pendulum effects

NEW INSIGHTS INTO THE BIOMECHANICS OF WALKING AND STEPS IN THE SKI TURN

Comments made by S.S. Komissarov in his paper, Dynamics of carving runs in alpine skiing. The centrifugal pendulum in conjunction with a critical examination of the biomechanics of the walking cycle and subjective on-snow experiments I did last ski seaon has given me insights into the mechanism that enables fluid dynamic skiing with directional control.

A telling statement by Komissarov is that in the fast skiing typical of FIS WC racers’ rhythmic carving turns are still possible but balanced carving turns are no longer possible. Komissarov further states that during rhythmic carving turns a skier is never in balance. I would modify this statement to posit that during rhythmic carving turns a skier is only in what can be described as a state of dynamic balance wherein neurobiomechanical processes effect tight control over variances in the orientation of the transverse plane of the base of the edged outside ski as it pertains to the alignment of the vector of opposing applied and reaction forces for a few milliseconds.

These insights explain why static balance exercises done on one foot, lateral side to side jumps where a subject lands and balances on one foot and even one ski turn exercises don’t equate with the dynamic mechanism responsible for the fluid movement of dynamic skiers.

A critical examination of the walking (aka gait) cycle raised issues that as far as I know may never have been explored. These issues have potential implications for the role of steering in the alignment of the pendulum vector of COM with the transverse aspect of the outside ski as it pertains to the edge angle in carving and the stiffening of the outside foot and leg that enables powerful carving forces to applied to the outside ski that when released act can act as catapult mechanism to propel the skier into the next turn.

Pelvic rotation appears to be a key component in the dynamic processes of both walking and skiing.

In my next post I will start to explain how I believe pelvic rotation in walking relates to pelvic rotation in a ski turn and what the differences are.

 

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 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.

PUTTING THE MECHANICS AND PHYSICS IN THE BIOMECHANICS OF SKIING

Balance is an internal response to an external disturbing force

Recently, I came across an excellent article by Ian Griffiths titled, ‘Putting the mechanics back into ‘biomechanics‘. Ian is a Sports Podiatrist in the UK. His article was published on http://www.running-phyiso.com/mechanics/ on February 4, 2015.

According to the introduction by PhysioTom, Griffiths has been challenging misconceptions around pronation and foot function for some time. Griffiths states that the aim of writing his blog is to try and put some mechanics back into discussions of biomechanics. According to Griffiths if those discussing biomechanics, “don’t do physics”, then it may be time to choose another speciality. I whole-heartedly agree. From what I have read in the literature over several decades, it is not so much a case of putting the biomechanics back into discussions of the biomechanics of skiing, but more a case of putting the mechanics into discussions of the biomechanics of skiing where little or no component of mechanics currently exists. It is not possible to engage in a meaningful discussion of the biomechanics of ski technique, especially at the World Cup level, without including the components of mechanics and physics and especially the opposing forces acting across the inside edge of the outside ski of a turn.

Here is an article that I wrote on skier balance in February of 2002.

Good balance is everything in skiing. Few would argue the point. So why is it is that no one seems to be able to describe what good balance is? It’s pretty hard to know when a skier has good balance if no one knows what good balance is. It’s even harder to learn good balance for the same reason. If you watch skiers coming down a hill it’s obvious that some have better balance than others. How can you tell? They are typically much quieter and more fluid in their movements than less balanced skiers. And they don’t get tossed around as much in the bumps. More important, they determine where and how they want to move. Skiers who have good balance, are in control.

The ability to stand upright without falling over is a form of balance. But good ski technique is more than simply not falling over. Good balance allows a skier to resist the external forces of skiing as efficiently as possible by controlling their position on their skis. The muscle effort required to do so with good balance is a fraction of that required when balance is poor. The stress on a skiers’ body is equally reduced. If you want to become a good skier, then you have to know what good balance is and what you have to do to get it.

Webster’s dictionary defines balance as equilibrium or to remain in equilibrium. Not much help here. No wonder the term balance is nebulous in skiing.

The definition of equilibrium is more involved. But it provides clues as to what balance is (or should be). Webster’s defines equilibrium as: a state of balance between opposing forces or effects, the system involved (that’s the skier) undergoing no total change. This provides a better picture of balance because equilibrium in this context means that a skier can initiate a movement pattern and then return to the same body position from where the movement started. This implies the ability of the body’s balance system to maintain control of the movement of the joint system.

