Physics

THE ELEMENTS OF FLUID SKIING

A recent paper, Dynamics of carving runs in alpine skiing. The centrifugal pendulum by S.S. Komissarov, provided me with insights as to the differences between elite and lesser skiers.

Komissarov clarifies that the context of proficient skiers being well-balanced simply describes the observation that the skiers do not appear to be in danger of falling. The signature of good skiers is that they move effortlessly from turn to turn in a smooth, continuous rhythmic manner much like a metronome or inverted pendulum.

A key point is Komissarov’s comment that elite skiers somehow manage to ski faster than the theory of ideal carving predicts. He also states that the fluidity of the pendulum action of the elite skier does not actually require a forceful participation from the skier and that the skier has to make sure that they do not inhibit this natural process but just “get on board and enjoy the ride!”.

The reference to fluid skiing being a natural process requiring no forceful (conscious) participation from the skier in terms of the associated neurobiomechanics responsible for the pendulum action is one reason why I am shifting the focus of my blog away from ski technique (which is consciously mediated process) to the elements of fluid skiing and especially factors that interfere with the natural processes that enable humans to ski as easily as they walk so that analyses can focus on the why not the what.

 

THE MECHANICS OF PLATFORM ANGLE: PART 2

I believe the single most important factor affecting a skiers’ balance and directional control of a ski is the ability of the balance system to effect dynamic balance of the angle of the platform of an edged ski with the surface of the snow . So I am going to focus my efforts on explaining the mechanism of dynamic balance.

Skiing is an interaction of the skier with the snow. Since the interface of the interaction is the inside edge of the ski and the snow I’ll start here with an explanation of the principles of the associated mechanics.

Snow = Ground

Snow is an extension of ground. Hence we speak in terms of ground reaction force or GRF. In moving over the snow a skier is interacting through a layer of snow with the ground. In the name of consistency firm pistes will be the reference for the discussion of forces.

The interface with the ground is the base plane of a ski in particular the juncture of the transverse base and sidewall planes. In this interaction the angle of the base plane  with the surface plane of the snow is the plane of balance for the skier in terms of the management of angular forces.

Edging Forces are not Normal

In order for the platform of a ski to hold and not slip under the weight of a skier the edge and the adjacent portion of the base  must cut a step or ledge into the surface of the snow. But the portion of the ski that must cut a step into the surface of the snow is not a knife edge. It is more like a knife blade on the flat with the blade aligned perpendicular to the angle of attack of the force applied by the skier.

Typical force diagrams in technical discussions of skiing only show opposing angular forces with a platform perpendicular to the vector of the forces. The opposing angular forces R and GRF are said to cause the platform of the ski to cut a step or ledge into the surface of the snow as shown in the graphic below. One problem is that sketches such as the one below don’t show the perpendicular (Normal) or horizontal (Shear or Slip) components of the opposing angular forces R and GRF.

When an applied force is Normal to a surface (perpendicular) the penetration or cutting action is maximal.  But when a force applied to a surface is less than perpendicular it will have components of Normal and Shear or Slip forces such as shown in the graphic below. At an angle of attack of 45° the Normal and Shear components of the applied and GRF forces will be equal. But as the angle of attack decreases and becomes more aligned with the plane of the surface the Shear or slip components will increase in magnitude and the Normal components of the applied and GRF forces will decrease in magnitude.

As this happens the tendency of an applied force acting on a body like the platform of a ski that would cause it to penetrate into the surface of the snow and cut a step will decrease as the angle of the platform with the snow increases. As the platform angle increases so will the tendency of the ski to slip and not hold an edge. The components of opposing applied and GRF forces acting on a solid body or surface are determined mathematically by sine/cosine. They are not negotiable. Nor can their impact on ski platform mechanics be ignored.

 

In my next post I will discuss the real force that makes the platform of a ski cut a step or groove into the surface of the snow that the edge of the ski tracks in.

THE MECHANICS OF PLATFORM ANGLE: PART 1

In order to engage in an interactive productive dialog on issues pertaining to ski technique and related equipment a frame of reference based on validated, non-negotiable principles of physics, mechanics and (neuro)biomechanics as well as a schedule of defined reference terms such as exists in the sciences of mechanics, anatomy and physics is essential. Defined technical reference terms help ensure all participants in a discussion are on the same page.

I decided to start the new direction of The Skier’s Manifesto with a critical examination of the mechanics of platform angle starting with a schedule of the technical terms associated with platform angle and their definitions. Additional technical terms and their definitions will added in future posts according to the content of the discussion. The intent at this point is to start with a basic discussion of forces applied to a rigid body and/or surface (in this case, the surface of the snow) and then expand the scope of the discussion in future posts. Agreement on terms and definitions is important. So please comment if you feel one or more the following terms are inappropriate or inaccurate or should be expanded and/or refined.

