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 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. , 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).
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.
- 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
- 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
- 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.
- 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
- 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