THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: IMPULSE LOADING


In this post, I will discuss the role of impulse loading, in the perspective of phases of a turn cycle, in creating a platform under the body of the outside ski on which a skier can stand and balance on.

Impulse Loading

Impulse loading is crucial to the ability to establishing a platform under the body of the outside ski by cantilivering GRF, acting along the running surface of the inside edge, out under the body of the ski to create a stable platform for the skier to stand and balance on.

Maximization of dynamic stability while skating is crucial to achieve high (vertical) plantar force and impulse. (1)

Impulse in particular has been identified as an important performance parameter in sprinting sports as skating. (1)

The preceding statements apply equally to skiing.

The most important aspect of alternating single limb support locomotion is the ability to rapidly develop a stable base of support on the stance or support leg from which to initiate precise movement. Dr. Emily Splichal refers to this process as Time to Stabilization. The ability to balance on the outside ski of a turn is unquestionably the single most important aspect of skiing. Time to Stabilization, especially in GS and SL , is where races are won or lost. Here, the time in which to maximize dynamic stability on the outside foot and leg on the outside ski is in the order of 20 milliseconds (2 one-hundredths of a second); less than a rapid blink of the eye.

The Mid Stance, Ski Stance Theory

The predominant position within the ranks of ski industry is that skiing is a mid stance activity in terms of the stance phases of the gait cycle. In the mid stance phase of the gait cycle, tension in the longitudinal arch (LA) resulting from passive tensioning of the plantar ligaments is minimal and the foot is continuing to pronate. Mid stance, as the assumed basis for ski stance, appears to have served as the rational for the assumed need to support the LA with a custom footbed or orthotic (usually in neutral STJ) and immobilize the joints of the foot with a custom fit liner. Hence, the theory that the foot functions best in skiing when its joints are immobilized. I am not aware of any studies, let alone explanations based on principles of applied science, that supports this theory. To the contrary, the available evidence suggests that immobilizing the joints of the foot, far from making it function best in skiing, has the exact opposite effect.

Wearing ski boots for a few hours can lead to a weakening of the muscles that operate within the ankle joint. This works as though one joint was excluded from the locomotive function.

………. according to Caplan et al. [3], the muscle groups that determine strength and are responsible for the function of stability in the ankle joint are very sensitive to changes caused by immobilisation. They found that immediately after immobilising the ankle joint for a week, the balance parameters were 50% lower than before the immobilisation.

 The problem with the mid stance, ski stance theory, is that impulse loading cannot not occur until late stance when arch compression, fascial stiffening of the forefoot and torsional stiffening of the subtalar and knee joints, is maximal.

One factor that has been shown to reduce arch compression is arch supportive insoles and orthotics. A study done in 2016 (1.) compared the effect of half (HAI) and full insoles (FAI) on compression loading of the arch to compression loading of the arch that occured in a standardized shoe (Shoe-only). Two separate custom insoles were designed for each participant. The first insole was designed to restrict arch compression near-maximally compared to that during shod running (Full Arch Insole; FAI) and the second was designed to restrict compression by approximately 50% during stance (Half Arch Insole; HAI). The Full Insole (black) most closely resembles the type of arch support used in ski boots to support the foot. The bar graph below shows the resulting reduction compression. I have overlain the FAI bar to illustrate how it compares to Shoe Only compression. This kind of study can now be done and should be done in vivo in skiing – during actual ski maneuvers where the effect of insoles and custom fit liners on the physiologic function of the foot and lower limb as a whole can be studied and assessed.

Two pressure studies done in 1998 by a team from the University of Ottawa (2, 3), that used elite skiers as test subjects, found large variations in pressures applied to the ball of the foot observed in the data that suggested some factor, or combination of factors, was limiting the peak force and impulse in terms of the vertical force that skiers were able to apply to the sole of the boot and ski. The researchers suggested a number of potential factors but did not investigate them.

These highest pressures reach up to 30 newtons per square centimetre. Force-time histories reveal that forces of up to 3 times body weight can be attained during high performance recreational skiing (my emphasis added).

Conclusions/Discussion:

It is quite likely that the type of equipment (skis and boots) worn by the subjects had an effect on the values obtained (my emphasis added).

A factor that was not controlled during data collection was the equipment worn by the subjects. The skiers wore different boots, and used different skis, although two of them had the same brand and model of skis and boots. It still has yet to be determined if that factor had any effect on the results. A point that all the skis that the subjects used had in common is that the skis were all sharp side-cut skis (also called shaped skis). Another equipment variation which may have affected in-boot measurements, is that some subjects (n=5) wore custom designed footbeds, while the other did not (my emphasis added).

