# 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

# THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: THE TURNTABLE EFFECT

Neither the Two Phase Second Rocker (heel to ball of foot rocker) described in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: HEEL/FOREFOOT ROCKER (1.) or the Rotating Turntable Effect described in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: THE ROCKER/TURNTABLE EFFECT (2.) are new. They have been the trademark technique of the world’s best skiers for decades. But the ability to engage the associated mechanics and biomechanics requires what amounts to a perfect storm that typically occurs early in the development of a young skier. More than raw athletic talent, discipline and dedication, the ski boot appears to be the critical factor that determines who acquires the ability to engage these effects.

In working with skiers and racers who are gifted natural athletes, it has been my consistent finding that a change in ski boots that compromises neuromuscular function will result in the body adopting compensatory mechanisms that can reduce competence on skis to survival reactions. Given sufficient time, the survival mechanism will become imprinted until a point is reached where it is accepted as normal by the body. Even after the cause is corrected, it can take years of retraining to erase and replace survival motor patterns. A good example of this is what happened to Mikaela Shiffrin at the start of the 2014-2015 World Cup after changes were made to her boots in the fall of 2014. Fortunately, she was able to revert to her previous boots over Christmas and quickly restore her former competitive competence.

Four synergistic mechanisms associated with the mechanics of edge change result in the creation of a platform under the outside ski that a skier can stand and balance on. These are:

1. The Two Phase Second Rocker (heel to ball of foot rocker) Mechanism
3. The Over-Center mechanism, and
4. Open and Closed Chain Whole Leg Rotation; The Rotating Turntable Effect.

The most critical and seemingly least appreciated and understood mechanism in skiing is the mechanics and biomechanics of whole leg rotation.

LeMaster recognized the role of whole leg rotation in skiing in his book Ultimate Skiing when he stated under Twisting Actions (p 13) that torques play important roles in turning skis and holding them on edge. In Chapter 7, Turning the Skis (p 107), LeMaster states, Rotating the leg inward generally rolls the ski on its edge, too, combining the increase in the edge and platform angles—often a desirable combination while acknowledging that leg rotation is powerful and can produce large torques through the whole turn. But LeMaster does not describe the mechanics associated with whole leg rotation in this context.

### The Center of Rotation

Whole leg rotational force is applied to the femur primarily by the gluteus medius.

The most important source of rotational power with which to apply torque to the footwear (ski boot) is the adductor/rotator muscle groups of the hip joint. – US Patent 5,265,350 MacPhail

Rotation of the femur is transferred through the tibia where it is applied through its lower or distal aspect to the talus that forms the ankle joint with the tibia.

The graphic below shows a skeleton of the foot aligned on a fixed reference axis (dashed line).The graphic below shows the same skeleton of the foot rotated 15° medially (towards the center of the body) against the fixed reference axis (dashed line).

The graphic below shows the relative displacement of the heel and forefoot in relation to the fixed reference axis (dashed line).

The graphic below compares the displacements of the heel and limit of the forefoot at the end of the second toe with horizontal lines in the center of the graphic. The lines show that the end of the second toe displaces almost 4 times as much as the rearmost end of the center of the heel during whole leg rotation of the foot. Hence the advice in my post, THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: PRESS AND POINT THE BIG TOE (3.), to point the big toe in the direction you want to go.

### Open Chain Rotation vs Closed Chain Rotation

• Open Chain Rotation – occurs when the foot can rotate in the horizontal plane in conjunction with the rotation of the whole leg from pelvis. In ski technique, this is referred to as steering.
• Closed Chain Rotation – occurs when the foot is fixed on its long axis and whole leg rotational force is applied to the foot from pelvis.

Open Chain whole leg rotation acting about the axis of the ankle joint in combination with a Two Phase Second Rocker induced Over Centre mechanism are prerequisites to the application of Closed Chain Rotation. The emerging profile created by the steering angle of the outside ski as it crosses the fall line below a gate yields important clues as to the technique of a racer.

In my next post, I will discuss Closed Chain Rotation applied to the outside ski in a turn and the transfer path of torques applied to the foot by the leg through the boot-binding interface to the ski.

