Skier Balance

IS DYNAMIC SKIING A FORM OF WALKING?

The text below is from a sub page I put up on the home page in 2014 in which I posited that elite skiers use the same hard-wired processes as walking.

It was only recently after I connected pelvic alignment with the ball of the outside foot of a turn achieved by steering the foot into position with COM to create an alignment with the fall or gravity line did I finally put the last piece of the puzzle in place.


As bipeds, we propel our bodies forward by moving from one fascially tensioned base of support with foot to core sequencing on one foot to another fascially tensioned base of support with foot to core sequencing.

Dynamic skiing uses the same basic pattern. In skiing, we need to establish a fascially tensioned base of support with foot to core sequencing on one foot in order to be able to move with precision to another fascially tensioned base of support with foot to core sequencing on another foot. As far back as the 70’s, the famous French ski technician, Patrick Russell, said that the key to effective skiing is to ‘move from ski to ski’. What Russell was really alluding to is the process of alternating single limb support.

Ever since alpine skiing became formally established, it has been known that the best skiers move from the outside ski of one turn to the outside ski of the next turn. Although this may sound simple enough, the key to being able to effectively move from ski to ski (foot to foot) is the ability to establish a fascially tensioned base of support with foot to core sequencing one one foot and then use it to move the body or Centre of Mass to the new outside foot (current uphill ski) of the next turn. Good skiers do this so seamlessly that turns seem to have no beginning or end. The turns just flow together. When viewed in the context of stance and swing phases, the resemblance to walking becomes apparent

How to make skiing as intuitive as walking is what this blog is about. I devoted an entire series of patents to this subject commencing with US Patent No. 5,265,350 and associated international patents on the elements of a minimal ski boot necessary to accommodate the process of establishing a fascially tensioned base of support with foot to core sequencing on one foot and transitioning seamlessly back and forth between bipedal and monopedal stances.

The ability to balance multi-plane torques on the outside leg of a turn is, and continues to be, the secret of the worlds’ best skiers including Toni Sailor, Nancy Greene Raine, Pirmin Zubriggen and, today, Mikaela Shiffrin, Lindsey Vonn and Ted Ligety to name but a few.


A REVIEW OF GAIT CYCLE AND ITS PARAMETERS – Ashutosh Kharb1, Vipin Saini2 , Y.K Jain3, Surender Dhiman4 – https://ijcem.org/papers72011/72011_14.pdf

Dynamic loading of the plantar aponeurosis in walking – Erdemir A1, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA. – https://www.ncbi.nlm.nih.gov/pubmed/14996881

Active regulation of longitudinal arch compression and recoil during walking and running – Luke A. Kelly, Glen Lichtwark, and Andrew G. Cresswell – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4277100/

The Foots Arch and the Energetics of Human Locomotion – Sarah M. Stearne, Kirsty A. McDonald, Jacqueline A. Alderson, Ian North, Charles E. Oxnard & Jonas Rubenson – http://www.nature.com/articles/srep19403

Shoes alter the spring-like function of the human foot during running – Kelly LA1, Lichtwark GA2, Farris DJ2, Cresswell A2. – J R Soc Interface. 2016 Jun;13(119). pii: 20160174. doi: 10.1098/rsif.2016.0174. – https://www.ncbi.nlm.nih.gov/pubmed/27307512

The Science of the Human Lever: Internal Fascial Architecture of the Forefoot with Dr. Emily Splichal – https://www.youtube.com/watch?v=_35cQCoXp9U

 

 

 

 

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.

 

NABOSO: FEEL THE FORCE

To Dr. Emily Splichal

In recognition of Dr. Emily Splichal’s contribution to my knowledge and through the knowledge gleaned from the use or her pioneering NABOSO surface science technology I am dedicating this post to her as my teacher, mentor and inspiration. Thank you Dr. Splichal.


In this post I am going to discuss how NABOSO surface science technology gave me the feedback mechanism to confirm the optimal ramp angle I needed to transition to a higher level of skier performance.

