Human nature being what it is, there will undoubtedly be those who will argue that skiing with an unbalanced base of support is not a bad thing and that it may even be better in some ways than being able to develop a dynamically tensioned base of support from which to move from ski to ski; something our bodies are designed to do. In fact, I have had some suggest that in some ways an unbalanced base of support in the context of skiing on two skis may even be safer than skiing with a dynamically tensioned base of support with the force concentrated on the outside ski of a turn.
Pronation and supination are terms often incorrectly applied to eversion and inversion. Pronation refers to the movement of the foot complex as a whole, and consists of a combination of eversion through the subtalar joint, internal rotation of the tibia, and other movements occurring across the transtarsal joints and through the forefoot. This term is most commonly applied to the motion of the foot in gait, and occurs through the first half of the stance phase, as the centre of pressure moves through the lateral aspect of the heel, to the medial side of the forefoot and the first metatarsal head. The shank rotates forward (dorsiflexes) over the ankle, the tibia rotates internally (ergo – into the turn) and the medial arch of the foot becomes compressed (ergo – the inner or longitudinal arch decreases in height)”.
– G. Robert Colborne, Ph.D.
It should be noted that the arch of the foot does not ‘collapse’ when it is compressed. Instead, the arch gets more rigid and more capable of transferring force from the base of the tibia (shin) to the head of the first metatarsal (aka – ball of the foot) and from there to the inside edge of the outside ski. It should also be noted that pronation is most commonly applied to gait (ergo – walking and running) and not quiet standing on two feet where it is commonly applied in relation to neutral alignment and the fitting of custom insoles pertaining to skiing where pronation is typically not relevant.
Pronation primarily consists of a combination of eversion through the subtalar joint and internal rotation of the tibia. Supination primarily consists of a combination of inversion through the subtalar joint and external rotation of the tibia. These rotations are not mutually exclusive of each other. The two rotations are mechanically coupled and occur on approximately a 1:1 ratio. This means that for every degree of rotation about the long axis of the foot and ski out of or into a turn there will be a corresponding vertical axial rotation of the tibia out of or into the turn. Further, the foot can invert (sole turns inward – 30 degrees), 1.5 times as much as the foot can evert (sole turns outward – 20 degrees). In addition, the natural flow of the foot is from a state of supination to a state of pronation. In skiing, pronation of the outside foot of a turn leads to a dynamically balanced base of support. Supination does not.
The foot has two major functions: support and propulsion. It must be a mobile structure to allow stability on uneven surfaces and a rigid structure to support the body in standing or allow the forefoot to act as a lever in propelling the body forward.
– The Joints of the Ankle by Verne T. Inman.
Although Wright and Desai described the importance of ankle and coupled subtalar rotation at length in a paper published in 1964 it was Verne. T. Inman, Professor Emeritus who described the subtalar joint as a torque converter in converting rotational moments of force about the long axis of the foot into corresponding vertical axial moments of rotation of the tibia in his seminal work The Joints of the Ankle published in 1976, some 38 years ago. So the hard coupling of rotation of the foot about its long axis in conjunction with vertical axial rotation of the leg is not something that was discovered yesterday or is in any way controversial. Here is a video clip that shows the torque converter mechanism in the subtalar joint.
The implications to skiing are that any moment of force that tends to cause the foot to rotate into or away from a turn will unavoidably tend to cause the tibia to rotate in the same direction. More recently, Benno M. Nigg has further elaborated on the coupling mechanism in his book, I will expand on this mechanism in future posts and explain the protective effect of pronation and a dynamically balanced base of support on the outside ski of a turn.