# PLATFORM

Have you seen what Sassy Sally can do? She can stand on one foot. Totally amazing. They should make standing on one foot an Olympic event. Only the best athletes in the world can do this!

Are you laughing yet? You should be. Standing on one foot is something anyone can easily do unless they have significant pathological conditions or external influences that preclude it. Yet standing on the outside ski (foot) of a turn is typically regarded as the exclusive domain of the world’s best skiers.

Dots 1 to 3 set out the conditions under which a skier can engage the external forces to drive edging and carving forces. The ability to do this allows skiers to develop a platform under the outside foot to stand on in a turn. This is the secret of the world’s best skiers.

When I first started skiing, I recall hearing comments like, “He knows how to stand on his skis. That’s why he’s such a great skier!” How about, “He knows how to stand on one foot? That’s why he’s such a great walker!” In skiing, it’s not a case of a skier knowing how to stand on their outside foot. This is something we are designed to do. It is more a case of being able to stand on one foot, something the ski boot intentionally interferes with. In order to stand on the outside foot of a turn, the foot must be able to pronate. Why? At initiation, the foot has to be able to rotate on 2 axes (eversion and internal rotation of the leg) in order to develop a platform for the outside leg to stand on in the turn. Rotation about  2 axes in eversion and internal axial rotation of the leg is called pronation.

In quiet standing, the force of gravity (usually represented as G or W) pulls the Centre of Mass downward towards the centre of the earth.  G or W is an attractive or pulling force that tends to topple CoM. When G pulls CoM downward through the feet it forces the multi-axial joint system of the ankle to rotate so as to bring the 3 points of the tripod-like arches of the foot into stable contact with the ground. When the feet are rotating about 2 axes, we say that they are ‘seeking ground’ or ‘seeking ground reaction force’. Postural responses require ground reaction force. Only after the feet have acquired stable 3 point contact with the ground are the lower limbs able to support CoM and execute the postural responses  required to maintain CoM within the limits of the base of support in the feet.

In skiing, the resultant force R acting on the skier is also causing the feet to seek ground (snow) in an attempt to develop a platform that equates with ground in terms of a continuous source of ground reaction force. When R is aligned as shown in the sketch below it causes the ski to rotate about its inside edge into the turn (i.e. – evert). The black portion of the ski-platform represents the width of the waist or narrowest part of the sidecut. The grey portion (ends) of the ski-platform represents the extremes of the shovel and tail.

As the ski rotates into the turn about its inside edge, the sidecut of the ski cuts into the snow surface compressing it under the base of the ski.  The off axis vector of R with the base of the ski pulls CoM through a point between the principle points of reaction force at the pivot of the inside edge and the load at the extremes of the sidecut at the shovel and tail. The arc of the sidecut compresses the snow causing it to act in the capacity of ground. This can only happen when the vector of R is at angle less than 90 degrees to the transverse plane of the base of the outside ski and only at the inside edge. Any time R acts at an angle of 90 degrees or greater to the transverse plane of the base of the outside ski, the pull of  R on CoM  will cause the foot or feet to seek full contact with the snow surface. Until such time as the foot or feet have achieved full contact with the snow, unbalanced moments of force or torques will be present across the ankle joint and the knee and the body will be unable to generate postural responses that maintain CoM within the limits of the base of support in the foot or feet.