Optimal Ramp Angles starts with Stance Training

My transition started with refinements to my stance that came from incorporating Dr. Splichal’s principles of foot-to-core sequencing (that connects the feet with the pelvic core) and body fascial tensioning (that unifies the body). Prior to these changes my stance is what I would now define as good but not optimal. The huge improvement resulting from the refinements served as the impetus for a series of posts on the sequencing process required to assume a fascially tensioned stance with foot to core sequencing. I called this the SR Stance. The reason I chose this name was to draw reader attention to the stance posts by making the stance seem innovative, but not intimidating.

KIS is the Stance Kiss of Death

In reviewing material on ski technique, a skier’s stance is described as anything from an athletic stance, a relaxed stance, a ready stance, a balanced stance, a centered stance or a whatever feels good stance. A focus on selling skiing as easy with the KIS principle (Keep It Simple) has resulted in stance being perceived as less than critical to good technique. This leaves most skiers with the impression that a ski stance should feel similar to a relaxed upright stance on two feet with weight equally distributed between both feet and the heels and forefoot of each foot. This is interpreted by skiers as meaning they are balanced or in balance. So it follows that in actual skiing there should be even ‘pressure’ everywhere with no sensation of pressure on any specific area of the foot.

If I ask a typical skier to stand on a ramped surface and assume their ski stance they will find the sweet spot where their weight feels evenly distributed and identify it with their ski stance regardless of the  angle of the surface

So the first challenge to transitioning to a higher level of skier peformance is accepting that a strong ski stance must be learned and consistently rehearsed by doing drills as I do every time I go skiing. It’s like pre-flight check. NABOSO provide the conscious and subconscious CNS feedback that tells me when I am cleared for take off.

The NABOSO Effect

In my post NABOSO PROPRIOCEPTIVE STIMULATION INSOLES, I stated that the principle proprioceptive neural activity associated with balance responses occurs across the plantar plane. It is strongest in the 1st MPJ (big toe joint) and big toe. The fast acting small FA II nerves in this area are activated by pressure and skin stretch both of which occur in the late phase of Mid Stance. Optimal ramp angle is critical because it maximizes both pressure and skin stretch thereby potentiating the sensory input required to initiate controlled movement.

Assuming a NABOSO is trimmed, if necessary, to fit a shoe, there will be a positive effect on plantar proprioceptive stimulation. But my experience to date has been that the plantar proprioceptive stimulation will be much more pronounced in a minimal, zero drop shoe with adequate width for fascial forefoot tensioning and correct alignment of the big toe.

The big breakthrough for me came after I started using NABOSO insoles in shoes with different heel raises (drops). It turned out that I had the highest perception of  pressure under the ball of my foot in late mid stance phase with shoes with zero ramp (drop). When I put NABOSO insoles in my ski boots to test them I could hardly perceive any pressure under the ball of my outside foot during skiing no matter how I adjusted my stance or the tensions in my boot closures. This told me that my ramp angle of almost 3 degrees was far too great. As soon as I reduced the angle to 1.2 degrees (which is what I tested best at on my dynamic ramp angle device) it is no exaggeration to state the the whole world changed. But the transition effect didn’t kick into high gear until this ski season after my brain had time to delete a lot of the bad programming from the old ramp angle.

NABOSO 1.0 on the left. NABOSO 1.5 on the right. I use 1.5 in my ski boots. I purchase the large size and trim to fit.

Tentative Conclusions

  • A system that provides continuous subconscious sensory input to the CNS with the ability to consciously sense sensory input during drills in executive mode is important.
  • Stance training should be incorporated into racer training programs at an early stage and optimal stance ramp angle identified and implemented.
  • Once optimal ramp angle has been implemented the boot should be set up to the skier’s functional specification which I will discuss in future posts.
  • Stance ramp angle should be retested on a periodic basis to confirm the requirements have not changed.
  • Adjustments should be made as soon as possible after the end of a competitive season and no further changes made during the subsequent competitive season.

