energy transfer

THE SKI BOOT FLEX INDEX INSTABILITY PROBLEM

It has been known for decades that an unbalanced moment of force or torque will be present on the outside ski when the center of pressure of the load applied to the ski by a skier is acting along the center of the transverse axis of the ski where it is offset from GRF acting along the inside edge. Ron LeMaster acknowledges the existence of an unbalanced moment of force on the ouside ski in both The Skier’s Edge and Ultimate Skiing (Edging the skis). LeMaster states in Ultimate Skiing;

The force on the snow is offset from the center of the skier’s and creates a torque on it that tries to flatten the ski.

Ron didn’t get the mechanics right. But he correctly shows the unbalanced torque acting on the ankle joint. LeMaster tries to rationalize that ice skates are easy to cut clean arcs into ice with because the blade is located under the center of the ankle. While this is correct, ice skaters and especially hockey players employ the Two Stage Heel-Forefoot Rocker to impulse load the skate for acceleration. Hockey players refer to this as kick.

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP, Robert Colborne said:

…..In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

The load or weight of COM is transferred to distal tibia that forms the ankle joint. This is the lower aspect of the central load-bearing axis that transfers the load W from COM to the foot. What happens after that depends on the biomechanics. But the force will tend to be applied on the proximate center of the stance foot. This is a significant problem in skiing, (one that LeMaster doesn’t offer a solution for) when the ski is on edge and there is air under the body of the ski. The unbalanced torques will move up the vertical column where they will manifest at the knee against a well stabilized femur.

But this unbalanced torque creates another problem, one that is described in a paper published in 2005 by two Italian engineers (1.) that describes how this load deforms the base of the boot shell.

The Italian study found large amounts of deformation at mean loads of up to 164% body weight were measured on the outer ski during turning. The paper suggests that the ski boot flex index is really a distortion index for the boot shell. The lower the flex index, the greater the distortion potential.

For the ski-boot – sole joint the main problem is not material failure, but large amounts of local deformation that can affect the efficiency of the locking system and the stiffness of the overall system.

Values of drift angle of some degree (>2-3°) cannot be accepted, even for a small period of time, because it results in a direct decrease of the incidence of the ski with the ground.

My post GS AND KNEE INJURIES – CONNECTING THE DOTS (2.) cites studies that found that knee injuries are highest in GS in the shortest radius turns where peak transient forces are highest.

As shown in Figure 2a FR (sum of centrifugal and weight forces) and F GROUND (ground reaction force) are not acting on the same axis thus generating a moment MGR that causes a deformation of the ski-boot-sole system (Figure 2b) leading to a rotation of the ground reaction force direction. The final effect is to reduce the centripetal reaction force of the ground, causing the skier to drift to the outside of the turn (R decreases, causing the drift event).

An imperfect condition of the ski slope will emphasize this problem, leading to difficulties maintaining constant turning radius and optimal trajectory. The use of SGS ski-boot in competitions requires a particular focus on this aspect due to the larger loads that can be produced during races.

I have added a sketch showing that the moment arm M R created by the offset between the F Ground and F R is in the plane of the base of the ski where it results in an Inversion-lateral rotation torque.

The importance of sole stiffness is demonstrated with a simplified skier model…..…ski boot torsional stiffness with respect to ski longitudinal axis in particular is very important as it deeply influences the performance of the skier during turning…. A passage over a bump or a hollow may generate a sudden change in ground reaction force that may lead to a rapid change in the drift angle delta. The ski boot must be as stiff as possible going from the lower part of the boot to the ski (i.e. lower shell-joint-sole system)

As explained in the method section using the simplified model, values of some degree cannot be accepted, even for a small period of time, because the skier stability and equilibrium could be seriously compromised especially when the radius of curvature is small. A non perfect condition of the ski slope will emphasize the problem, leading to big difficulties for maintaining constant turning radius and optimal trajectory.