In 2013 (4), a study presented at the European Congress of Sports Science in Barcelona, Spain that used special hockey skates that I prepared to maximize peak force and impulse using principles described in my blog compared peak and impulse forces of elite skaters in the skates I prepared (NS) to peak and impulse forces seen in their own skates (OS). The skates I prepared were used as a standardized reference similar to the protocols where baseline data obtained barefoot is used to assess the effect of specific footwear on physiologic function. The bar graphs below compare NS (the skates I prepared) to OS (the subjects own skates).

The researchers noted:

Thus, the results of this study show that direct measurement of these dynamic variables may be important indicators in evaluating skating performance in ice hockey as it relates to skate design or skill development.

Peak force and impulse are associated with high peak tension in the LA created by Achilles to forefoot load transfer.

I expect that similar results would be seen in ski boots.

The Phases of a Ski Turn Cycle

In order to appreciate the dynamics of impulse loading in skiing, I have modelled the phases of a turn cycle into 2 main phases with associated sub phases. The graphic below shows the Loading (1 – yellow) and Stance (2 – red) Phases of the outside (left) foot in a turn cycle with sub phases. The actual turn phase starts at the juncture of the traverse and from fall line and ends when the skier starts to extend the inside (right) knee. I will discuss the turn cycle in detail in a future post. My long-held theory, which was partially validated with the 1991 Birdcage studies, is that ski movements should employ the same hard-wired patterns as walking and running and that skiing should as instinctive and transparent.

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. (5)

The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. (5)

 

Peak Force and impulse loading occurs at ski flat between edge change (red circle). This is what I refer to as the Moment of Truth. Moment, in this context, being a moment of force or torque. The manner in which the torque acts in the sequence of events surrounding edge change determines whether GRF is cantilevered under the base of the ski or whether it acts to rotate the ski (invert) it out of the turn.

 

 

In my next post, I will discuss the 2-step rocker impulse mechanism that cantilevers GRF acting along the running inside edge of the outside ski out under the body of the ski.


  1. The Foot’s Arch and the Energetics of Human Locomotion: Sarah M. Stearne, Kirsty A. McDonald, Jacqueline A. Alderson, Ian North, Charles E. Oxnard & Jonas Rubenson
  2. ANALYSIS OF THE DISTRIBUTION OF PRESSURES UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS: Dany Lafontaine, M.Sc., Mario Lamontagne, Ph.D., Daniel Dupuis, M.Sc., Binta Diallo, B.Sc.. Faculty of Health Sciences1, School of Human Kinetics, Department of Cellular and Molecular Medicine, Anatomy program, University of Ottawa, Ottawa, Ontario, Canada. 1998
  3. ANALYSIS OF THE DISTRIBUTION OF PRESSURE UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS: Dany Lafontaine, Mario Lamontagne, Daniel Dupuis & Binta Diallo, Laboratory for Research on the Biomechanics of Hockey, University of Ottawa, Canada – Proceedings of the XVI International Symposium on Biomechanics in Sports (1998), Konstanz, Germany, p.485.
  4. A Novel Protocol for Assessing Skating Performance in Ice Hockey: Kendall M, Zanetti K, & Hoshizaki TB School of Human Kinetics, University of Ottawa. Ottawa, Canada – European College of Sports Science
  5. Dynamic Sensorimotor Interactions in Locomotion: SERGE ROSSIGNOL, RE´ JEAN DUBUC, AND JEAN-PIERRE GOSSARD Centre for Research in Neurological Sciences, CIHR Group in Neurological Sciences, Department of Physiology, Universite´ de Montre´al, Montreal, Canada – 2006 the American Physiological Society

 

 

8 comments

  1. Dear Dave,

    Thanks for the wonderful article. On rereading I am discovering some questions I had about the graphs as well as how and what is being measured.

    First Bar Graph.
    Half and Full Insoles
    On the y axis what is “Maximum Arch Compression”? What is being measured (start and end points)? I’m understanding greater compression is deemed better but am unclear why because I don’t know what is going on with the foot in this experiment. Is what is being measured the short foot or hard arch? I’m guessing the arch support restricts the foot somehow.

    Second display of 2 Bar Graphs.
    1st bar graph – Comparison of Peak Forces
    I am not sure I understand what is being displayed in the x axis; comparison of 4 skaters each tested with their and your skate boots? Peak force I’m guessing is the maximum pressure applied by something to the boot sole, skate blade to ice, for or aft of the blade one edge or the other? I can’t read the unit of measure what is it?

    2nd bar graph – Comparison of Impulse Forces
    Again I can’t make out the unit of measure but here impulse force seems to represent some calculation of the time the force was applied relative to change in velocity times mass.

    My take away is this is sort the reverse of the logic adding crumple points in a car for passenger safety. Your skate boot design is like the rigid frame of old style cars providing les duration (time) of Impulse yielding greater force so long as mass and change in velocity are held constant. In reality the mass of skater remains the same and in theory so dose the change of velocity at which his/her muscles perform.

