LIGETY’S MOMENT OF TRUTH – STEP 2


The last post on this subject established that Ted Ligety starts standing on his uphill ski when it is still on its current (uphill) edge. The NY Times video, Ted Ligety on GS, has an animated sequence that shows how Ligety’s Centre of Mass rotates about the uphill edge of the inside ski as he progressively loads it. Loading the inside ski while it is still on its inside edge causes the transverse aspect of the ski to rotate away from the turn. For reasons that I will explain in future posts, it is not possible to develop a dynamically balanced base of support on the inside ski of a turn. I define balance in the context of a dynamically balanced base of support because it involves the simultaneous balancing of the moments of force across the 3 degrees of freedom (D0Fs) of the joints of the lower limb system. Given that Ligety’s inside turn (left) foot is ahead of his outside turn (right) foot and the angle of the left boot shaft is controlling the angle of flexion of his ankle, Ligety’s Centre or Pressure on his left foot will be under the heel and on the proximate centre line of the ski. It will stay there until his foot is flat or (ergo – plantigrade) on the snow surface. It is at this point that what I refer to as the Moment of Truth occurs. The animation in the NY Times video got the part right that shows pressure (Centre of Pressure) acting on the proximate transverse centre line of Ligety’s left foot. What the NY Times didn’t show, and what is the most critical part of Ligety and Shiffrin’s technique, is what happens (or should happen) in the very brief interval during which Ligety’s foot is flat on the snow surface between edge changes and, thanks to Newton’s Laws, he is standing at a right angle to the slope. The NY Times also did not show the geometry of Ligety’s GS ski and its relationship with the key mechanical references of the foot. Twenty-seven years ago, in 1987, Professor Dr. M. Pfeiffer at The Institute for Athletic Sciences at the University of Salzburg in Salzburg, Austria said, “Correct positioning of the foot is more important than forced constraint and “squeezing” the foot. Pfeiffer also said, “If muscle function is inhibited in the ankle area (which is the role of the modern form-fitted ski boot), greater loads will be placed on the knee”. It appears that Dr. Pfeiffer’s words fell on deaf ears.

In order to use the mechanics and biomechanics I am about to describe, racers like Ligety and Shiffrin need to maintain pressure on the active foot and associated ski by maintaining continuous contact with the snow surface and making smooth, uninterrupted transitions between turns, something Ligety was not able to do in the second run of the Olympic Men’s Slalom because of the way the course was set. It is also why Shiffrin got into trouble when she got caught back at one point in the Women’s Slalom. The technique they both use allows the elastic qualities of the soft tissues in the foot and leg associated with monopedal stance to absorb balance disturbing and/or potentially injurious shocks that would otherwise travel up the leg. Here is what I said about monopedal stance in US Patent No 5,265,350, which was filed and granted in the major industrial nations of the world and published worldwide on or about November 30, 1993.

“A dominant position on the outside foot in the arc of a turn affords more efficient and precise control of the instrument since the inner limb, being relatively passive, is utilized primarily for the purpose of assisting balance.

“The most important source of rotational power with which to apply torque to the footwear is the adductor/rotator muscle groups of the hip joint. In order to optimally link this capability to the footwear, there must be a mechanically stable and competent connection originating at the plantar processes of the foot and extending to the hip joint. Further, the balanced position of the skier’s centre of mass, relative to the ski edge, must be maintained during the application of both turning and edging forces applied to the ski. Monopedal function accommodates both these processes

“In skiing, the mechanics of monopedal function provide a down force acting predominantly through the ball of the foot (which is normally almost centred directly over the ski edge). In concert with transverse torque (pronation) arising from weight-bearing on the medial aspect of the foot which torque is stabilized by the obligatory internal rotation of the tibia, the combination of these forces results in control of the edge angle of the ski purely as a result of achieving a position of monopedal stance on the outside foot of the turn. (COMMENT – These biomechanics can only be set up when the new outside ski of a turn is flat on the snow surface).

‘The edge angle can be either increased or decreased in monopedal function by increasing or decreasing the pressure made to bear on the medial aspect of the foot through the main contact points at the heel and ball of the foot via the mechanism of pronation. As medial (ergo – into the turn) pressure increases, horizontal torque (relative to the ski) increases through an obligatory increase in the intensity of internal rotation of the tibia. Thus, increasing medial pressure on the plantar aspect of the foot tends to render the edge-set more stable. The ski edge-set will not be lost until either the state of balance is broken or the skier relinquishes the state of monopedal function on the outside ski. (COMMENT – This biomechanical configuration will be lost if the pressure on the outside ski (foot) diminishes beyond a specific threshold level and especially if the outside ski separates from the snow even momentarily.

One of the issues that the NY Times video, Ted Ligety on GS didn’t touch on that is critical to the technique I describe is ski geometry, especially the relationship of ski geometry to the key mechanical references of the skier’s foot. The  animation in the NY Times video appears to show a ski with little or no sidecut. The table below compares sidecuts of skis over the history of skiing as a formal sport. Note that while shovel and tail dimensions have progressively increased, one factor has remained almost static, the width of the waist of the ski. In my next post I will explain why.

Ski Geometries