This excellent paper by the two Italian engineers concludes with the following statements:

Authors pushed forward the integration of experiments and modeling on ski-boots that will lead to a design environment in which the optimal compromise between stiffness and comfort can be reached.

The possibility of measuring accurately the skier kinematics on the ski slope, not addressed in the presented study, could represent a further step in the understanding of skiing dynamics and thus could provide even more insightful ideas for the ski-boot design process.

I first recognized the shell deformation, boot board instability issue in 1980, at which time I started integrating rigid structural boot boots into the bases of boot shells I prepared for racers. The improvement in ski control and balance was significant. The instability of  boot boards associated with shell/sole deformation with 2 to 3 degrees of drift at modest loads of up to 164% body weight has significant implications for footbeds.

  1. AN INNOVATIVE SKI-BOOT: DESIGN, NUMERICAL SIMULATIONS AND TESTING – Stefano Corazza 􀀍 and Claudio Cobelli Department of Information Engineering – University of Padova, Italy – Published (online): 01 September 2005 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/
  2. http://wp.me/p3vZhu-zx

WHY THE OPTIMAL STANCE FOR SKIING STARTS IN THE FEET

In this post, I am going to discuss why the optimal stance for skiing is dependent on the loading sequence of the new outside foot of turn, how this must start in the transition phase and why it is critical to the rocker impulse loading mechanism that engages the shovel and inside edge of the outside ski at edge change. This issue was introduced in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: TIMING OF EDGE CHANGE. The rocker impulse loading mechanism and the ability to balance on and control the outside ski is dependent on the ability to rapidly tension the biokinetic chain that stiffens the forefoot and torsionally stiffens the ankle and knee joints. This process enables top down, whole leg rotational force, into the turn, to be effectively applied to the foot and ski from the pelvis.

A Middle Ground on Stance

Although there is much discussion in skiing on the subject of stance, it is rare for discussions to include, let alone focus on, the foot.

The red rectangle in the graphic below shows the mid stance phase in the 8 component Gait Cycle.

A common position amongst the various authorities in skiing on stance, is that it is represented by the mid stance phase of the Gait Cycle. The 8 component Gait Cycle is the universal standard for discussion and analysis of gait in human movement. During the turn phase, the sole the outside foot or stance foot is in substantially constant contact with the zeppa or boot board. Since the ski stance does not involve initial heel contact or terminal phases, it was reasonable to conclude that skiing must be a mid stance activity.

Assuming that stance skiing is a mid stance activity also meant that the joints of the foot are mobile and the foot is still pronating and dissipating the shock of impact. The fact that the foot is not yet fully tensioned in mid stance, while still pronating, appears to have led to the conclusion that the foot is unstable and in need of support. Towards this end, form fitting footbeds, liners and, more recently, form-fitted shells were introduced and soon became standard. I described what has become known as the Holy Grail of skiing; a perfect fit of the boot with the foot and leg; one that completely immobilizes the joints of the foot in my post, A CINDERELLA STORY: THE ‘MYTH’ OF THE PERFECT FIT.  This objective, precipitated the premise that forces are best applied to the ski using the shaft of the ski boot as a handle with the leg acting as a lever. In this paradigm, the foot was relegated to a useless appendage.

The Missing Ninth Component – Late Stance

The problem with the assumption that mid stance is the defacto ski stance is that it has only recently been suggested that a critical ninth component, Late Stance, is missing from 8 components of the Gait Cycle.

Although it has been known for decades that the foot undergoes a sequential loading/tensioning process that transforms it from what has been described at initial contact as a loose sack of bones, into a rigid lever in terminal stance for propulsion, the effect of fascial tensioning on late stance has remained largely unexplored until recently when the exclusive focus on the rearfoot began to shift to the forefoot. I discuss this in BOOT-FITTING 101: THE ESSENTIALS – SHELL FIT.

