The position of some in the ski industry is that the foot functions best in skiing when its joints are immobilized in a neutral position; ergo, when the foot is ‘captured’. Neutral in this context refers to a neutral configuration of the subtalar joint. The subtalar joint or STJ is the joint between the talus and calcaneus (talocalcaneal joint). This important joint underlies the ankle joint (tibio-talar joint). It functions by allowing eversion and inversion of the foot. When the STJ is in so-called neutral, the foot is neither supinated (inverted – sole turned inward), nor pronated (everted – sole turned outward). When viewed from the rear, the heel bone will appear to be vertical to the ground or supporting surface when the STJ is in neutral.
I have yet to find an explanation as to how immobilizing the foot in a neutral configuration, effectively rendering the lower limbs dysfunctional, would make the foot function best in skiing. There are significant implications to not only preventing the foot from functionally pronating, but becoming functionally pronated very quickly. I use the term “without delay” in reference to the ability of the foot to pronate in a timely manner within the confines of a ski boot.
At the initiation of the transition phase between turns, the center of the force applied to a ski by the inside foot of a turn will be under the proximate centre of the base of the heel bone. This is the default location of what will become known as the Centre of Pressure (CoP) at foot flat when force is applied to the snow by the heads of all five metatarsals and the base of the heel bone. In a ski with a width underfoot of 65-67 mm, the head of the first metatarsal of a size 9 US men’s foot will be substantially aligned above the inside edge of the outside ski. But the proximate centre of the force applied to the ski under the base of heel bone will be offset 30-35 mm to the outside turn aspect of the centre of the head of the first metatarsal (dimension X in sketch below). The fore/aft offset between the centers of the head of the first MT and the base of the heel bone will be approximately 60 mm (dimension Y in sketch below).
In the transition phase, the ski goes flat between edge changes. At this time, CoP must be able to move from the base of the heel bone to the centre of the head of the first MT in a fraction of second through the mechanism of pronation (dashed red line).
The kinematics is essentially the same as in the gait sequence: supination to pronation. The unweighted swing foot is supinated: the ankle is plantarflexed, the foot is inverted and the leg is externally rotated. The foot makes contact with the ground on its lateral border and everts about the STJ as the foot pronates. CoP moves forward in a sweeping arc towards the head of the first MT. When the head of the first MT is maximally loaded the skier is in monopedal function.
Monopedal function is a physiologic state wherein balance is achieved with the weight of the body borne on the medial plantar aspect of one foot. Foot pronation is the vital integral function of monopedal stance and is essential for balance and control in the sport of skiing
The ability to balance on one foot provides superior balance and control in sports such as skiing and skating.
Should CoP fail to complete the transitory shift to the head of the first metatarsal before the ski acquires a significant edge angle, an inversion moment arm will be set up that will cause the center of force to return to its default position on the centre axis of the foot and, by association, the centre axis of the ski. The resulting inversion moment of force across the STJ will be translated into an obligatory vertical axial moment of force that will rotate the leg as a unit externally; ergo, towards the outside of the turn. In the scenario where a ski is on edge and external forces are at play that are pulling the COM of a skier towards the outside of a turn, there can be no neutral ground, no neutral STJ. There can only be eversion moments of force or inversion moments of force acting across the inside edge of the outside ski. The multi-axial joint system will do what it is designed to do: rotate simultaneously in 2 planes until the 3 points of the arch have made contact with a supportive surface and a stable source of ground reaction force has been acquired.
Whether this mechanism causes or contributes to injury is not conjecture. Any unbalanced moment of force across the STJ is a predisposition to injury of the lower limbs.