# STANCE HACK: TUNE UP YOUR FEET

### Biohacking Your Body with Barefoot Science

“…… hacking” or finding a way to more efficiently manipulate human biology.  This can include areas of sleep, nutrition, mental health, strength, recovery. (1)
– Dr. Emily Splichal – Evidence Based Fitness Academy

Last ski season, I developed some simple cues or hacks to help skiers and racers quickly find the body position and joint angles required to create the pressure under the outside foot with which to impulse load the outside ski and establish a platform on which to stand and balance on through the turn phase –  THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: IMPULSE LOADING

The primary source of information that helped me develop these cues are the exercises developed by Dr. Emily Splichal. Her exercises also helped me to appreciate the extent to which traditional supportive footwear with raised heels and cushioned soles has damaged my feet and deadened the small nerves responsible for maintaining upright balance and the ability to initiate precise movement. Since implementing Dr. Splichal’s evidence based science, I am not only skiing at a level beyond what I considered possible, I am starting to walk naturally for the first time in my life.

The information contained in Dr. Splichal’s videos will challenge everything you know or thought you knew about what we have been conditioned to believe about our feet and the footwear we encase them in. Contrary to what we have been told, cushioning under the feet does not reduce impact forces on the lower limbs and protect them. Instead, it actually increases impact forces while slowing what Dr. Splichal refers to as the time to stabilization; the time required to stabilize, stiffen and maximally protect the joints of lower limb from impact damage – THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: TIMING OF EDGE CHANGE

### The Best Surfaces to Train On

A good place to start is to learn which surfaces are best to train on. Again, while it may seem logical and intuitive that surfaces with cushioning are best because they will protect the body from shocks, studies show the exact opposite to be true. Over time, support and cushioning in shoes can diminish the sensitivity of the rich small nerve matrix in the feet that acts as a neural mapping system for balance and movement. In her YouTube video, Best Surfaces to Train On ), Dr. Splichal discusses the effects of different surfaces on plantar small nerve proprioception and explains how barefoot training is a form of small nerve proprioceptive training designed to activate the plantar foot. Balance training is best done barefoot.

### The Power of Plantar Proprioceptors

Watching Dr, Splichal’s webinar presentation Understanding Surface Science: The Power of Plantar Proprioceptors – https://youtu.be/t5AU-noqMFg will further your appreciation of the power of plantar proprioception.

### First Stance Hack – Plantar Foot Release for Optimal Foot Function

Dr. Splichal’s 6 Minute Plantar Foot Release for Optimal Foot Function – https://youtu.be/zyrKgFwsppI will dramatically improve foot function.
Dr Splichal explains how to use RAD rollers (golf ball or other firm balls will also work) to optimize foot function by releasing tissues in the plantar foot by applying pressure to the 6 areas shown in the graphic below.
Dr. Splichal advises to focus on using a pin and hold technique  (not rolling the foot on the balls) to apply pressure to these 6 spots on each foot holding for about 20 seconds on each spot with each of the three different sized rounds for a total time of about 6 minutes. The foot release should be done 2 times and day and prior to each training session.
In my next post I will talk about the second Stance Hack: Pressing Down on the Big Toe to Impulse Load the Ski and Power the Turn

1.  https://barefootstrongblog.com/2017/04/28/biohacking-your-body-with-barefoot-training/

# THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: TIMING OF EDGE CHANGE

In my US Patent 5,265,350 (November 30, 1993), I stressed the importance of avoiding any structures in the ski boot that would delay or especially prevent, the loading sequence that enables a skier to rapidly assume a position of balance in monopedal stance on the outside ski at ski flat that occurs between edge change. The 2 paragraphs of text below are excerpted from the patent.

The avoidance of any obstruction (in the ski boot) is required in order to ensure that a monopedal stance will be attained without interference or delay. Such interference would be deleterious to the user and is, therefore, undesirable.

In order for the user to enjoy maximum control of the ski, it is important that these forces be transferred as directly as possible and without delay. As previously stated, this is an object of the invention. It is also important that forces exerted by the ski on rigid base 2100 be transferred as directly as possible and without delay to the foot of the user so that appropriate muscle action can be accurately and quickly stimulated which would act to make corrections which influence the relative position of the joints in order to maintain the user’s state of balance.

