Clarke’s Third Law

Conspicuous hardly begins to describe what I was feeling.  In the early morning rush of skiers grabbing a quick caffeine rush at the Wizard Grill, amid tables full of Ski School twinks waiting to see whether they were going to have any work for the day, an attractive woman was carefully stringing computer cables up the inside legs of my ski pants.  Things like that draw attention even at the base of Blackcomb on a Monday morning.

One end of the cables were attached to pressure sensing insoles in my ski boots, the other to a data recording box I was trying to figure out exactly were to attach.  About the size of an epic Michener paperback, it was just too big to slip into any of my pockets.  Finally clipped to the waist of my pants, it was, in turn, coupled to a high-powered flash unit strapped to my arm, both of which were fired by a button left dangling pretty much nowhere.

Robocop.  I couldn’t get the image out of my head, although at least one person who asked what all the hardware was about accepted my answer that it was a control mechanism to power my artificial leg.

David MacPhail grabbed the digital video camera and we headed up Blackcomb to take some measurements.  Dave — who I’d been working with to document some background on the Rise boot he’s been developing — had only recently launched Synergy Sports Consultants.

I wasn’t clear where exactly he was taking me or what we were going to accomplish, but a more willing guinea pig would have been hard to find.  In the nether world of ski theory, and more particularly in the areas of skiing biomechanics and modeling, Dave MacPhail is riding the cutting edge.  His work with National Team skiers and his understanding of exactly happens to the human body when it straps on a pair of skis has brought him an international reputation as an authority in the field.

On a clear slope under the Solar Coaster, Dave skied ahead to set up the video shot.  Sophie — who’d wired me up — rechecked the cable connections, set a baseline measurement for each of my unloaded feet and told me to point the flash unit down the hill at the camera.

As they signaled their readiness to each other, Sophie fired the flash and told me to ski down toward Dave.

Making my best ski school turns, I skied for the camera.  We repeated the process a few times and then we went back down to the Daylodge to…well, I wasn’t sure to do exactly what.

What, turned out to be mind blowing.  The unit strapped to my waist was a Pedar foot pressure data recorder from the Novel company of Munich, a techy little piece of equipment that, until last year, was the size of a small desk.  On a PCMCIA flash card, the unit was capable of recording about 10 minutes worth of data.  Fed by 80 pressure sensors arrayed throughout the insoles in my boots that each took 50 measurements per second, the Pedar tracked pressure across time as my feet worked to move me like a skier.

Downloaded onto a laptop computer and run through the company’s software, the data could be displayed as images of my left and right foot, colour-coded across the sensing mechanisms to display the changes in foot pressure as I made turns.  With lower pressure readings showing up as black squares and higher pressure lighting up bright pink, the readout was a moving kaleidoscope of colour as it played back my runs down the mountain.

On each colourful foot profile, a small dot traced a red line showing my centre of pressure at any moment in time.  A good skier using foot pressure the way they’re supposed to, would, over the course of a run, track a red line from the ball of their foot back toward their heel.  The track would be true and relatively straight with few variations.  That’s what the tracing on my right foot looked like.  The track of pressure of my left foot looked like someone who had never seen an Etch-A-Sketch grabbed both knobs and started twisting them randomly.

The difficulties showing up in my left foot readout were verified when Sophie explained the graphic display at the top of the screen.  “This line graph shows change in pressure over time for each foot.  When you make a good turn, like you’re doing with your right foot, the graph of pressure shoots up dramatically at the start of the turn, drops down slightly to a plateau, then falls away as you unweight the foot at the beginning of the next turn.  Your left foot comes on very gradually.  Something’s blocking your foot function,” she explained.

The final diagnostic piece of the puzzle — at least as far as the technology end of things went —was put in place when Sophie downloaded the images from the digital video camera and synchronized them with the Pedar display.  There I was, making graceful turns and there was the readout of what my feet were doing — or not doing, as it turned out.

“Neat,” I said.  “Now what?”
“Now you find out what Synergy is all about,” said Dave.

Synergy — small “s” — is about joint action of different substances producing an effect greater than the sum of the effects of all the substances acting separately.

The whole being greater than the sum of the parts.