In my next post I will discuss Dr. Splichal’s protocol for using NABOSO insoles and matts in training.


Disclosure

I am not involved in any form of business association or affiliation or any have business interest or investment with Dr. Splichal/NABOSO/EBFA. Nor do I receive any form of compensation from the sale of NABOSO. Prior to marketing her NABOSO insoles Dr. Splichal provided me with a small sample of NABOSO material at her cost to cut insoles from for testing.

 

 

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 11

In my preceding post I said that after a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that rotational (steering) force should be applied to an isolated area of the inner shell wall of the ski boot by the medial aspect of the head of the first metatarsal. The reason I conducted a thorough investigation and analysis of the forces is that as a problem solver this is my standard protocol. Protocol aside, the need for a thorough investigation of every aspect affecting athletic (skier) performance was known as far back as 1983.

….. quality teaching – coaching of neuromuscular skills in physical education should always be preceded by an analytical process where the professional physical educator synthesizes observations and theory from scientific and technical perspectives……

There are many sports skills which require that sports objects, implements, equipments, and apparatus be utilized. (implements such as ski boots and skis)

All factors must be studied in terms of the skill objective. If problems are noted in the performance of the skill, where did they originate? Within the performer? Within the sport object? Both? What precise changes must be made to obtain the skill objectives?

The directions for improvement given to the performer must be based on scientific and technical analysis of the total skill.

Analysis of Sport Motion (May 1, 1983): John W. Northrip

Planes of Forces

The ability to conduct a thorough investigation and analysis of the forces associated with platform angle mechanics and biomechanics requires as a minimum, a basic understanding of the engineering aspects of the associated forces. In the case of platform mechanics and biomechanics, knowing the plane or planes in which a force or combination of forces are acting is essential.

The Force Plane in the Perfect Fit

The objective of achieving a perfect fit of the foot and leg of a skier is create a perfect envelopment of the foot and leg of a skier with the rigid shell wall of a ski boot so that force is applied evenly to the entire surface of the foot and leg to create a unified mass with the ski so that the slightest movement of the leg will produce edging and steering forces. In this format force(s) applied to the base plane by the leg will be distributed to a broad area with limited ability to apply coordinated forces to a specific area of the ski. Sensory input is also limited by the uniform force applied to all apects of the foot by the perfect fit format creating what amounts to the skiing equivalent of the Bird Box.

Platform Planes

In the mechanics and biomechanics of platform angle there are potentially three horizontal planes in which forces can be applied as shown in the graphic below. The left hand image shows the rotational force applied to a torque arm plane elevated about the base and plantar planes. In the perfect fit format in the right hand image the leg is shown as a rigid strut extending to the base plane where rotational force is applied.When the foot and leg of a skier are perfectly fit within to the rigid shell of a ski boot any force applied by the leg can only applied to the base plane of the ski where the force is distributed over a broad area. Steering and edging forces applied to the ski by the leg lack precision because they cannot be applied to specific areas or applied in a coordinated manner.

In the above graphic the whole leg rotational effort applied to the base plane by foot in the two examples is shown with no resistance. In my next post I will discuss what happens when resistance is added that opposes the rotational force applied to the base plane.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 10 – SUPPLEMENTAL INFORMATION


Because of the complex issues I am about to start discussing in the next series of posts I am providing supplemental reference information to assist the reader in understanding the issues associated with platform angle mechanics and biomechanics and underlying process of dynamic stability.

Background of events leading up to the outside ski platform ground balance solution

In late 1989, after gaining valuable insights from the medical textbook, The Shoe In Sport, I had formulated a hypothetical model that explained the macro details of the mechanics and biomechanics of platform angle and the mechanism of user CNS postural balance control.

Insights from The Shoe in Sport:

Correct positioning of the foot is more important than forced constraint and “squeezing” the foot.

Forward sliding of the foot should not be possible. 

From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). 