What I was really referring to is what Dr. Emily Splichal describes as Time to Stabilization.

The window for stabilization for optimal loading and energy transfer is very narrow and occurs as a skier approaches the fall or rise line at the point where a turn will start. The graphic below shows the Stabilization Zone for optimal loading and energy transfer to the outside ski shown circled in pink.

The timing of impulse loading is critical. The loading impulse is applied by a short, rapid knee extension made just as the ski is about to go flat on the snow between edge change in combination with forward movement of CoM in relation to the outside foot. Extending the knee tensions the hamstrings and gastrocnemius. This will cause the ankle extend slightly creating rocker-action impulse loading of the forefoot, especially the 1st MPJ or ball of the foot.

Dr. Splichal has graciously given me permission to republish her recent post. This may well be one of the most important articles ever written pertaining to skiing and ski technique.

### Time To Stabilization & Athlete Injury Risk

by Dr Emily Splichal – Evidence Based Fitness Academy

A majority of my podiatry practice is built around treating athletes and chronic athletic injuries.   From professional dancers to marathon runners all athletes – regardless of sport or art – require the same thing – rapid stabilization for optimal loading and energy transfer.

Why is rapid stabilization so important?

During dynamic movement such as walking, running or jumping (or skiing – my addition), the ability to rapidly load and unload impact forces requires a baseline of stabilization.   With a rate of impact forces coming in at < 50 ms during walking and < 20 ms during running it is no wonder the rate of stabilization must be fast!

To put this a little bit more in perspective.   Our fast twitch muscle fibers don’t reach their  peak contraction till about 50 – 70ms.   So if impact is coming in at rate < 20 ms during running and your hip / knee / ankle and foot are not already stable before you strike the ground – it is too late!     It physiologically is not possible to react to impact and stabilize fast enough.

A client or athlete who is reacting to impact forces will often present with ITB syndrome, runner’s knee, peroneal tendinitis, stress fractures, shin splints – and that’s just naming a few!

Considering Time to Stabilization (TTS)

In my workshops I often say that “we are only as strong as we are stable” or that “stability is the foundation through which strength, force and energy is generated or transferred”.

The precision, accuracy and anticipation of stabilization must be so well programmed into the nervous system that peak stability is happening before contact with the ground.   This is referred to pre-activation and is associated with a faster TTS.

The opposite of pre-activation stabilization is reactive stabilization and is how many – if not most – of my patients or people in general are moving.   When we think of the rate of neuromuscular coordination even a small delay (think milliseconds) will result in tonic (exaggerated) muscle contractions, micro-instability and inefficient loading responses eventually leading to neuromuscular and connective tissue fatigue and injury.

So how can you improve client and athlete TTS?

1. Pre-activate base to center stabilization pathways aka foot to core sequencing

This is THE basis to EBFA Certifications Barefoot Training Specialist and BarefootRx.   With our feet as our base, the activation and engagement of our feet to the ground is key to center or core stabilization.    Fascially, the feet and core are connected through the Deep Front Line and must be integrated and sequenced as part of a proper warm-up or movement prep.

2. Consider surface science to optimize foot feedback

All surfaces are designed differently with certain surfaces actually blocking and damping the critical proprioceptive input between foot and ground.    When we think of softer surfaces and mats, research has shown a direct correlation between softer surfaces and delayed / prolonged loading responses.

Harder surfaces.  Surfaces that allow the transmission of vibration.  And surfaces with textures allow more accurate and precise proprioceptive input.   Thus led to the innovation of Naboso Technology by EBFA Founder Dr Emily Splichal

Ideally if Step 1 – pre-activation of our stabilization pathway could be done on a Naboso surface this would be ideal.    More information can be found at www.nabosotechnology.com

3. Footwear to allows optimal feedback and foot function

If we follow Steps 1 & 2  and activate the neuromuscular system barefoot and from the ground up we then want to ensure this carries over as soon as we put on our shoes (or ski boots – my addition) and begin our sport or activity.

Imagine if you activate the proper neuro pathways but then put your client into a thick cushioned shoe (or ski boots – my addition).  This essentially shuts off and defeats the purpose of Step 1 & 2.   We need to ensure a proper shoe is worn to allow this carry over into sport.    So think flexible, minimal cushioning. possible textured insoles (check out Naboso Insoles launching Spring 2017)

The textured insole in the shoe above is NABOSO technology.