From a perspective of physics, Webster’s defines equilibrium as the state of a system in which the net force is zero and which may be either static or dynamic. Skiing is a dynamic activity because the external forces acting on a skier are constantly changing. Here, balance is a process and not a static condition because the body must change in response to changes in external force. Balance in this situation is the equivalent of a stalemate in a tug-of-war. Both sides are pulling at each other. But neither side is moving. In the human system, balance is an internal response to an external force that challenges equilibrium because the body has no direct control over the external force. But in the balance tug-of-war scenario, the balance system must have the reserve power to give up or take territory at will in order maintain equilibrium.

Balance is controlled by the balance system of the CNS. Its job is to maintain an upright position and prevent falls that could cause injury, especially to the brain. It does this by gathering information from a wide variety of sources distributed throughout our body. Vision is important to the process. So are the tensions sensed in the muscles and joints. But in standing upright, some of the most critical information about balance comes from the feet. In skiing, the feet are where everything happens.

Since walking and running are a series of intentional falls and recoveries, the balance system has a complex job. It has to continuously analyze the flood of information it receives and then compare it to movement patterns stored in its memory bank. In fractions of a second it has to decide if movements are putting the body in danger of falling. If so, it must respond with appropriate action.

The problem with standing erect (Figure 1) is that humans are vulnerable to toppling. This makes balance strategies intended to prevent falls extremely complex. The brain tells us the body that we are standing perfectly vertical in a static position. But this is an illusion. The human system incorporates an ingenious strategy to address the problem of standing upright. Instead of trying to maintain a perfectly vertical position, the body is configured with our weight slightly in front of our ankle (Figure 2). Although we don’t sense it, we are in effect leaning slightly forward. In this position, there is a constant tendency for gravity to cause us to fall on our face. The balance system counters this tendency with the muscles in the back of our leg that push down on our forefoot. This action pushes us backwards; just enough to prevent a forward fall. But not so much that we would fall over backward. Staying upright involves a constant cycling of this back and forth movement called sway.

Activities like skiing make the job of the balance system even more complicated because it has to use movement patterns from hard-wired activities like walking and running to assess the risk of falling on skis. The important thing to know about the balance system is that it will always produce the best balance solution it can for a given situation. So if a skiers’ balance is poor on skis then the balance system is probably doing the best it can with what it has to work with. Simply trying harder to have better balance will usually make things worse. The solution is to find out what is causing problems for the balance system and take steps to correct the situation.

In Figure 1 the ankle is shown with cross hairs through a circle. This is the main joint the body rotates about in an upright posture. The ankle is the key joint in balance in skiing since this is the point where the most significant external forces act.

Figure 1Figure 2 below shows the ankle extended up to the center of mass (black and red circle). The center of mass or COM represents the net position of the weight of your body. It is where you are in relation to the ground.

Figure 2

Figure 3 below shows a simple model of the foot with a strut extending upward from the ankle to the center of mass. This model is similar to the one the human system uses to maintain an upright position. If we were to turn the model upside down it would become apparent that our body is configured like an upside down pendulum hinged at its base at our feet. This arrangement was first suggested by Dr. David Winter at the University of Waterloo in Canada.

Figure 3

An external force W (gravity) pulls the weight of the COM forward and downward, towards the ground. In effect, gravity is trying to topple COM. An internal force M (muscle) pulls against the rotation of the body caused by gravity W. The opposing arrows through the center of the COM show the direction of forces and relative strength. The external force W tries to rotate the body clockwise. It is shown as a negative (-) force. The action of these forces is shown at the ankle. Force W is said to tend to disturb equilibrium. The internal force M is controlled by the balance system. It acts to oppose force W by pulling COM counterclockwise. It is shown as a positive (+) force.  Equilibrium exists when M – W = 0. Therefor M =W. This is called the balance equation.

When an external force acts on our body it tends to cause rotation at the joints of the foot, knee and hip. Our balance system senses the direction and strength of the external force and signals muscles to pull in the opposite direction with equal force. This prevents the joints from rotating. If the external force changes in any way, the balance system responds to match the change. The important point is that for balance to exist in this relationship, the balance system must be in control of the relationship of the 2 opposing forces. This requires that the balance system sense the slightest change in the direction and strength of an external force and respond immediately with opposing muscle action. One factor that helps balance is that the pull of an external force usually tends to stretch a muscle as it is contracting. This is called eccentric contraction or what I wrefer to it as elastic tension. The reason it is so important to good balance is that the harder and faster an external force pulls against muscle in eccentric contraction the harder the muscle pulls back. In other words the pull of an external force can actually make muscle stronger and faster. In eccentric contraction muscle can produce up to 8 times as much power as it can in concentric contraction.