As recently as 2004, Achilles/PA loading of the forefoot was poorly understood. Under Background, a 2004 study (2.) on the role of the plantar aponeurosis in transferring Achilles tendon loads to the forefoot states:

The plantar aponeurosis is known to be a major contributor to arch support, but its role in transferring Achilles tendon loads to the forefoot remains poorly understood.

The study found:

  • Plantar aponeurosis forces gradually increased during stance and peaked in late stance.
  • There was a good correlation between plantar aponeurosis tension and Achilles tendon force.
  • The plantar aponeurosis transmits large forces between the hindfoot and forefoot during the stance phase of gait.
  • The varying pattern of plantar aponeurosis force and its relationship to Achilles tendon force demonstrates the importance of analyzing the function of the plantar aponeurosis throughout the stance phase of the gait cycle rather than in a static standing position.

Changes in Muscle-tendon unit (MTU) and peak EMG increased significantly with increasing gait velocity for all muscles. This is the first in vivo evidence that the plantar intrinsic foot muscles function in parallel to the plantar aponeurosis, actively regulating the stiffness of the foot in response to the magnitude of forces encountered during locomotion. These muscles may therefore contribute to power absorption and generation at the foot, limit strain on the plantar aponeurosis and facilitate efficient foot to ground force transmission.

Transmits large forces and foot to ground force transmission means large downward forces directed at the ground or to a ski and from there to the snow.

Although I did not understand the esoteric details of fascial tensioning back in 1993, I was sufficiently aware of the relationship between peak tension in the plantar aponeurosis (PA), to be able to construct a simple model that illustrates how peak PA tension results in peak Achilles tension and how this causes the soleus muscle to go into isometric contraction, arresting further forward movement of the shank. I discuss this in detail in my series of posts on the SR Stance.

The photos below shows the simple model I made in 1993. Simple models of this nature are finding increasing use today to model what are called Anatomy Trains.

In late stance, the foot gets shorter in length and the arch gets higher and tighter as intrinsic tension transforms the foot from a mobile adapter in early stance into a rigid lever in late stance so it can apply the high force to the ground necessary for propulsion in the terminal stance phase that occurs at heel separation. The graphic below shows how the arch height h to foot length L ratio increases as the foot is getting shorter and the arch gets higher in late stance.

What has only recently being recognized is that the fascial tension that occurs in stance maximizes balance responses, neuromuscular efficiency and protection of the lower limbs through a process of  foot to core sequencing; one that stiffens the forefoot and torsionally stiffens the joints of the ankle and knee.

Loading/Fascial Tensioning Speed

A 2010 study (4.) found:

Early-stance tension in the PA increased with speed, whereas maximum tension during late stance did not seem to be significantly affected by walking speed. Although, on the one hand, these results give evidence for the existence of a pre-heel-strike, speed-dependent, arch-stiffening mechanism, on the other hand they suggest that augmentation of arch height in late stance is enhanced by higher forces exerted by the intrinsic muscles on the plantar aspect of the foot when walking at faster speeds.

…… or, by more rapid, forceful impulse loading at ski flat – see SUPER PETRA VLHOVA’S EXPLOSIVE IMPULSE LOADING IN ASPEN SLALOM

A 2013 study (3.) found:

Although often showing minimal activity in simple stance, the intrinsic foot muscles are more strongly recruited when additional loads are added to the participant.

A 2015 study (5.) found:

Changes in Muscle-tendon unit (MTU) and peak EMG increased significantly with increasing gait velocity for all muscles. This is the first in vivo evidence that the plantar intrinsic foot muscles function in parallel to the plantar aponeurosis, actively regulating the stiffness of the foot in response to the magnitude of forces encountered during locomotion.

These muscles may therefore contribute to power absorption and generation at the foot, limit strain on the plantar aponeurosis and facilitate efficient (vertical) foot to ground force transmission.

…….. or foot to ski to snow force transmission.