The comment about the importance of correct positioning of the foot and the ski boot  representing an interface between the human body and the ski gave me insights that led to the discovery of key mechanical of the foot whose position in relation to the inside edge and X-Y axes of the ski affects the transfer and control of steering and platform forces to the ski and control.

When I wrote the application for US Patent No 5,265,350 in late 1991 and early 1992 I described the mechanics and biomechanics of plantar angle in great detail knowing this information would be freely available to the entire world to use once the patent was published. The only exception was the information covered by claims. Known mechanics and biomechanics are not in themselves patentable.

Patents and Research

It is important to note that patents, even when granted, do not apply to the use of a patented device for the purpose of pure research. Knowing this at the time I wrote the patent, I described the Birdcage research vehicle in sufficient detail with many figures to enable the device to be constructed at minimal cost so research could be conducted by others as soon as possible for the purpose of advancing the knowledge base and science of alpine skiing.

The following unedited text is excerpted from the patent.

……. the teaching of this (patent) application is that force must be applied and maintained only to specific areas of the foot and leg of the user while allowing for unrestricted movement of other areas.

The performance of such mediums (skate blades and skis) is largely dependent on the ability of the user to accurately and consistently apply forces to them as required to produce the desired effect.

In addition, in situations where the user must interact with external forces, for example gravity, the footwear must restrain movements of the user’s foot and leg in a manner which maintains the biomechanical references with the medium with which it is interacting.

Precise coupling of the foot to the footwear is possible because the foot, in weight bearing states, but especially in monopedal function, becomes structurally competent to exert forces in the horizontal plane relative relative to the sole of the footwear at the points of a triangle formed by the posterior aspect and oblique posterior angles of the heel, the head of the first metatarsal and the head of the fifth metatarsal. In terms of transferring horizontal torsional and vertical forces relative to the sole of the footwear, these points of the triangle become the principal points of contact with the bearing surfaces of the footwear. 

The most important source of rotational power with which to apply torque to the footwear is the adductor/rotator muscle groups of the hip joint. In order to optimally link this capability to the footwear, there must be a mechanically stable and competent connection originating at the plantar processes of the foot and extending to the hip joint. Further, the balanced position of the skier’s centre of mass, relative to the ski edge, must be maintained during the application of both turning and edging forces applied to the ski. Monopedal function accommodates both these processes. 

Yet a further problem relates to the efficient transfer of torque from the lower leg and foot to the footwear. When the leg is rotated inwardly relative to the foot by muscular effort a torsional load is applied to the foot. Present footwear does not adequately provide support or surfaces on and against which the wearer can transfer biomechanically generated forces such as torque to the footwear. Alternatively, the footwear presents sources of resistance which interfere with the movements necessary to initiate such transfer. It is desirable to provide for appropriate movement and such sources of resistance in order to increase the efficiency of this torque transfer and, in so doing, enhance the turning response of the ski.

In skiing, the mechanics of monopedal function provide a down force acting predominantly through the ball of the foot (which is normally almost centred directly over the ski edge). In concert with transverse torque (pronation) arising from weight bearing on the medial aspect of the foot which torque is stabilized by the obligatory internal rotation of the tibia, the combination of these forces results in control of the edge angle of the ski purely as a result of achieving a position of monopedal stance on the outside foot of the turn. 

The edge angle can be either increased or decreased in monopedal function by increasing or decreasing the pressure made to bear on the medial aspect of the foot through the main contact points at the heel and ball of the foot via the mechanism of pronation. As medial pressure increases, horizontal torque (relative to the ski) increases through an obligatory increase in the intensity of internal rotation of the tibia. Thus, increasing medial pressure on the plantar aspect of the foot tends to render the edge-set more stable.

There are many figures that illustrate the concepts expressed in the above text which I will include in future posts.

The photo below shows the strain gauges (black disks) fit to the 1991 research vehicle. These gauges recorded first metatarsal forces under and to its inner or medial aspect and the outer and rearmost aspects of the heel bone.

I’ve learned a lot since the above information was made public after the patent was issued on November 30, 1993.