Dr. Emily Splichal, Podiatrist and Human Movement Specialist, is the Founder of the Evidence Based Fitness Academy and Creator of the Barefoot Training Specialist®, BarefootRx® and BARE® Workout Certifications for health and wellness professionals. With over 15 years in the fitness industry, Dr Splichal has dedicated her medical career towards studying postural alignment and human movement as it relates to foot function and barefoot training.

Dr Splichal actively sees patients out of her office in Manhattan, NY with a specialty in sports medicine, biomechanics and forefoot surgery. Dr Splichal takes great pride in approaching all patients through a functional approach with the integration of full biomechanical assessments and movement screens.

Dr Splichal is actively involved in barefoot training research and barefoot education as it relates to athletic performance, injury prevention and movement longevity. Dr Splichal has presented her research and barefoot education both nationally and internationally, with her Barefoot Training Specialist® Program in over 28 countries worldwide and translated into 9 languages.

Due to her unique background Dr Splichal is able to serve as a Consultant for some of the top fitness, footwear and orthotic companies including NIKE Innovations, Trigger Point Performance Therapy, Aetrex Worldwide, Crunch Fitness and Sols.

Degrees/Certifications: Doctor of Podiatric Medicine (DPM), Master’s Human Movement (MS), NASM-CES, NASM-PES, NSCA-CPT

# THE MECHANICS OF BALANCE + BIOMECHANICS ON THE OUTSIDE SKI: WHERE IS GROUND?

This is a revised version of a post I published on February 28, 2017.

It was my intent to discuss the key move in the First Step to Balance on the Outside Ski; Impulse Loading of the Forefoot. However, it has become apparent that it is necessary to preface this subject with a discussion on the source of ground in relation to the outside foot in order to impart an appreciation of why a mechanism is required to extend ground from the running edges of the ski in order to create a platform for a skier to stand and balance on when the outside ski is on its inside edge.

In typical discussions of ski technique and the mechanics, biomechanics and physics of skiing, the prevailing mental model assumes that a skier is in balance (see REVISION TO FEATURE POST: CLARIFICATION OF DEFINITION OF SKIER BALANCE) if they are able to stand upright and exercise a degree of control over their skis. In balance studies performed in gait labs, ground reaction force in the form of stable surface under the entire area of the foot or feet for subjects to stand balance on is assumed. There is no basis to assume this is the case when a ski is on edge because the source of ground is on the wrong side of the platform underfoot.

### Mental Models

Mental models are a form of cognitive blindness. Once people assume they know something, they not only don’t question what they believe, they filter out information that conflicts with their mental model. And they typically fail to see the real issue even when it is in plain sight.

A man should look for what is, and not for what he thinks should be.

–  Albert Einstein

Even though I quickly became a competent skier soon after I started skiing,  I struggled to hold an edge on firm pistes and especially glare ice. It was disconcerting to see elite skiers hold an edge on ice with minimal effort while making controlled turns. When I sought the advice of the experts, they claimed that holding an edge on ice was matter of sharp edges and/or driving the knees into the hill. When I protested that after trying both and found it harder to hold an edge, the experts claimed that the ability of some skiers to do what I couldn’t was due to superior technique. They were just better skiers. No further explanation was needed.

The inability of experts to explain why a small number of skiers seemed able to balance on their outside ski and hold an edge even on ice provided me with the impetus to look critically at this issue with the objective of formulating an explanation based on principles of applied science.

The only plausible explanation for the ability of a skier to be able to stand and balance on their outside ski when it is on its inside edge is that some source ground (reaction force) must be present under ski that they are using to stand and balance on. Hence, the question, Where is the Ground?

On very hard pistes, ground as a source of reaction force, is limited to the running portion of the inside edge of the outside ski and the small portion of the base adjacent the edge with the edge and base supported on a small shelf cut into the surface of the snow/

In Figure 2.11 on page 26 of his book, Ultimate Skiing, LeMaster explains how the sidecut of a ski creates a smaller radius turn as the edge angle increases.

