World Cup Racing posts

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 12

At this point my discussion of the mechanics and biomechanics of platform angle is at what I can appropriately call the moment of truth. Moment in the context of the mechanics and biomechanics of platform angle means moment of force or torque; platform angle involves the ability of the CNS of a skier to control torques across the inside edge of the outside ski so the skier can stand and balance on the platform.

What is Balance?

That balance is the single most important factor in human movement, especially movement associated with athletic performance, is undisputed. In complex activities like skiing that involve movement in 3 dimensional space in a dynamic physical environment, optimal balance is critical. But what constitutes balance? In order to know if a skier is has optimal balance or is even in balance one has to know what balance is and what factors enable or compromise balance (i.e. postural) responses and  especially the factors that enable optimal balance.

The Balance Zone

A skier is in balance when the CNS is able to maintain the position of a skiers’ COG within the limits of a narrow band close to the inside edge of the outside ski during the load phase of a turn. The load phase of a turn occurs in the bottom of a turn when the force exerted on the platform by the COM of a skier must be balanced against the external resultant force of gravity and centrifugal force. In the load phase, the CNS must maintain COG within the forward limit of the Balance Zone within close proximity to the ball of the foot. When balance is challenged COG must not exceed the rearmost limit of the Balance Zone that lies just in front of the ankle joint. The Balance Zone and its limits are shown in the graphic below. If COG exceeds the limits of the Balance Zone shown in pink, the skier will lose their state of balance and with it dynamic control of the platform underfoot.  They will also suffer a lose of dynamic stability in the joint system of the lower limb.

The Balance Plane

In the ski system platform the plantar plane under the plantar aspect (sole) of the foot is the interface of CNS mediated balance activity. When the coordinated, concurrent forces are applied at the main force transfer point of the foot that I call the Center of Control, shown in the preceding graphic, the applied forces will manifest in more than one plane as shown in the graphic below.Force Fa applied under the head of the first metatarsal will be distributed over an area around its center.  When the force applied in the plantar plane is transferred through the structure of the platform to the base plane the center of force will maintain its position. But when the force area of distribution will increase as shown in the pink zones under the head of the first metataral and the base plane. In free rotation of the ski, resistance from the force of friction Ff will be minimal as will any force applied in the torque arm plane by the eccentric torque arm. Rotational force will be largely confined to the base plane.

The Missing Force Factor: Sidecut

In the free rotation, the effect of the sidecut of a ski is not a significant factor in terms of a source of resistance. But as the transverse aspect of the base plane of the ski acquires an angular relation with surface of the snow the resistance created by GRF acting at the  limit of sidecut at the shovel sets up an interaction between the rotational force applied to the inner wall of the boot shell adjacent the medial aspect of the head of the first metatarsal with the resistance created by GRF at the limit of sidecut at the shovel. In the graphic below I have connected the  2 dots of the platform ground effect problem with a line drawn between the two points.The graphic below shows a schematic of the mechanical aspects of the opposing moment or torque arms between the two dots that I connected in the preceding graphic. The inside edge below the head of the first metatarsal acts as a pivot in conjunction with the Center of Force applied 90 degrees to the transverse aspect of the base plane for the plaform to rotate about as the ski goes on edge.

As the base plane of a ski acquires an angular relationship with the snow the torque arm rotating the ski goes into what cane best be described as turbo torque boost. Whole leg rotational force continues to rotate the whole ski but the eccentric torque arm engages and applies a high torsional load that winds the body of the platform about the shovel. This mechanism has to be considered in the perspective of the of the inertia from the movement of the skier driving the cutting action of the shovel.  The graphic below shows the opposing how opposing torsional forces at the limit of sidecut and applied by the application of for by eccentic torque arm to the vertical shell wall by the medial aspect of the head of the first metarasal act to apply a upward force that extends to the outboard end of the plantar plane of the platform.  This is the mechanism that enables elite skiers to balance on their outside ski and initiate precise movement from from a dynamically stable platform.I first solved basic mechanics and biomechanics of the outside ski balance problem 30 years ago. The degree of difficulty was not great. Solving the problem took diligence and persistence in researching all the relevant aspects and identifying all significant forces and associated planes.

I’ll let the readers ponder the informaton in this for a while after which I will be happy to respond to questions and comments.

WHY SHIFFRIN AND HIRSCHER ARE DOMINATING

Existing footwear does not provide for the dynamic nature of the architecture of the foot by providing a fit system with dynamic and predictable qualities to substantially match those of the foot and lower leg.

MacPhail, US Patent 5,265,350 – November 30, 1993

Of all the figures who have influenced the development of the plastic shell ski boot over the years, the Australian, Sven Coomer, stands tall as one of the most significant and innovative. More recently, Coomer was involved with the development of Atomic’s race boot, the Redster, used by Marcel Hirscher and Mikaela Shiffrin. Coomer claims that the Redster allows the skier’s forefoot to flex and move naturally within the confines of the shell.

A 2014 article by Jackson Hogen quoted Coomer as saying:

This liberation of the previously stunted, frozen and crushed forefoot is what allows for the subtle edging and foot steering that initiates the slalom turns of World Cup champions Marcel Hirscher and Mikaela Shiffrin. (1.)

Four years, later Hirscher and Shiffrin are dominating the technical disciplines of the World Cup circuit.

The ability to establish balance on the outside foot and ski in milliseconds is dependent on the ability of the forefoot to fully spread and acquire fascial tensioning that extends to the ankle and knee. This is called time-to-stabilization. Although Coomer doesn’t mention them, a myriad of other factors are also critical; including the alignment of the big toe on the long axis of the foot and the optimal ramp angle.

Coomer suspects that if racers would only fit their boots more accurately, coupled with a dynamic molding inner boot medium between the foot and shell, and without down-sizing into short, narrow, thick-sidewall shells, their results just might improve. (1.)

In order to realize their maximum potential it is critical that racers and even recreational skiers have a ski boot fit with dynamic and predictable qualities that substantially match those of the foot and lower leg. Yet Coomer readily acknowledges:

Many racers believe they need downsized, super-stiff, ultra-narrow boots. The most accomplished alpine ski boot designer of the plastic era, Sven Coomer, believes that’s changing.(1.)

But then, he seems to retract his optimism when he says that after forty-five years as the Cassandra of the ski boot world, he knows all too well that just because you can prove you’re right, it doesn’t mean your advice will be heeded.

My observation is that since Hogen’s 2014 article, the situation with downsized, hyper-restrictive ski boots that severely compromise the dynamic nature of the architecture of the foot, has gotten worse. I have seen instances where after having ski boots properly fit, it took several full seasons for the competence of the balance to be fully restored after a skier or racer’s feet and legs were constrained for years in ski boots that were too small and too tightly fit.

Marcel Hirscher and Mikaela Shiffrin have heeded Coomer’s advice. Others choose to ignore him at their own peril. In so doing, they handicap their efforts and limit their race results.

In my next post I will start a series of posts on how to build a ski boot from the snow up; one that provides a fit with dynamic and predictable qualities that substantially match those of the foot and lower leg.


  1. The Master Boot Laster by Jackson Hogen: The International Skiing History Association – Article Date: Tuesday, June 3, 2014