In Part 10, I will discuss the mechanism by which forces applied by the ball of the foot to what I call the Control Center of the platform provide quasi ground under the outside foot and leg in the load phase of a turn for a skier to stand and balance on.

SKIER BALANCE: IT’S ABOUT BALANCING OPPOSING TORQUES

The subject of my 4th post published on May 14, 2013 was the role of torques in skier balance. That this was one of my most important yet least viewed posts at 109 views suggests that the role of torques in skier balance is a concept foreign to skiers especially the authorities in the ski industry. This post is a revised version supplemented with information results from a recent study on balance control strategies.


While everyone recognizes the importance of good balance in skiing, I have yet to find an definition of what is meant by good balance, let alone a description of the neurobiomechanical conditions under which a skier is in balance during actual ski maneuvers. In order to engage in a meaningful discussion of balance, one needs to be able to describe all the forces acting on the skier, especially the opposing forces acting between the soles of the feet of the skier and the snow surface (ergo – applied and ground or snow reaction forces). Without knowing the forces involved, especially torques, any discussion of balance is pure conjecture. In 1991,  I formulated a hypothetical model that described these forces.  I designed a device with biomedical engineer to capture pressure data from the 3-dimensional forces (torques) applied by the foot and leg of the skier to the internal surfaces of the boot during actual ski maneuvers.

Test subjects ranged from Olympic and World Cup champions to novice skiers. By selectively introducing constraints that interfered with the neurobiomechanics of balance even a World Cup or Olympic champion calibre skier could be reduced to the level of a struggling beginner. Alternatively configuring the research device to accommodate the neurobiomechanical associated with skiing enabled novice skiers to use  balance processes similar to those of Olympic champions. To the best of my knowledge, no one had ever done a study of this nature before and no one has ever done a similar study since.

When analyzed, the data captured using the device called into question just about everything that is accepted as fact in skiing. This study was never published. For the first time I will present the data and describe the implications in future posts. We called the device shown in the photo the Birdcage. It was fully instrumented with 17 sensors strategically placed on a 3 dimensional grid.

Birdcage

The Birdcage instrumentation package was configured to detect coordinated neuromuscularly generated multiplane torques that oppose and maintain dynamic balance against external torques acting across the running surface of the inside edge of the outside ski in contact with the source of GRF (i.e. the snow).

  1. plantarflexion-dorsiflexion
  2. inversion-eversion
  3. external/internal vertical axial tibial rotation

Ankle torques are applied to the 3 points of the tripod arch of the foot (heel, ball of big toe, ball of little toe) and can manifest as hindfoot to rearfoot torsion or twisting wherein the forefoot rotates against the rearfoot.

A recent study (1.) on the role of torques in unperturbed (static) balance and perturbed (dynamic) balance found:

During perturbed and unperturbed balance in standing, the most prevalent control strategy was an ankle strategy, which was employed for more than 90% of the time in balance.

In both postures (unperturbed and perturbed) these strategies may be described as a single segment inverted pendulum control strategy, where the multi-segment system is controlled by torque about the most inferior joint with compensatory torques about all superior joints acting in the same direction to maintain a fixed orientation between superiorsegments.

The alignment of opposing forces shown in typical force representations in discussions of ski technique is the result of the neuromuscular system effecting dynamic balance of tri-planar torques in the ankle-hip system.

NOTE: Balance does not involve knee strategies. The knee is an intermediate joint between the ankle abd hip and is controlled by ankle/hip balance synergies.

The ankle strategy is limited by the foot’s ability to exert torque in contact with the support surface, whereas the hip strategy is limited by surface friction and the ability to produce horizontal force against the support surface.

Ankle balance strategies involve what are called joint kinematics; 3 dimensional movement in space of the joint system of the ankle complex. Contrary to the widely held belief that loading the ankle in a ski boot with the intent of immobilizing the joint system will improve skier balance, impeding the joint kinematics of the ankle will disrupt or even prevent the most prevalent control strategy which is employed for more than 90% of the time in balance. In addition, this will also disrupt or even prevent the CNS from employing multi-segment balance strategies.