The Optimal Ski Stance is Unique

While the optimal stance for skiing has the greatest similarity to the late phase of stance, I am not aware of any stance that has requirements similar to the ski the stance where a specific loading sequence precedes rocker impulse loading as the outside ski changes edges in the top of a turn.

As with the gait cycle, the movement pattern associated with a turn cycle also involves loading and swing phases.

Time To Cascade

There are two intertwined rocker mechanisms that impulse load the forefoot at ski flat between edge change. These rocker mechanisms rely on what the 3 components of what I refer to as the Time To Cascade which is only possible when the plantar aponeurosis is rapidly fascially tensioned.

  1. Time to Fascial Tension which affects,
  2. Time to Stabilization which affects
  3. Time to Protection which protects the lower limbs 

In my next post, we will Meet the Rockers and continue with the discussion of the mechanics of balance on the outside ski.

  1. http://musculoskeletalkey.com/gait-and-gait-aids/
  2. Dynamic loading of the plantar aponeurosis in walking –Erdemir A1, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA. J Bone Joint Surg Am. 2004 Mar;86-A(3):546-52.
  3. Dynamics of longitudinal arch support in relation to walking speed: contribution of the plantar aponeurosis – Paolo Caravaggi, Todd Pataky, Michael Gu¨ nther, Russell Savage and Robin Crompton – Human Anatomy and Cell Biology, School of Biomedical Sciences, University of Liverpool, Liverpool, UK – J. Anat. (2010) 217, pp254–261
  4. The foot core system: a new paradigm for understanding intrinsic foot muscle function – Patrick O McKeon1, Jay Hertel2, Dennis Bramble3, Irene Davis4 Br J Sports Med doi:10.1136/bjsports-2013-092690
  5. Active regulation of longitudinal arch compression and recoil during walking and running Kelly LA, Lichtwark G, Cresswell AG – J R Soc Interface. 2015 Jan 6;12(102):20141076.

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.  

dancer

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”.

acle

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.

To learn more about foot to core sequencing please view HERE

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.

IMG_1753

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)

IMG_1767

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

 

 

 

BOOT-FITTING 101: THE ESSENTIALS – SHELL FIT

In this post, I am going to discuss the process I follow to assess what I call the essential foot to shell clearances. This is a 2-step process.

Step 1 – Establish the clearances between the structures of the foot and the inner wall of the boot shell required for the foot to function.

Step 2 – Establish the physical connections between discrete restraint force transfer areas of the foot and the inner walls of the boot shell required for the effective force transfer to the ski, for containment of the foot required to support the processes of balance and for the coupling of the foot to specific mechanical references in the boot shell related to the running surface of the ski.

As a prelude to discussing shell fit, it is necessary to point out that a major shift is occuring in the area of focus on the human foot.

Until recently, most discussions on the human foot have focussed almost exclusively on the rearfoot; the ankle complex, the tibial-talar and sub-talar joints, ankle dorsiflexion and plantarflexion, ankle mobility, inversion, eversion, etc. This limited focus has been at the expense of an appreciation and understanding of the role of the forefoot and the complex lever mechanism that enables the first MTP joint to apply large forces to the ground. A study (1) published in 2004 commented:

The plantar aponeurosis (plantar fascia) is known to be a major contributor to arch support, but its role in transferring Achilles tendon loads to the forefoot remains poorly understood.

 Fascia is a sheet or band of fibrous tissue such as lies deep to the skin or invests muscles or various body organs.

The most plausible reason why the role of the  plantar aponeurosis in transferring Achilles tendon loads to the forefoot is poorly understood is that it has not been given much attention until recently.  

The above cited study concluded:

Plantar aponeurosis forces gradually increased during stance and peaked in late stance.

The almost exclusive focus of attention on the rearfoot has led to assumptions about the function of the foot as a system which are only now being called into question and found to be erroneous or invalid. One result is the erroneous assumption that the arch of the human foot is weak and collapses under the weight of the body. This has spawned a lucrative market for custom made arch supports intended to provide what is perceived as needed support for the arch of the foot.