In a theological context, synergy is a doctrine that human effort cooperates with divine grace in the salvation of the soul.  I’ve often thought of skiing as a salvation of the frozen Canadian soul and certainly a day in the high alpine making perfect turns in all conditions is as close to divine grace as most of us will ever come.  But it was the more secular meaning of the word Dave had in mind in naming the company.

“The whole concept of Synergy probably came into my mind 25 years ago.  I started thinking about something called bio-integration, bringing people with different important skills together to work holistically on making your body work right.  Five years ago, we couldn’t have launched Synergy because the technology wasn’t quite there.  We needed more sophisticated software and I could see the time coming closer to when we’d reach a point where a lot of things in athletics that are mysteries now were going to be revealed by being able to plug in sensors at key points of interface.  Now, we’re starting to get there.”

But data is just data without something to make it sing.  And that’s where the principals of Synergy begin to make the concept work.  Joanne Younker is Synergy’s president. She’s been working with Dave for 12 years on both the Rise boot and putting together a biomechanical model of how people ski, how joints and muscles and nerves and bones work together to overcome our natural tendency to fall down when the earth starts to slide out from under our feet at an accelerating rate.

Joanne’s a level IV CSIA instructor and a level II CSCF coach and a personal trainer when she’s not on skis.

Sophie Cox and Joanne Younker

She’s been a keen skier since she was fourteen and a student of kinaesthetics since 1989 when she blew her back out squatting improperly in the weight room, an injury leading to temporarily paralysis and a burning desire understand how her body works.

“Working with David, and studying the biomechanics of skiing, I can look at someone skiing and understand what they’re doing wrong and, more importantly, probably why they’re doing it.  That is, what muscles aren’t functioning right or what functions are blocked.  Working with this technology, I can validate my diagnosis with hard data.”

Using a set of dry-land kinaesthetic exercises, Joanne led me through a session designed to help me experience the “feel” of having the right muscles firing and applying pressure with the correct area of my feet.  Once I’d managed to do these correctly, she had me stand on the Pedar’s insoles outside my ski boots.  Connected to the computer, they gave me a real time display of where, in turn, I was applying pressure with each foot. Running me through the exercises again, I could use the display to associate that “feel” with a visual representation of correct pressuring.  There was no guesswork.  When I lit up the right area of the pressure pads, I was having my feet do exactly what they should do to initiate a good turn.

The final step of the exercises was to slip the insoles back into my ski boots and repeat the exercises again.  Within the confines of my boots, I could watch as I pressured the ball of my foot and got my bulk into the right plane of alignment.  I was surprised — as is virtually everyone else who has gone through this exercise — at how far forward I really needed to bring my centre of mass to consistently apply pressure where needed.

All of this might have taken a lot longer to happen if the third member of the Synergy team hadn’t walked into town by accident.  Sophie Cox finished her B.Sc. at the University of Brighton School of Podiatry in, England, in the summer of 1998 and was working in a Podiatry clinic in London.  Her mother brought home a bottle of Whistler spring water  — the same water that gets flushed down toilets in Function Junction, ironically — and she was taken with the idea of goofing off for a year in Whistler.  After some web surfing, she decided to take a job as a bootfitter at Can-Ski and really learn how to ski and party, Whistler style.

A colleague in Boston mentioned the groundbreaking work Dave had been doing in biomechanics and planes of movement associated with skiing to her and she attended a presentation Dave made last March to the Congress of the Canadian Sports Medicine Association.  “After Sophie met David and explained what she’d been doing with the Pedar, he was really excited.  He called me up and said, ‘I’ve met the third person!’ and we went from there,” Joanne explained.

After a summer back in England working , Sophie returned this fall to work with David and Joanne on the biomechanics of skiing and help launch Synergy.  What she brings to the table, in addition to the technology, is an in-depth understanding of the structures and movement of the foot and ankle joints and a wealth of knowledge in diagnosing problems related to feet and lower limbs.

“I look at a skier’s mechanics, what they can and can’t do, and try to decipher why they can’t do it.  Sometimes it’s bad motor skills and that’s Joanne’s part.  But if she’s trying to teach them a skill and they just don’t have the biomechanical capability to do it, that’s where I come in.  I can determine the physiological problem and refer them on to a physio or bootfitter or local podiatrist.”