Regardless of which strategy is employed by the central nervous system (CNS), motion and torque about both the ankle and hip is inevitable, as accelerations of one segment will result in accelerations imposed on other segments that must be either resisted or assisted by the appropriate musculature. Ultimately, an attempt at an ankle strategy will require compensatory hip torque acting in the same direction as ankle torque to resist the load imposed on it by the acceleration of the legs. Conversely, an attempt at a hip strategy will require complementary ankle torque acting in the opposite direction to hip torque to achieve the required anti-phase rotation of the upper and lower body.

Balance is Sensory Dependent

As a final blow to skier balance supporting the arch of the foot and loading the ankle impairs and limits the transfer of vibrations from the ski to the small nerve sensory system in the balls of the feet that are activated by pressure and skin stretch resulting in a GIGO (garbage in, garbage out) adverse effect on balance.

Spectral analysis of joint kinematics during longer duration trials reveal that balance can be described as a multi-link pendulum with ankle and hip strategies viewed as ‘simultaneous coexisting excitable modes’, both always present, but one which may predominate depending upon the characteristics of the available sensory information, task or perturbation.


  1. Balance control strategies during perturbed and unperturbed balance in standing and handstand: Glen M. Blenkinsop, Matthew T. G. Pain and Michael J. Hiley – School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK – Royal Society Open Science

SKIER BALANCE: EDGE ANGLE

In his book, Ultimate Skiing, Ron LeMaster says  that once the forces of skiing are understood, everything else makes sense. I would qualify this statement and say that until the forces of skiing are understood, nothing makes sense and any assumptions are nothing less than wild guesses.

In his book, Le Master says 2 things that I believe  are fundamental to skiing:

1. Most of the expert skiers’ weight is balanced over the outside ski and,

2. The edge angle of the ski with the applied force must be 90 degrees or smaller.

In point 2, it is my position that the angle must be less than 90 degrees. Although he doesn’t appear to say so, I am assuming that LeMaster is referring to the edge angle of the outside ski of a turn.

On page 20 of his book, FIGURE 2.3, a and b show two different alignments of opposing forces, R and S. In Figure a the angle of the transverse aspect of the base of the ski with the snow reaction force S is 90 degrees. In Figure b the angle of the transverse aspect of the base of the ski with the snow reaction force S is greater than 90 degrees.  LeMaster doesn’t provide details on what force R represents. As explained my my last post, in order to assess balance in skiing as it pertains to a dynamically balance base of support one needs to know the relationship of Centre of Mass (CoM) and Centre of Pressure (CoP) with the Snow Reaction Force. One also needs to know where the ball of the foot and centre axis of the foot are positioned in relation to the inside edge of the outside ski.

While all of the above are relevant, the key issue, which neither LeMaster’s book or any other literature I have read on ski technique doesn’t touch on, is the nature of the forces associated with a dynamically balanced base of support that would act to cause the transverse aspect of the ski base to assume  an angle with the applied force that is less than 90 degrees. Here LeMaster provides all kinds of clues that he is standing on the answer, but not seeing it. For example, on page 44 (Janica Kostelic) he describes how one of Kostelic’s strengths was her ability to maintain contact with the snow through the transition (from one turn to another) with perfect flexion moves so she could get pressure on the edge early in the turn. He goes on to say that Kostelic also knew when and how to use her inside ski to advantage. LeMaster is so close, yet so far.

As I will explain in future posts, Kostelic and all the other great skiers including Mikaela Shiffrin and Ted Ligety, more than simply getting pressure on the new outside ski early in the turn, apply forces to the new outside ski (current inside ski) that sets up and over-centre mechanism that allows them to use external forces acting on them to drive 3-dimensional forces into the new turn. They set this mechanism up in the transition from the current outside ski to the new outside ski, before they apply steering forces to the ski .


References: Ultimate Skiing – Ron LeMaster