Technical Terms associated with Platform Angle

  • Platform Angle: the angle of the transverse aspect of the body of the ski underfoot with the surface of the snow.
  • Edge Angle: the angle of the edge of the ski in relation to the plane of the transverse aspect of the body of the ski adjacent the edge.
  • Force: an unopposed interaction that will change the motion of an object. A force has both magnitude and direction, making it a vector quantity.
  • Force Vector: the magnitude and direction of a force.
  • Applied Force: a force applied to a rigid body or surface.
  • Reaction Force: a force that opposes a force applied to a rigid body or surface.
  • Normal Force: a force acting perpendicular to a rigid body or surface that is resisting a force applied to it.
  • Angular Force: a force applied to a rigid body or surface that is not normal (perpendicular) to the rigid body or surface to which the force is applied.
  • Angle of Attack: the angle an angular force forms with the rigid body or surface to which it is applied to.
  • Resultant Force: also known as Net Force, is a single force associated with torque obtained by combining a system of forces and torques acting on a rigid body.

Technical discussions of the forces associated with the angle of the platform with the snow typically show opposing resultant and ground reaction forces implying a state of balance of the forces acting on platform created by the outside ski underfoot.

Schematic diagrams showing forces acting on the platform created by the body of the ski underfoot often show two opposing forces in alignment with each other acting close to or at the axis point created by the inside edge of the outside ski. Or diagrams may simply show opposing forces aligned with each other implying the existence of a state of equilibrium.

In my next post I will discuss whether the above force diagrams accurately reflect a state of equilibrium of the forces acting on the platform of the outside ski. Please join the conversation.

THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: FIRST STEP

In this post, I am going to begin the first of what I expect to be a series of posts on the Two Step Process to Balance on the Outside Ski.

Prelude

Before I start, I am going to caution the reader that they should not expect that the ability to learn and engage the processes responsible for balance on the outside ski to be easy to understand or quick to learn.  Many obstacles stand in the way of the ability to balance on the outside ski. As Benno Nigg’s experiments in the early ’90s at the Human Performance Laboratory at the University of Calgary demonstrated, the human body is highly adaptable. If a person puts their feet in footwear that prevents natural barefoot function, the body will find a best case work around compromise.

This is what happens to skiers when they put their feet in ski boots. As the Polish study showed, over time, the body will adapt. But adaptation always comes at a price.  Some skiers may adapt to constraints of a ski boot to the point where they are considered expert skiers by the prevailing standards. But they typically reach a point where they can no longer advance. Given same ability, the least compromised skiers become the best.

The problem faced by skiers who wish to learn balance on their outside ski (foot) is that the ingrained motor patterns their brain has created as a work around to address the limitations caused by their ski boots can be exceedingly difficult to erase. A skier will typically make some progress only to have their brain revert to motor patterns that have worked in the past when it senses danger. When this happens, the odds are great that even the most athletically gifted skier may have to relearn skiing to some extent. I have seen many graphic examples of this pattern over the past several years in skiers and racers I have worked with.

WARNING: The Mechanics of Balance on the Outside Ski is Not Simple

About the simplest way I can describe the mechanics is that the First Step involves a heel to 1st MPJ rocker loading mechanism while and the Second Step involves an intertia-driven turntable, over-centre mechanism. The mechanics is unified sequence of events. The reason I have broken it two steps is to make it easy to understand the critical nature of the first part of the sequence.

More than 25 years ago, I tried to make the First Step simple and easy to understand with the model I fabricated shown in the photo below and that graphic illustration that follows that shows how the Achilles tendon tensions the Plantar Aponeurosis (aka the Plantar Fascia) and enables foot to pelvic core sequencing. Note the annotation in graphic to Late Stance and (SR) Ski Stance Zone.

In my demonsrations, I  would drop the model on a table from a height of a few inches.  The rotation of the leg of the model would be quickly arrested by simulated isometric contraction of the Achiles. The model and the demonstration failed to garner attention or interest because the importance of the forefoot to foot function was not on the radar screen. Instead, the focus was on the hindfoot and addressing the known looseness of the forefoot associated with the mid stance phase of gait. A late stance phase was not yet part of the gait cycle narrative. The importance of late stance and fascial tensioning of the forefoot to foot function and foot to core sequencing has only recently been recognized.

sr-tripod-demo

Plantar Apo Dynamics

First Step

The First Step is to tension the biokinetic chain that extends from the MPJs of the foot to the pelvis. The timing of this event, which is critical, will be discussed in a later post.

The key move is the loading of the outside foot. This should happen in the top of the turn as the fall line is approached. This is the point where a skier should become the tallest in relation to the snow. At the end of a turn (in the bottom) is where a skier should be lowest.