    Am I following you the way you want to be understood that your skate boot design promotes a hard Longitudinal Arch LA and perhaps stiffer skate boot that complements the foot’s gait biomechanics? And further this rigidness acts like the I-beam frame of an old car therefore time of event is decreased thus force has to increase for the equation to balance?
    Impulse = Force*time = Mass*(change in Velocity)

    The result of your skate boot design is more force that equates to more GRF (snow or ice reactive force) which could be beneficial for skiers or skaters alike.

    Thanks for doing this it works my brain, Thomas

    1. Hi Thomas, I am way behind on blog activity. So I will make a start towards addressing the issues you raise.

      First Bar Graph.
      Half and Full Insoles
      > Arch height has a direct relationship with arch compression-tension. This is the same relationship of bow truss height to bottom chord tension in an architectural truss. The mechanism that creates compression is passive in that it is results from intrinsic ligament tension that causes achilles tension to peak which, in turn, causes the soleus to go into isometric contraction. The rate at which COM is moving forward (anteriorly) that results in peak force and impulse. As I pointed out, it is only recently that the recruitment of intrinsic-extrinsic muscles in stiffening the biokinetic chain and dramatically increasing peak plantar (downward directed) force and impulse has been recognized. It was the irrational and unwarranted conclusion that mid stance represented typical arch stiffness that served as the unwarranted conclusion that the foot was intrinsically weak and needed support in general from external interventions and further, that the arch should be static, not dynamic. The barefoot compression levels recorded with markers that indicate arch height reveal that functional arch compression and achilles forefoot loading is far greater than erroneously assumed.

      Second display of 2 Bar Graphs.
      1st bar graph – Comparison of Peak Forces
      I am not sure I understand what is being displayed in the x axis; comparison of 4 skaters each tested with their and your skate boots? Peak force I’m guessing is the maximum pressure applied by something to the boot sole, skate blade to ice, for or aft of the blade one edge or the other?
      > Peak force is the maximum force applied by the first MTP to the base of the skate. Impulse is the average force over a standard time. Impulse is a measure of the ability to effect a change in momentum, in the subject case, momentum of the skater. The skaters were able to apply an average of 100% greater average force over the same time frame compared to the impulse they could apply with their own skates. In both hockey skating and skiing this translates into greater acceleration.

      I can’t read the unit of measure what is it?
      > I will have to review the original data. The issue is that the skaters averaged a 100% improvement in both peak force and impulse compared to their own skates. There was no breaking or adjustment period. The skates are inanimate. They do not perform or function. They only create an environment that influences the user’s ability to perform. So the improvement seen in the data can only be attributed to the skate enabling a higher level of performance.

      My take away is this is sort the reverse of the logic adding crumple points in a car for passenger safety.
      > Close. Introducing cushioning under the sole foot between it and the ground has the effect of increasing the time to stabilization and with it, the ability to achieve maximal balance and initiate precise movement. This has the effect of attenuating both peak force and impulse. Your skate boot design is like the rigid frame of old style cars providing les duration (time) of Impulse yielding greater force so long as mass and change in velocity are held constant. In reality the mass of skater remains the same and in theory so does the change of velocity at which his/her muscles perform.

      I will continue tomorrow.

      1. Dear Dave,

        So great to hear your insights. I have been pondering them since.

        With regard to the first bar graph your response seem o get at the hart of the development of Vladimir Janda’s short foot. In my antidotal experience there are at least two significant factors involved here with regard to fitting a new ski boots. One would be roll arch supports play in comprising what you describe as:
        “Arch height has a direct relationship with arch compression-tension. This is the same relationship of bow truss height to bottom chord tension in an architectural truss. The mechanism that creates compression is passive in that it is results from intrinsic ligament tension that causes Achilles tension to peak which, in turn, causes the Soleus to go into isometric contraction.”
        In other words if the arch is inhibited from compressing (by and arch support) it by definition can not create compression-tension.
        A second factor developing the kinetic chain is bought about by the COM moving anteriorly (shank angle between 10%20% AKA 100-110) with the Achilles ET all going into isometric contraction causing a hard stop. This is similar to (if not the same as but alerted by the boot and ski holding the foot flat on the ground) the beginning of the propulsion phase of gait. These combined factors (free arch & adequate shank angle) result as you say, in “… the recruitment of intrinsic-extrinsic muscles in stiffening the biokinetic chain and dramatically increasing peak plantar (downward directed) force and impulse….” ?What is not tested for is adequate Shank angle but it’s presence is implicit in the results?