“The only way of discovering the limits of the possible is to venture a little way past them into the impossible.”  Clarke’s Second Law

For me, the proof of what Synergy was offering was back out on the slopes.  I practiced and visualized what Joanne had shown me, let Sophie make a few modifications to my left footbed and got wired up again a few days later.  Back at the computer after two or three runs, I sat in rapt amazement at the difference.

On the Pedar’s readout, the front of my feet were lighting up at the initiation of each turn.  The tracking line of the centre of force had moved inward — indicating a much stronger pronation, getting the ski on its edge — and my left trace looked like something made by a functioning foot instead of a peg leg.

I know what you’re thinking; almost anyone can help me be a better skier.  That’s like crowing about doubling your money when you only have fifty cents to start with.  But what about good skiers?  What can all this do for them?

In the fall of 1991, during dry-land training in Banff, Rob Boyd blew a disc at the L-5, S-1 joint in his back.  An ensuing laminectomy restricted his mobility and left some nerve damage on his right side— although not enough to keep him off the podium from time to time for the next six years.  “I learned to compensate using different muscle patterns,” he said.

Three years off the World Cup Circuit now, and away from the daily coaching, Rob wasn’t happy with the way he was skiing this season, nor was he happy with his finishes in the early Ford Pro Series downhill races.  “I saw Jim DeMarco, M.D. wired up to this thing one day and started thinking maybe Dave — who had done a lot of boot work for Rob in the past — could do some testing on me and help me find some answers.”

Sophie and Joanne ran Rob through a gait test, using the pressure pads inside his running shoes while he walked the treadmill at Meadow Park.  “What we saw,” Sophie related, “was Rob had some blockage in the way his foot was functioning.  He wasn’t pushing off the ball of his foot with any force at all but compensating through other muscle patterns.”

“Right away, from what we saw on the data, my suspicions were confirmed that my right side wasn’t working well,” Rob added.

What they saw when Rob was hooked up to the Pedar for the first time on the slopes was even more surprising.  His heels lit up like a Christmas tree and he was almost never pressuring the front of his boot.  His left turns were strong and crisp but his right turns were nowhere near the same intensity.  “Yeah, that was surprising to see.  It felt like I was skiing alright and using the balls of my feet but I wasn’t even close,” Rob said.

Dave went to work on Rob’s boots, Sophie made some modifications to his footbeds and Joanne got him started on a series of patterning exercises and visualization techniques.  “I could really feel the difference when I started concentrating on using my foot more.  That and the changes in my boot environment made a big difference.  I could feel it right away at Sugarbush (Vermont).  My skis were gliding on the flats; just floating,” Rob said.  He could also see the results in his times: second on his first run and fourth on his second.

“The next step will be to set Rob up with a physiotherapy regimen with Allison MacLean,” Joanne said.

And that’s where the remaining synergy of Synergy comes into play.  The company’s goal is to actively work with bootfitters, physiotherapists, chiropractors and other specialists in the community who can treat the whole person.

Allison is just beginning to work with the Synergy people and is excited about the “integrated approach” they’re trying to bring to problem solving.  “The data gathering and testing they’re doing is interesting,” she told me.  “It’s hard sometimes to know exactly what’s not functioning in the case of lower limb injuries and whether what your treatment is as effective as it could be.  When they send someone to me, we’ve got a pre-treatment set of data we can compare to post-treatment performance to really know whether what we’re doing is effective.”

“Every other person you bring into this adds something to the mix and produces even more beneficial results,” Dave explained.  “Sophie and Joanne and I, working together, have a much greater impact than any one of us could have on our own.  That’s the genesis behind Synergy.  But we want to bring the best resources we can to bear and make it so everybody looks like a hero.”

This obviously includes some of the best bootfitters in town.  George McConkey is sold on the idea.  “What Synergy is doing validates a lot of my own ideas about foot function and bootfitting,” he said.  “I still believe 99% of most peoples’ problems are in their boot and with any luck, what we’re starting to see in the way of data coming out of this will get the manufacturers interested in designing boots that work.”