It is not possible to replicate the loading move except when skiing because of the dynamic nature of the 3-dimensional forces associated with ski maneuvers. But the forefoot loading move that creates fascial tension the forefoot is essentially the same move we make when we move forward on the stance foot in walking in preparation to take a step. Once the foot has adapted to the ground, forward rotation of the shank (ankle flexion) is arrested by isometric contraction of the calf muscle. At this point, further forward movement of the torso occurs through knee extension in what amounts to a heel to ball of the foot rocker mechanism; i.e. a forward and downward action that applies force to the ground to prime the energy return foot spring in preparation to propel the body forward.
One way to get a feel for this mechanism is to stand sideways across the bottom of a stair and place one foot on the first tread about a whole foot length ahead of the foot on the floor. The knee of the leg on the floor should have slight bend so the calf muscle is in isometric contraction (SR Stance). The angle of the shank of the foot on the tread should be a little less than 90 degrees in terms of dorsiflexion. From this base position, the torso is projected forward in order to achieve a position of balance over the foot on the first tread. This is roughly what the loading move should feel like in skiing that is made as the fall line is approached. Once a feel for this has been acquired I can discuss how this integrates with rotation of the leg.
It is important to not have the ankle flexed for the above exercise because the ski boot limits ankle flexion. At the start of the transition at the end of a turn, the weight will be under the heel of the inside (uphill) foot. It is also important that the calf muscle of the foot on the stair tread go into isometric contraction so that further forward movement of the torso occurs through knee extension.
In a ski turn, the forefoot loading move is one of a quick heel to 1st MPJ forward rocker knee extension pulse that loads the ball of the foot (1st MPJ). Loading of the 1st MPJ (ball of the foot) is caused by forward movement of the torso (CoM), not plantarflexion. This loading move is made in the top of a turn as the fall line (aka rise line) is approached. The window in which to make this move is narrow and the time required  to complete the move, brief.
If you watch video of Shiffrin slowed to 0.25 normal speed or step the video in frame-by-frame, you will clearly see her make this loading pulse which usually involves a lifting of the fore-body of the old outside ski due to swing leg reaction force.
In my next post, I will discuss Step Two.

 

 

 

 

 

 

 

 

 

 

 

THE EFFECT OF PENDULUM ACTION ON THE INSIDE SKI

Part 1 left off with the inside ski flat on the surface of the snow after it had completed its rotation about its current (uphill) edge when pressure was applied to the ski by stepping on it. The current or uphill edge was the point where snow reaction force S was acting. The pressure W, applied under the heel of the inside leg and foot, on the proximate center of the ski, was offset from S resulting in a moment arm that tended to rotate the ski downhill. This rotation was opposed by a force exerted against the inner aspect of outside of the boot shaft  by the inside leg being abducted (moved outward) by the thigh as shown in the insert in the graphic of Ana Fenninger below.


Fenniger Abduct

When the ski rotates into contact with the snow surface, rotational momentum wants to maintain the rotation.

Rotational Momentum

 

If the piste is firm or icy, there will be little or no penetration into the surface as the ski moves beyond full contact with the snow surface as it changes edges.

 

 

Ski Flat SRF

One way or another, there will be a translation of the plane of the base  of the ski into a different plane as it changes edges and begins to rotate about the inside edge of what will become the outside ski of the new turn. Translation is part of the event that I call Roll Over.

 

Translation

If the pressure stays in the center of the ski as it changes edges and translation starts, there will be a problem. Even though COM will eventually pass the axis of rotation of the new edge, translation will be resisted by the Pressure applied to the center of the ski. This is the literal Moment of Truth. If the Pressure stays on the center of the ski, force exerted on the inside of the inner side of the boot shaft will cause translation to occur against the Pressure that continues to rotate the ski out of the turn. What has to happen for Pressure and Translation to be in phase, so Roll Over can occur, will be the subject of Part 3.

EXTENSION/PENDULUM EFFECTS

Recent World Cup results in Are, Sweden in the women’s slalom and men’s GS have created a perfect seque to discuss the technique that all racers on the podium have in common, the extension/pendulum effect technique. The women’s slalom was a virtual horse race photo finish. And although Shiffrin didn’t make the podium, her results sent a clear message that any rumours of her competitive demise have been greatly exaggerated. While the women’s slalom was almost a dead heat, Austrian Michael Hirscher literally blew away Ted Ligety in the GS by 1.2 seconds. With all top technical racers rapidly adopting the extension/pendulum effect technique, reaching the podium is fast becoming a case of who is doing it the best.

The reason for the sudden seismic shift in technique is that until the emergence of Ligety and Shiffrin extension had long been associated with up-unweighting. In Burke Mountain Academy’s YouTube video, Get Over It, Mikaela Shiffrin comments, “The first time I did the get over it drill I wasn’t a big fan of it……..because my coach called it up and over…. and I thought, well…., you’re not supposed to be moving up”. (http://www.youtube.com/watch?v=Bh7KF49GzOc&sns=em)

There are a two main reasons why extending on the inside leg at the end of a turn is so effective.

