Before I can discuss the role of pronation in enabling a skier to develop a dynamically tensioned base of support, I need to clarify the implications of a neutral foot.
The 2 big buzz words in boot-fitting are neutral foot and neutral alignment meaning that the knees track straight forward during ankle flexion.
The ankle complex consists of two major joints. The base of the tibia forms a joint with a bone called the talus. The resulting joint is called the tibial-talar joint, commonly referred to simply as the ankle joint. A second joint below the talus is the sub-talar joint. This joint underlies the tibial-talar joint. It allows the foot to rotate about its long axis in eversion and inversion. The tibial-talar joint is the joint that flexes the foot in plantar-flexion (toes move away from the shin) and dorsi-flexion (toes move closer to the shin).
Due to the confusion and misinformation that has arisen surrounding pronation, a common perception exists that anything other than flexion of the ankle is abnormal and problematic. The solution is footbeds that are claimed to maintain the foot in a neutral axis and alignment procedures typically follow that are claimed to correct abnormal tracking of the knees and ensure that they move straight ahead on a neutral axis when the ankle joint is flexed.
None of these positions are supported in sound principles of biomechanics. But I will save further discussion of this issue for a future post. The image below shows the same neutral configuration of the feet in bipedal neutral stance as my last post. In this image I show the straight ahead (neutral) excursion trajectories of Centre of Mass and Centre of Pressure in each foot. I also show the axis of the ankle joint as square (at right angles) to the excursion trajectories. This does not happen in reality. But in order to differentiate between neutral Sub-Talar Joint and pronation I need to indulge the neutral camp.
The limits of the Base of Support that define what is called the sway zone is shown in grey. Because of the large area of the Base of Support at one time or another some formulators of ski methodologies have advocated a wide track stance with weight and steering on both skis because it is more stable than a narrow stance where the weight is predominantly on the outside ski of a turn. In fact, as I will show in future posts, a wide track stance is not only highly unstable, it precludes the ability to develop a dynamically tensioned base of support on which to move from one foot to another foot.