Scott Humby, one of the owners of Fanatyk Co., isn’t so sure what’s going on is going to shake up the industry, but he sees potential benefit.  “I think what they’re doing can help you by really proving what’s going on in your boots.  If it make you feel better about your skiing; you’ll ski better.  If, as bootfitters, we’ve done all we can for someone and they’re still struggling, we’ll definitely send them on to Synergy because there may be something we’re just not seeing.  There’s a huge benefit in being able to refer someone on to a team of specialists.”

It seems axiomatic that what Synergy is doing is the way sports will go in the future.

The advances in sports in the last 25 years have largely come about because of a refinement in coaching techniques and technological innovations in equipment.  But most of what’s being done on the coaching front still relies on what a coach can see and how he or she interprets that visual data.  The advances in coaching and teaching in the next 25 years will probably be realized through the application of measurement technologies only now being brought into the field.

Some people in town and on the mountains think what Dave’s up to is another bit of high-tech quackery, other’s are true believers.  But whether coaches and instructors and others who guide athletes embrace the kinds of tools is probably more a matter of when, not if.  Elite athletes will demand it; the wired generation coming up will assume its presence. And guys like me who just want to get better and shorten the distance between muscle pattern and muscle memory will embrace it the same way we embraced those shapely new skis we can’t live without.

In the meantime, Arthur C. Clarke’s Law of Revolutionary Ideas is probably apropos:

Every revolutionary idea — in science, politics, art or whatever — evokes three stages of reaction. They may be summed up by the three phrases:

1. “It is completely impossible — don’t waste my time.”

2. “It is possible, but it is not worth doing.”

3. “I said it was a good idea all along.”

#### Watch out for number three.

author- J.D. Maxwell

reprinted with the permission of Whistler Piquenewsmagazine

published on February 18, 2000

In this post, I will discuss the role of impulse loading, in the perspective of phases of a turn cycle, in creating a platform under the body of the outside ski on which a skier can stand and balance on.

Impulse loading is crucial to the ability to establishing a platform under the body of the outside ski by cantilivering GRF, acting along the running surface of the inside edge, out under the body of the ski to create a stable platform for the skier to stand and balance on.

Maximization of dynamic stability while skating is crucial to achieve high (vertical) plantar force and impulse. (1)

Impulse in particular has been identified as an important performance parameter in sprinting sports as skating. (1)

The preceding statements apply equally to skiing.

The most important aspect of alternating single limb support locomotion is the ability to rapidly develop a stable base of support on the stance or support leg from which to initiate precise movement. Dr. Emily Splichal refers to this process as Time to Stabilization. The ability to balance on the outside ski of a turn is unquestionably the single most important aspect of skiing. Time to Stabilization, especially in GS and SL , is where races are won or lost. Here, the time in which to maximize dynamic stability on the outside foot and leg on the outside ski is in the order of 20 milliseconds (2 one-hundredths of a second); less than a rapid blink of the eye.

### The Mid Stance, Ski Stance Theory

The predominant position within the ranks of ski industry is that skiing is a mid stance activity in terms of the stance phases of the gait cycle. In the mid stance phase of the gait cycle, tension in the longitudinal arch (LA) resulting from passive tensioning of the plantar ligaments is minimal and the foot is continuing to pronate. Mid stance, as the assumed basis for ski stance, appears to have served as the rational for the assumed need to support the LA with a custom footbed or orthotic (usually in neutral STJ) and immobilize the joints of the foot with a custom fit liner. Hence, the theory that the foot functions best in skiing when its joints are immobilized. I am not aware of any studies, let alone explanations based on principles of applied science, that supports this theory. To the contrary, the available evidence suggests that immobilizing the joints of the foot, far from making it function best in skiing, has the exact opposite effect.

Wearing ski boots for a few hours can lead to a weakening of the muscles that operate within the ankle joint. This works as though one joint was excluded from the locomotive function.

………. according to Caplan et al. [3], the muscle groups that determine strength and are responsible for the function of stability in the ankle joint are very sensitive to changes caused by immobilisation. They found that immediately after immobilising the ankle joint for a week, the balance parameters were 50% lower than before the immobilisation.