  1. It sets up an inverted pendulum that rotates the skier as a vertical unit about the inside edge of the inside ski out of the old turn and into the new turn. This creates rotational momentum that rotates the skier and ski past ski flat onto its new inside edge while establishing a base for dynamic inclination into the new turn.
  2. It creates a gravity like force at ski flat that enables the central load-bearing axis to transfer the load W to the foot and induce pronation. In the New York Times video, Ted Ligety describes his extension  as “creating pressure”.

But it is a serious error to assume that the racers who are currently reaching the podium with the extension/pendulum effect technique have the optimal equipment setup and/or the optimal expression of the technique. While the racers who were on the podium in Are are getting the pendulum effect component of the technique right, only a few are getting the configuration at ski flat right. When they do, it is hit and miss. Almost no racer is getting the movement sequence at what I call Ligety’s Moment of Truth, right. The most plausible reason for this is interference caused by the structures of  ski boot with the  joint actions of the foot and leg essential to the technique. The key to effective use of the extension/pendulum effect technique is the ability to rapidly transfer the load at ski flat from the central load-bearing axis to the forefoot  and especially to the ball of the great toe, something most boots intentionally prevent.

The racer in the photo below shows what the optimal form of the extension/pendulum effect technique should look like after the start of the transition phase. The relaxed, focussed look on her face indicates that she is in a flow state (DOT  13).

DeeDee

While the stance associated with technique will look familiar, most will probably not recognize the racer. The reason she is not familiar is that I took this photo more than 25 years ago, during off season training in Hintertux, Austria. The racer is former National Alpine Canada Team member, Diana DeeDee Haight. Her technique is all the more remarkable given the fact that her GS skis are 70 mm wide underfoot with minimal sidecut compared to current GS skis with their 65 mm or less width underfoot. I started working with DeeDee at Nancy Greene Raine’s request. DeeDee who was only 13 at the time was training at the Toni Sailor Summer Ski Camp in Whistler. She was the first racer I worked with. Although she had all the earmarks of a champion, I immediately recognised that her boots were limiting her potential.

In 1978 I had her change to a Lange boot. DeeDee has chunky, peasants feet that are significantly wider than the US men’s size 6 Lange shell I had sized her in. Her big toe is large and straight. The shell deflected it towards the middle of her foot. I knew this would adversely affect her balance. I am certain most would be horrified at what I did to expand the shell of her boots sufficiently for her foot to sit naturally in it with the ball of her big toe against the inner wall and with her big toe straight. I also reduced the forward lean of the cuff and adjusted the cuff cant to a more upright position so it would work with her morphology. This involved disassembling the boot, welding the rivet holes  closed and re-assembling the parts. By the time I was finished, her Langes were nothing like the boots that came out of the factory. Instead, they were more akin to a NASCAR race car that bears only a superficial resemblence to the stock factory dealer show room version. But as the saying goes, “That’s racing”.

Starting in the 1981-1982 World Cup season, DeeDee began to use an improved version of the tongue system that Podborski was using. It is similar to the one shown in the photo below except that the system was in two separate components, connected to each other with a thin leather strap. This version was closer to the one shown in the patent figure that follows with the exception of the rearward extension of the forefoot element under the inside ankle bone. This was eliminated after it was found to cause interference with pronation.

IMG_3705

US 4,534,122 1

A large gap between the forefoot and shin components ensured that the physiologic function of DeeDee’s ankle joint was not inhibited and that the load on her shin stayed centred at the upper aspect of the front of the boot cuff. When I took the photo of DeeDee in 1989, she had been using the improved version of the tongue system for 8 years. Like a Formula One driver and his or her crew chief, DeeDee was always involved in the preparation and modification of her boots. As a racer, these  were her race vehicle. She understood what I was trying to achieve and provided me with valuable feedback on whether my efforts needed fine tuning and especially when they failed to meet our objectives and expectations.  DeeDee’s technique evolved through her innate mechanism of alternating single limb support and the postural responses associated with the processes of balance and acceleration. To the best of my knowledge, no one taught her to ski this way. Instead, her technique evolved because the environment in her ski boot was conducive to the innate processes that enabled her CNS to connect with a contiguous source of GRF at the snow. Her CNS simply did what it is designed to do.

In my opinion DeeDee had the potential to become one of the greatest female technical skiers in World Cup history. Unfortunately for her, the focus of the team in her era was on speed events, especially downhill, where DeeDee was outside her comfort zone. A series of serious injuries eventually led to a decision to retire.

In my next post, I will provide my analysis  of the key events in the extension/pendulum effect technique using screen shots from the women’s slalom and men’s GS at Are, Sweden.