In boot-fitting, the process of fascial tensioning, in which the height of the arch decreases and the forefoot splays, has been misinterpreted as an indication of a collapsing (implied failure) of the arch due to its inability to support the weight of the superincumbent body during skiing maneuvers. This has led to an almost universal perception and acceptance in skiing of custom arch supports as essential foundations for the foot and the most important part of a ski boot.

The Fascial Tension/SR Stance Connection

Plantar aponeurosis forces peak in late stance in the process of fascial tensioning where they act to maximally stiffen the foot in preparation for the application of propulsive force to the ground. When fascial tensioning of the plantar aponeurosis peaks, forward rotation of the shank is arrested by isometric contraction of the Achilles tendon. This is the shank angle associated with the SR Stance.

Immobilize – Support – Stabilize

Discussions of foot function in the context of the foot to shell clearances necessary for foot function and especially fascial tensioning, tend to be obscured by a consistent, persistent narrative in the ski industry spanning decades that the foot should be supported, stabilized and immobilized in a ski boot. Foot splay, associated with fascial arch tensioning, is viewed as a bad thing. Efforts are made to prevent foot splay with arch supports and custom formed liners in order to the fit the foot in the smallest possible boot size in the name of optimizing support.

In the new paradigm that exists today, the foot is increasingly viewed in the context of a deeply-rooted structure. In the design and fabrication of footwear, attention is now being directed to the accommodation of the  fascial architecture  and the importance of fascial tensioning as it pertains to the science of the human lever mechanism of the foot.

Fascial Tensioning and the Human Foot Lever

Fascial tensioning is critical to the stiffening of the foot for effective force transmission and to foot to core sequencing.

The body perceives impact forces that tend to disturb equilibrium as vibrations. It damps vibration by creating fascial tension in the arches of the foot and the lower limb. Supporting the structures of the foot, especially the arch, diminishes both the degree and speed of fascial tensioning to the detriment of the processes of balance and the ability to protect the tissues of the lower limbs through the process of damping of impact forces.

Dr. Emily Splichal has an excellent webinar on The Science of the Human Lever – Internal Fascial Architecture of the Foot as it pertains to foot to core sequencing – https://youtu.be/_35cQCoXp9U

The DIN Standard is Not a Foot Standard

A major problem for the human foot in a ski boot is the DIN standard toe shape. DIN stands for ‘Deutsches Institut für Normung’ which means ‘German Institute of Standardization’.

The DIN toe shape creates a standard interface for bindings. In a strong, healthy foot, the big toe or hallux should be aligned straight ahead on the center axis of the boot/ski. But as an interface for the human foot, the DIN standard toe shape of a ski boot is the equivalent of a round hole for a wedge-shaped peg.

The graphic below shows a photograph of a foot overlaid over a photograph of the ski boot for the same foot. The outline of the wall of the boot is shown in red. Even though the length of the boot shell is greater than the length of the foot, the big toe will be bent inward by the wall of the shell using the one finger space behind the heel shell length check.

screen-shot-2017-02-09-at-3-55-38-pm

The Importance of Foot Splay

The progressive fascial tensioning that occurs as CoM advances over the foot transforms foot into a rigid lever that enables the plantar foot to apply force the ground or to a structure underneath the plantar foot such as a ski or skate blade. Forefoot splay is important to the stiffening of the forefoot required for effective plantar to ground force transfer.

Ski boot performance is typically equated with shell last width. Performance boots are classified as narrow. Such boots typically have lasts ranging from 96 mm to 99 mm. Narrow boots are claimed to provide superior sensitivity and quick response, implying superior control of the ski.