The problem with the mid stance, ski stance theory, is that impulse loading cannot not occur until late stance when arch compression, fascial stiffening of the forefoot and torsional stiffening of the subtalar and knee joints, is maximal.

One factor that has been shown to reduce arch compression is arch supportive insoles and orthotics. A study done in 2016 (1.) compared the effect of half (HAI) and full insoles (FAI) on compression loading of the arch to compression loading of the arch that occured in a standardized shoe (Shoe-only). Two separate custom insoles were designed for each participant. The first insole was designed to restrict arch compression near-maximally compared to that during shod running (Full Arch Insole; FAI) and the second was designed to restrict compression by approximately 50% during stance (Half Arch Insole; HAI). The Full Insole (black) most closely resembles the type of arch support used in ski boots to support the foot. The bar graph below shows the resulting reduction compression. I have overlain the FAI bar to illustrate how it compares to Shoe Only compression. This kind of study can now be done and should be done in vivo in skiing – during actual ski maneuvers where the effect of insoles and custom fit liners on the physiologic function of the foot and lower limb as a whole can be studied and assessed.

Two pressure studies done in 1998 by a team from the University of Ottawa (2, 3), that used elite skiers as test subjects, found large variations in pressures applied to the ball of the foot observed in the data that suggested some factor, or combination of factors, was limiting the peak force and impulse in terms of the vertical force that skiers were able to apply to the sole of the boot and ski. The researchers suggested a number of potential factors but did not investigate them.

These highest pressures reach up to 30 newtons per square centimetre. Force-time histories reveal that forces of up to 3 times body weight can be attained during high performance recreational skiing (my emphasis added).

Conclusions/Discussion:

It is quite likely that the type of equipment (skis and boots) worn by the subjects had an effect on the values obtained (my emphasis added).

A factor that was not controlled during data collection was the equipment worn by the subjects. The skiers wore different boots, and used different skis, although two of them had the same brand and model of skis and boots. It still has yet to be determined if that factor had any effect on the results. A point that all the skis that the subjects used had in common is that the skis were all sharp side-cut skis (also called shaped skis). Another equipment variation which may have affected in-boot measurements, is that some subjects (n=5) wore custom designed footbeds, while the other did not (my emphasis added).

In 2013 (4), a study presented at the European Congress of Sports Science in Barcelona, Spain that used special hockey skates that I prepared to maximize peak force and impulse using principles described in my blog compared peak and impulse forces of elite skaters in the skates I prepared (NS) to peak and impulse forces seen in their own skates (OS). The skates I prepared were used as a standardized reference similar to the protocols where baseline data obtained barefoot is used to assess the effect of specific footwear on physiologic function. The bar graphs below compare NS (the skates I prepared) to OS (the subjects own skates).

The researchers noted:

Thus, the results of this study show that direct measurement of these dynamic variables may be important indicators in evaluating skating performance in ice hockey as it relates to skate design or skill development.

Peak force and impulse are associated with high peak tension in the LA created by Achilles to forefoot load transfer.

I expect that similar results would be seen in ski boots.

### The Phases of a Ski Turn Cycle

In order to appreciate the dynamics of impulse loading in skiing, I have modelled the phases of a turn cycle into 2 main phases with associated sub phases. The graphic below shows the Loading (1 – yellow) and Stance (2 – red) Phases of the outside (left) foot in a turn cycle with sub phases. The actual turn phase starts at the juncture of the traverse and from fall line and ends when the skier starts to extend the inside (right) knee. I will discuss the turn cycle in detail in a future post. My long-held theory, which was partially validated with the 1991 Birdcage studies, is that ski movements should employ the same hard-wired patterns as walking and running and that skiing should as instinctive and transparent.

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. (5)

The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. (5)

Peak Force and impulse loading occurs at ski flat between edge change (red circle). This is what I refer to as the Moment of Truth. Moment, in this context, being a moment of force or torque. The manner in which the torque acts in the sequence of events surrounding edge change determines whether GRF is cantilevered under the base of the ski or whether it acts to rotate the ski (invert) it out of the turn.

