THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 6



In my last post I identified whole leg rotation of the head of the femur at its joint in the pelvis as the source of the rotational force acting 180 degrees to the transverse plane of the platform. In the technical terms of skiing whole leg rotational force is called steering.

When I started skiing in 1970 the ability to hold an edge on hard pistes and especially ice was the exclusive domain of elite skiers. Back then, the majority of skiers and racers were still skiing in low cut leather or plastic boots with the shaft not much higher than the ankle bones.

Debates raged in ski magazines as to the reason why elite skiers were able to easily hold an edge on ice while the majority of skiers struggled. The question was posed as to which came first. Did elite skiers edge first and then turn their skis or did they turn their skis and then edge? The consensus was that the best way to hold an edge and not to slip was to establish edge grip early and not slip when the forces increased. Recovering an edge once a ski started to slip was next to impossible. 

Since holding an edge during a turn involves movement of the skier there was no static way to demonstrate how to hold an edge on ice. The only option was watch an elite skier and try and copy them. This was seldom successful because even elite skiers couldn’t describe what they were doing. Strength and athletic ability and/or level of fitness did not seem to be significant factors.  Even elite hockey players often struggled to hold an edge on skis. I had questions but few answers. Finally a female ski instructor gave me a valuable clue when she told me that she presses down hard on the ball of her outside foot to make her edges hold on hard snow.

Clues such as turning the skis and putting pressure on the ball of the outside foot pointed towards the mechanism of the mechanics of platform angle and dynamic balance. But before the mechanics could be explained the introduction of the high shaft rigid plastic ski boot distracted attention away from the problem. High stiff plastic ski boots made it easy for even a novice to stand, crank their knees into the hill and put their skis on edge. This turned out to be a good marketing tool because it made holding an edge appear easy even for a novice. But using the leg as a lever didn’t work except under ideal conditions.

When I tried using my leg to hold a ski on edge on ice I met with marginal success. Later, when I modelled the mechanics the combination of forces didn’t result in a mechanism that would enable a skier to cut a step into hard pistes so as to create a platform and control its angle.

But the crank the knee into the hill option prevailed and took root. It provided an easy way to demonstrate a complex issue. Once knee angulation became established the ski industry appeared to lose interest in trying to discover the real mechanism responsible for platform mechanics. In spite of a protracted effort I didn’t begin to understand the mechanism until about 1989 after getting some valuable clues from the chapter on the ski boot in the medical text, The Shoe In Sport (see my post – THE SHOCKING TRUTH ABOUT POWER STRAPS). But getting insights on the mechanism entailed making some significant discoveries that have only come to be recognized and studied in the l ast 10 years.

One discovery I made that was fundamental to understanding platform mechanics is that the Achilles tendon is capable of transferring large forces to forefoot as the pelvis moves forward in the stance phase of locomotion.

Steer onto the Platform

Although steering and edging are often discussed together they are typically considered different, but related, skills that are blended together. In fact, they are one and the same. Elite skiers steer their skis onto a platform but only if their equipment, in particular their ski boots, enables the requisite neurobiomechanics. 

The Center of Rotation of the Foot 

The turning effort from the pelvis is applied to the foot at the distal (farther end) of the tibia as shown in the graphic below. In terms of position on the running length of a ski this places the center of rotation on the rear half of the ski. The implications are that the forebody of a ski will rotate more across a skier’s line than the tail of the ski. In my foot, the center of rotation is approximately 12 cm behind the running center of the ski.
The femur has a typical range of rotation of 45 degrees in each direction (total ROM 90 degrees); 45 degrees medial (towards the transverse center of the body) and 45 degrees lateral (away from the transverse center of the body). 

If rotational effort is applied to the foot against a firm vertical surface the rear foot will be forced away from the surface.

The implications for skiing are that as the platform angle of a ski with the plane of the snow increases towards perpendicular (normal) to the slope the turning effort applied to the feet will direct the forebody into the surface of the snow. As a reader commented on a previous post on platform angle mechanics the tips (shovel or forebody) of the ski leads the charge. A carved turn starts at the tip with the edges engaging and cutting a step into the snow for the portion of the edge that follow to track in. The shovel leads the charge and starts the carving action. 

Mechanical Points of Force 

A final point for this post is the two key mechanical points where loads on the foot apply high force to the platform; one under the ball of the great toe (i.e. head of the first metatarsal) and the other under the heel in an area called the tuber calcaneum. These are the primary centres of force in skiing. 

The effect of any rotational force or steering to a ski is significantly affected in the carving or loading phase by where the center of force is located. This will be the subject of my next post.

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