In Figure 2.12 on the following page, LeMaster shows misaligned applied (green arrow) and ground reaction (purple arrow) forces creating an unbalanced moment of force (yellow counter-clockwise rotation arrow) that  rotates the ski down hill (out of the turn). LeMaster goes on to state that as the skier edges the ski more, the ski bites better. But he fails to offer an explanation as to how the skier can edge a ski more against an unbalanced moment of force acting to reduce the edge angle.

The mechanism that generates a moment of force that opposes the moment force shown by LeMaster in Figure 2.12 and has the effect of extending ground (reaction force) acting along the running length of the edge of the ski  is the subject of this series of posts.

### Edge Angle Sidecut FXs

A simple way to acquire an appreciation for the location of ground relative to the outside ski on edge is to make a simple model out of flexible piece of sheet plastic material a few mm thick.

The photo below shows a model I made from a piece of sheet plastic about 8 inches long. The upper portion of the plastic piece has a shorter sidecut with less depth than the sidecut in lower portion of the piece of plastic piece. Both the model and sketches that follow are for illustrative purposes to demonstrate the effects of sidecut geometry on edge angle and a source of ground. Although the basic principles are the same, it is not intended that they be viewed as an accurate representation of actual ski geometries  The symmetrical geometry is for the benefit of the simplifying what is already a complex issue.

There is a relationship between the depth and length of a sidecut in that the greater the ratio of the depth to the length of a sidecut, the lower will be the edge angle it forms with the surface in relation to the camber radius. In the sketch below, the upper rectangular ski shape will maintain a vertical relationship with a surface regardless of the camber radius.

There is also a relationship between the edge angle a ski with sidecut will form with a uniform surface and the radius of the camber with the edge angle formed with a uniform surface. The edge angle will increase (become more vertical) with a decrease in the radius of the camber. This explains why GS skis that are longer and have less sidecut depth than SL skis can attain much higher edge angles.

The photo below shows how the aspect of the model I made with the smallest sidecut ratio forms a steep angle with a uniform surface when bent to sufficient camber radius to allow the sidecut to become compliant with a uniform surface.

When viewed from the rear of the model, the location of ground in relation to the structure of a ski with sidecut and camber should become readily apparent.

The graphic below shows what a photo taken at a low enough vantage point to the snow would capture looking straight on at a ski carving a turn with its edge compliant with the surface of the snow. This may seem foreign, even extreme to some. But when the edge of a ski is compliant with a uniform surface, the curve of the sidecut becomes linear.

The left image below depicts the schematic model of the ski shown in the second graphic with the camber angle sufficient to make the edge in contact with the uniform surface compliant with it. The angled line represents the surface of the snow. The schematic model of the ski represents the proximate end profile associated with a high load GS turn. A photograph in Figure 1.18 on page 17 of the Skier’s Edge  shows a similar profile in Hermann Maier’s outside (left) ski which is at a very high edge angle.

The graphic on the right shows some penetration of the running surface of the edge of the ski in conjunction with the forces commonly shown in the prevailing mental model that are used to explain how forces acting on the outside are balanced.

The reality is of the applied forces acting on the ski are shown in the vertical profiles in the graphic below as captured by digitized force plate data. Once the foot is loaded on a surface there is what is called a Center of Pressure as shown by the peaks in all 3 graphs. But when the foot is in compliance with a uniform surface, some pressure is expressed by the entire contact surface of the foot. So, the point application of applied force in opposition to a point application of GRF as depicted in the right hand graphic above is a physical impossibility.

Viewing a transverse vertical profile of a ski on edge from the perspective of ground as a source of GRF for a skier to stand and balance on puts the issue of skier balance in a whole new, albeit unfamiliar, perspective. But it is a reality that must be dealt with in order to engage in realistic narratives on the subject. Overly simplistic explanations of skier balance attributed to a basic alignment of opposing forces do not serve to advance the sport of skiing as a credible science.

I concur with LeMaster’s position that the platform angle a ski forms with resultant and GR forces must be at 90 degrees or slightly less in order for the edges to grip. In my next post, I’ll start to introduce mechanical principles that explain how this can be accomplished.

It is the ability of racers like Mikaela Shiffrin to stand and balance on their outside that enables them to consistently dominate World Cup competition.