In my next post, I will discuss the 2-step rocker impulse mechanism that cantilevers GRF acting along the running inside edge of the outside ski out under the body of the ski.

1. The Foot’s Arch and the Energetics of Human Locomotion: Sarah M. Stearne, Kirsty A. McDonald, Jacqueline A. Alderson, Ian North, Charles E. Oxnard & Jonas Rubenson
2. ANALYSIS OF THE DISTRIBUTION OF PRESSURES UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS: Dany Lafontaine, M.Sc., Mario Lamontagne, Ph.D., Daniel Dupuis, M.Sc., Binta Diallo, B.Sc.. Faculty of Health Sciences1, School of Human Kinetics, Department of Cellular and Molecular Medicine, Anatomy program, University of Ottawa, Ottawa, Ontario, Canada. 1998
3. ANALYSIS OF THE DISTRIBUTION OF PRESSURE UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS: Dany Lafontaine, Mario Lamontagne, Daniel Dupuis & Binta Diallo, Laboratory for Research on the Biomechanics of Hockey, University of Ottawa, Canada – Proceedings of the XVI International Symposium on Biomechanics in Sports (1998), Konstanz, Germany, p.485.
4. A Novel Protocol for Assessing Skating Performance in Ice Hockey: Kendall M, Zanetti K, & Hoshizaki TB School of Human Kinetics, University of Ottawa. Ottawa, Canada – European College of Sports Science
5. Dynamic Sensorimotor Interactions in Locomotion: SERGE ROSSIGNOL, RE´ JEAN DUBUC, AND JEAN-PIERRE GOSSARD Centre for Research in Neurological Sciences, CIHR Group in Neurological Sciences, Department of Physiology, Universite´ de Montre´al, Montreal, Canada – 2006 the American Physiological Society

This is a quick post to comment on a gutsy move by Tina Weirather; one that probably caught most off guard after her very successful 2016-17 World Cup season and especially just before the upcoming Olympics.

I believe Weirather’s timing is impeccable.  Said Weirather;

……….I’ve spent a long time thinking about all these steps. I asked myself a lot of questions and balanced the risks as well as all the potential advantages and disadvantages. The most important questions were: “How can I be most successful, how can I ski the fastest, how can I evolve the most?” The answers got clearer and clearer with every day I tested, every conversation I had, and the more I listened to my gut.

The tests went really well……………”

When I worked with Provincial and Canadian Team racers, I always made boot changes as soon as possible after the competitive season ended. The changes were done in a structured, systematic manner involving one-on-one testing where changes were made to one boot at a time and then compared to the unchanged boot. Only when the changes were proven better when compared to the unchanged boot were changes made to the other boot. In setting up new boots, it was standard practice to swap the liners from the current boots into the new shells to confirm they were properly set up and do one-one-one testing that compared the new shells with the liners from the previous boots to the previous shell/liner combination.

### Always have an Escape Route

Even with a lot of testing that resulted in new boots that appeared to be an improvement, I always recommended that racers keep their old boots intact and with them during training right up until racing started. If last minute doubts arose, the best practice was to revert to the old proven setup. Recall Shiffrin’s disastrous start to the 2014-15 World Cup season after changes were made to her boots in the fall of 2014. Fortunately, Shiffrin was able to revert to her old boots, train in Italy over Christmas and get back on track in the New Year. Many racers are not so fortunate.

It was my policy to not make changes to a racer’s ski boots should during the competitive season unless there was no other option. Making an equipment change now, such as Weirather has done, provides a big window in which to make adjustments in technique and fine tune equipment before the start of competition.

### A Formula (One) for Success Team

Weirather impressed me when she said;

It took a while, but I’m now 100% convinced I’ve found my dream team: HEAD (new) Tech: Reini Berbig (new) Coach: Charly Pichler (new) Dryland training: Micha Eder / @rotorteam Sports therapist: Fabienne Frommelt Team: Swiss Ski WC 1 Manager: Christopher Holzknecht (new).

I have long maintained that in order to succeed, ski racers need to adopt the Formula One model where the racer drives the skis and a whole team works together to support the racer.

In important ways, I believe Tina Weirather is the role model for World Cup ski racers.