The outside bone-to-bone width shown in the photo below is not quite 109 mm. The boot shell has been expanded. The 2 red arrows show the 5th and 1st toe joints (metatarsophalangeal joint or MTP joint). A prime hot spot in less than adequate shell width in the forefoot, is the 5th MTP joint. Even a minimal liner will narrow the boot shell width by 3 to 4 mm.

mt-width

Shell Check: Start Point 

I start with a skier standing in both boot shells with the insole in place from the liner then have them claw each foot forward in the shells using their toes until they can just feel the wall of the shell with the outside (medial) aspect of the big toe when they wiggle the toe up and down. If there is a finger space behind the heel, the shell is in the ball park.

A second check is made with the skier standing on one foot. Some allowance for the correct alignment of the big toe  can be made by grinding the inside of the shell where it is forcing the big toe inward. When fully weighted, a fascially tensioned forefoot will splay approximately 3 mm for a female and 5 mm for a male.  The ball shaped protrusion of the 5th MTP joint is typically almost directly below the toe buckle of a 4 – buckle boot.

Once a skier can stand on one foot in each shell with adequate space for normal foot splay, the rear foot can be checked for clearance. The usual sources of problems are the inside ankle bone (medial malleolus) and the navicular and/or the medial tarsal bone. A good way to locate the prime areas of contact is to apply a thick face cream or even toothpaste to the inside ankle bones then carefully insert the foot into the boot shell, stand on it to make contact with the shell, then carefully remove the foot. The cream will leave tell tale smears on the boot shell which can then be marked with a felt pen.

Getting Step 1 successfully completed can involve alternating back and forth between forefoot and rearfoot clearance. Until, both areas are right, full normal foot splay may not occur. Step 2 is done in conjunction with liner modifications which can be a process in itself and is often the most problematic aspect of creating an environment in a ski boot that accommodates and supports foot function especially fascial tensioning.

  1. Dynamic loading of the plantar aponeurosis in walking – Erdemir A1, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA  – J Bone Joint Surg Am. 2004 Mar;86-A(3):546-52.

PRONATION AND THE BIOMECHANICS OF EDGE CONTROL

The stick man sketch below are Figures 23 A and 23 B from US Patent No. 5,265,350 (expired) awarded to the writer.  The stick man in FIG. 23 A is engaged in quiet standing with the weight equally distributed between the left and right feet. This is called Bipedal (two-footed) Stance. The force vector W emanating from the Centre of Mass or CoM  is the ‘disturbing force’ of gravity.  W is called a disturbing force because it is tending to disturb the equilibrium of the stick man and cause him to topple.

Bi-Monopedal

Gravity is an ‘attractive force’ like magnetism. CoM is where you are in relation to the supporting surface. In this case, ground. But W is not the force applied to ground by the stick man. The applied force occurs at the contact points of the foot or feet with ground.  In FIG 23 A,  W lies equidistant between the two feet in the transverse plane.

In normal Bipedal Stance, each foot supports equal proportions of the bodyʼs weight W, assuming equal leg lengths. Approximately 50 percent of the load is borne by the heel. The remaining 50 percent is borne by the heads of the long metatarsal bones. The load on the head of the first metatarsal (aka the ‘ball of the foot’) is twice that of each of the heads of the other four metatarsals. The anteroposterior (ergo, front to back) distribution  of the load through the foot is due to the position of the CoM of the body above.  The point on the foot where the centre of the applied force appears to act is called the Centre of Pressure or CoP. I say ‘appears to act’ because CoP could lie somewhere in the vault of the arches of the foot. In Bipedal Stance CoP lies on an axis that runs through the proximate centre of the heel and the head (ball) of the 2nd metatarsal. In ice skates, this is the ‘balance point’ where the ice blade is mounted. The forces shown as w2 are the centres of the ground reaction force or  that opposes CoP.

The footwear industry’s dirty little secret is that shoes are made on lasts that approximate the shape of the human foot in Bipedal Stance; standing on two feet and not moving. When you start to walk in a shoe, the structures deform and distort to accommodate changes in the architecture of the foot. Ski boots are worse. Not only are they built on lasts that are the approximate the shape of your feet and legs in quiet Bipedal Stance, the structures impede or even prevent the user from attaining a dynamically balanced base of support on one foot. Claims in relation to skiing are made that the human foot functions best in skiing when its joints are immobilized, preferably in a neutral position. In a neutral position, joint actions of the foot and knee and hip are limited to flexion and extension with transverse and orbital movement of the leg in hip joint within its normal range of motion.