        In my experiment this spring with new boots your theory seems to match the empirical evidence I have antidotal put together. Interrupting the body’s natural ability to locomote seems to require all manor of unnecessary ski technique. Yes, I get it, the improvements to the boot/foot bed as you say “… only create an environment that influences the user’s ability to perform.” Oh, but how it enhances one’s ability!

        Hope you are having as an enjoyable weekend as I enjoy your blog, Thomas

      2. Dear Thomas,

        Please accept my apologies for the lengthy lapse in blog activity. I am working on a lot of new things that I intend to post on soon.

        Now to your questions.

        >“Arch height has a direct relationship with arch compression-tension. This is the same relationship of bow truss height to bottom chord tension in an architectural truss. The mechanism that creates compression is passive in that it is results from intrinsic ligament tension that causes Achilles tension to peak which, in turn, causes the Soleus to go into isometric contraction.”

        >In other words if the arch is inhibited from compressing (by an arch support) it by definition can not create compression-tension.
        The ability of the arch system to create compression-tension is limited by the degree with which an arch support (read: arch height limiting external appliance) restricts the height of the arch. But the effects of arch supports are far worse than simply inhibiting compression/arch height reduction.

        A second factor developing the kinetic chain is bought about by the COM moving anteriorly (shank angle between 10%20% AKA 100-110) with the Achilles ET all going into isometric contraction causing a hard stop. This is similar to (if not the same as but alerted by the boot and ski holding the foot flat on the ground) the beginning of the propulsion phase of gait. These combined factors (free arch & adequate shank angle) result as you say, in “… the recruitment of intrinsic-extrinsic muscles in stiffening the biokinetic chain and dramatically increasing peak plantar (downward directed) force and impulse….” ?What is not tested for is adequate Shank angle but it’s presence is implicit in the results?

        > There is what amounts to a 2 stage lever that shifts gears as late stance approaches. I found some good information that I will post soon. There is also an excellent recently published paper on the subject of longitudinal arch loading-sharing. It has only recently been appreciated that shank angle movement is arrested by passive simultaneous plantar ligment(s)/Achilles peak tension. Since the process is not mediated by the CNS it can and will vary with foot to foot variances of plantar ligament integrity and Achilles length. While I didn’t fully understand the process in 1991 when the Birdcage tests were done, I understood it well enough to make the string model that showed how it works. I could adjust the achilles strings to change the timing. I also understood that a slight ramp under the foot in ski boot would increase impulse loading but that the window and margin of error was small.

        In my experiment this spring with new boots your theory seems to match the empirical evidence I have antidotal put together. Interrupting the body’s natural ability to locomote seems to require all manner of unnecessary ski technique. Yes, I get it, the improvements to the boot/foot bed as you say “… only create an environment that influences the user’s ability to perform.”

        > Now we are down to the meat in the sandwich as the saying goes. In 1980 when the in-boot system I invented allowed to Podborski with a partially healed reconstructed ACL after being told he was out for at least 9 months (no 1980-81 WC Season) and win the results far exceeded my expectations. It was then that I had an epiphany of the possibility of footwear technologies that enhanced barefoot function. I called this Better than Barefoot or Beyond Barefoot. Our feet are designed for skiing. But maybe it was possible to alter te way they function for specific activities.

        “Oh, but how it enhances one’s ability!”
        > That’s a big YES. I am currently testing a new footwear technology that does what the inventor calls bio-hacking. Is it ever! Wow! Stay tuned. If it works in ski boots even close to what it does for walking and running it will be THE Game Changer. I hope to blog on this technology soon.

  2. Thank you, David. If we could only teach this to the education professionals in the PSIA! Their concept of the fundamental of edging as a result of inclination and angulation is so yesterday and detrimental to their membership that I shudder reading it. We can only keep pushing and hopefully the truth will emerge from the thinkers in their ranks. Keep up your good work. Dr. Kim Hewson

  3. So are you saying you load the uphill edge of the new outside ski before the edge change? Thank you

    1. Yes.

      The loading of the inside foot starts in what is referred as the transition phase. The natural kinematic flow in the loading of the foot in the gait cycle is from an inverted position in the unloaded (unweighted) state to an everted position (supination to pronation). At the start of loading phase in a ski turn, the weight of the body is under the heel. The cuff angle of the shaft of the boot is holding the shank in a dorsiflexed angle. The inside ski is supported on its outside (uphill) edge. Transferring the load to the inside foot will cause an acceleration of the unsupported body of the ski under foot due to the offset of the applied force and GRF acting along the ski edge. Transferring weight from the outside (downhill) foot to the inside foot initiates what amounts to a guided fall of COM down hill in relation to what will become the new stance limb. The difference between a ski turn cycle and the gait cycle is the 3 dimensional movement of the upper body in space that creates the alignment of COM with the stance foot at ski flat.

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