Superior Dynamic Stability (Equilibrium) has always been the single most important factor responsible for the dominance of the World’s best skiers. It enables racers like Hirscher and Shiffrin to literally free fall, maximally accelerate under gravity then precisely land on and lock up the edges of their outside ski, establish a line and project their body towards the next gate in milliseconds and initiate a new free fall. Maximization of Dynamic Stability is crucial for a skier to set up a dynamically stable foundation in the outside ski to stand and balance on so they can establish the strongest possible position from which to generate the internal forces required to oppose the external forces acting on them.
Both skating and skiing are susceptible transverse instability manifesting as wobble oscillation (chatter) across the pivot formed by the skate blade or inside edge of a ski underfoot that challenges skater/skier Dynamic Stability. A number of quantifiable metrics are reliable indicators of the presence and degree of Dynamic Stability. A key metric is Peak (maximum) Force.
The graph below shows the peak forces of 4 competitive skaters in the 2012 University of Ottawa skate study in their own skates (OS) and the skates I prepared (NS).I have added green bars for the elite skiers with highest and lowest peak forces from the 1998 University of Ottawa pressure study for comparison purposes.
Of interest is the fact that the peak force of one of the elite ski instructors is almost 3 times the peak force of one of the other elite ski instructors. Given the small variances in peak Forces of the 4 competitive skaters in their own skates and the significant increase in peak Force seen in the skates I prepared (NS) it is reasonable to assume that some factor or factors are limiting the performance of the competitive skaters and one or more of the elite ski instructors in the 1998 study. The researchers recognized this in the 1998 ski pressure study (1.)
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.
A 2017 pressure study on giant slalom turns (3.) notes several limitations to the use of pressure analysis technology fit to ski boots to record pressures during skiing.
The compressive force is underestimated from 21% to 54% compared to a force platform, and this underestimation varies depending on the phase of the turn, the skier’s skill level, the pitch of the slope and the skiing mode.
The use of the term underestimated is out of context. When fit to a ski boot, pressure analysis technology records the plantar pressures imposed on the pressure insole. The researchers clarify this with the statement:
It has been stated this underestimation originates from a significant part of the force actually being transferred through the ski boot’s cuff.
In other words, interference with the application of plantar pressure by the structures of the ski boot is negatively affecting the ability of skier to create a foundation characterized by Dynamic Stability under the outside foot of a turn.
As a result, the CoP trajectory also tends to be underestimated along both the anterior-posterior (A-P) and medial-lateral (M-L) axes compared to force platforms.
As I will show in my next post, CoP trajectory is limited by the structures of a skate or ski boot, not underestimated by the pressure analysis technology which is only the messenger in the scheme of things.
Although a static physical environment is not the same as the dynamic physical environment associated with skating or skiing, pressure data captured on a force platform in a controlled laboratory setting can provide valuable baseline data on L-R symmetry that could explain the asymmetry seen in the large differences in the 1998 ski pressure study (1.) as shown in the table below.
What the pressure data is really showing is a L-R imbalance of Dynamic Stability.
Australian therapist and skier, Tom Gellie, posted on L-R pressure asymmetry on September 30 2018 on his FaceBook page, Functional Body.
Dynamic equilibrium is the most important aspect of skiing. Everything else is subordinated. Every aspect of skiing from equipment to technique should be assessed on its impact on the processes of Dynamic equilibrium. Ski design in particular needs to be analyzed especially as it pertains to sidecut geometry since it dictates the point where ground reaction force occurs and ground reaction force is fundamental to the initiation and maintenance of the processes of Dynamic equilibrium.
– M. Mester: keynote speaker at the first annual science symposium on skiing
……. to be continued in Part 4.
- ANALYSIS OF THE DISTRIBUTION OF PRESSURES UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS – Dany Lafontaine, M.Sc.1,2,3, Mario Lamontagne, Ph.D., Daniel Dupuis, M.Sc.1,2, 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, University of Ottawa, Ottawa, Ontario, Canada – 1998
- Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing: Thomas Falda-Buscaiot , Frédérique Hintzy, Patrice Rougier, Patrick Lacouture, Nicolas Coulmy – Published: May 4, 2017 https://doi.org/10.1371/journal.pone.0176975