The important point in the balance process shown in Figure 3 is that force M represents an internal response to an external force W. If the balance system can successfully oppose an external force it can usually protect the affected part from injury in addition to maintaining balance. Ski equipment, but especially ski boots can disrupt the balance system and cause a serious loss of muscle power. When the balance system controls the balance equation (equilibrium), it has the ability to adjust the position of the COM in relation to the base of support at the feet. It does this by either increasing or decreasing muscle force so as to create movement at a joint. When the intended position of the body has been attained, the balance system adjusts the muscle force to maintain the balance equation. When external forces exceed the opposing internal forces of the skier, equilibrium is lost.

EXTENSION PENDULUM/EFFECTS EVENTS

There are 9 specific events associated with the extension/pendulum effect technique.

  1. Transition – (aka Get Over It or Up and Over). As the turn phase ends, the racer shifts the load from the outside ski to the inside ski while it is still on its inside (current) edge by extending on the inside leg and moving the hips up and over the inside ski to bring COM over the inside foot and in front of the ankle joint (hence Up and Over).
  2. Pendulum Effect Initiation – As the racer begins to transfer the load to the inside ski, the action releases the load on the outside ski. This initiates load induced rotation of the inside ski into the new turn. The inside (uphill) edge of the inside ski acts as a pivot for the rotation. The column of the skier rotates about the pivot like an inverted (upside down) pendulum imparting rotational momentum that will take the ski and skier past ski flat and into the new turn.  The pendulum effect begins at the transition phase and ends when the transition phase for the next turn begins.
  3. Ski Flat – Ski flat provides a brief source of contiguous Ground Reaction Force that enables load transfer induced pronation.
  4. Coordinated Ankle Dorsiflexion/Knee Flexion/Hip Flexion – As the ski flattens on the snow, the racer assumes a position of balance on one foot (monopedal stance) made possible by coordinated ankle dorsiflexion/knee flexion/hip flexion. As ankle dorsiflexion progresses, it drives the intrinsic mechanism that causes the foot to pronate. As the foot pronates, the load shifts from the proximate anatomical centre of the foot to the proximate head of the first metatarsal and over the inside edge of the outside ski.
  5. Load Transfer Induced Pronation
  6. Edge Change
  7. Whole Leg Top Down Internal Rotation – As the ski edge changes and the ski rotates into the new turn, the racer rotates the leg internally (into the turn) with the powerful hip rotators. This rotates the ski horizontally about a centre between the outer aspect of the heel and the inner aspect of the head of the 1st metatarsal.
  8. Edge Set/Closed Kinetic Chain – As the ski rotates across the trajectory of the racer, the ski progressively acquires edge angle until a point is reached where it engages with the snow and creates a closed kinetic chain. When this happens, the edge ski presents a source of resistance to the skiers momentum as the skier aligns the resultant force R emanating from COM with the load W and both loads align through the proximate centre of the head of the 1st metatarsal. When the kinetic chain closes, internal rotation of the whole leg is converted through the torque converter in the subtalar joint into rotation of the foot and ski into the turn. Using this mechanism, the skier is able to wind the ski about its long axis into the turn like a corkscrew while controlling the amount of torque applied to the ski.
  9. Inside Leg Pelvis Position/Torque Control – The inside leg is used to restrain the pelvis against the torsional load arising from internal rotation of the outside leg and bottom up torsional loads from perturbations if Ground Reaction Forces.

A key aspect of the extension/pendulum effect technique is that when it is correctly executed at ski flat, it sets up a vertical force acting at right angles to the transverse aspect of the ski in opposition to the Ground Reaction Force at the portion of the inside edge 0f the ski under the centre of the head of the first metatarsal. The alignment is maintained from initiation of the new turn until it is released at the end of the turn when the transition phase commencement.

________________________________________

Biomedical perspective by Dr. Kim Hewson

This is a description of a fluid blending of biomechanical events in a ski turn.   A specific emphasis should be on responses to coupled dorsiflexion and foot pronation (no.4).  Simultaneously, a mandatory Internal rotation of the lower leg occurs.

After load transfer and edge change, further  internal leg rotation occurs  through the internal rotators of the pelvis and hip (no.8) rotating the leg into the turn.   The initial edge set also triggers a kinetic closed chain leg extension through the knee and hip from a solid platform at the foot.

Turn mechanics utilize muscle co-contractions in flexion, extension, internal and external rotation in a blending of stabilizing agonists and antagonists.

These fluid transfers of energy in a stacked(balanced) skeletal system result in minimal muscular effort while providing highly efficient turn mechanics.

Dr. Kim Hewson is a Telluride Ski School Alpine Instructor and Staff Trainer in the Biomechanics of Alpine Skiing

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.