A whole industry has been established on methods of immobilizing the foot and stabilizing it in a neutral position with custom formed boot shells, custom formed liners and custom formed footbeds and orthotics that significantly restrict or prevent pronation. The indirect effect of preventing pronation is that the position of CoP on the axis running through the proximate centre of the heel and head of the 2nd metatarsal becomes fixed.  For reasons that will be explained in future posts, this can have the effect of preventing the user from being able to establish the over-centre edge control mechanics that the best skiers use and especially an inability to establish a dynamically stable base of support on which to move from ski to ski.

THE MYTH OF PRONATION AND THE COLLAPSING ARCH

If you purchased a pair of ski boots or had work done by a boot-fitter you may have been told that your feet pronate and that this will cause your arch leg to collapse creating a myriad of problems including a lack of control and especially an inability to effectively transfer energy. You may have also been told that your foot functions best in skiing when it is immobilized, preferably in a neutral position, one that completely prevents pronation. All of the preceding are myths. But more than simply being untrue, they are potentially dangerous. ‘Pronation is bad’ is nothing more than a good story; one that most people buy into simply because they have no idea of what pronation is. So they tend to assume that the store clerk or boot-fitter actually knows what they are talking about.

When I first started modifying ski boots in 1973 in an attempt to help my own skiing I bought into the pronation is bad story after reading an article in a running magazine about how over-pronation (an invented term) was causing injuries in runners. The article implied that the human feet, far from being a marvel of complex engineering, are riff with problems like ‘pronation‘  that require corrective devices such as footbeds or prescription orthotics to make feet function properly. This made sense to me at the time, but only because of what I didn’t know. After reading the story in the running magazine, I had what I thought was an epiphany; ‘If foot defects can cause a myriad problems in running, imagine what havoc they must be wreaking in skiing’. Thinking I was on to something, I had prescription orthotics made for my wife and myself by a sports podiatrist. Later, I began making custom footbeds for ski boots. In the 1970s, I was one of the few boot-fitters in world making custom footbeds. And while the response of the majority of skiers I made them for was overwhelmingly positive there was one problem, my own custom footbeds didn’t work for me. In fact, footbeds and prescription orthotics made skiing much more difficult for me than insoles with no arch support. Unfortunately, the pronation is bad story caught on. Soon the evils of over-pronation morphed into the evils of pronation period; any and all pronation was bad and should be stopped or at least greatly restricted.

As Mark Twain quipped, “It ain’t what you know that gets you into trouble. It’s what you know for sure that just ain’t so”. For the past 30 years most of the world has known for sure that pronation is bad. Now a study published this month (1) confirms what I and others have maintained for years, not only is pronation not a predisposition to injury but there are significantly less injuries in those whose feet pronate.

Far from being a bad thing in skiing, pronation is the key to setting up the over-centre mechanism that makes the forces of skiing work for you instead of against you. So what is pronation?

In future posts I will explain what pronation is and why it is essential to sound ski technique and the ability to balance on the outside ski.

1.  British Journal of Sports Medicine (http://bjsm.bmj.com/content/early/2013/06/12/bjsports-2013-092202.abstract?sid=3e90eba8-f3ca-46f7-88e0-d8a28f83b729)

Sources of the story on the Danish study include:

A Popular Myth About Running Injuries – New York Times
http://well.blogs.nytimes.com/2013/06/26/the-myth-of-pronation-and-running-injuries/?_r=1

Speciality running shoes may not reduce injuries – Health – CBC
http://www.cbc.ca/news/health/story/2013/06/27/running-shoes-injuries.html