If there two issuse the ski industry appears to be in agreement on it is that the foot functions best when it is immobilized in neutral and that pronation is bad.
Pronation is a combination of eversion (the sole turns away from the left-right centre of the body) and vertical axial rotation of the leg towards the left-right centre of the body. In the context of skiing, if the outside foot of a turn were to pronate, the foot and leg would rotate into the turn. But this should not be allowed to happen because pronation is bad. Let’s find out if this is true.
First, let’s revisit the stick men figures in FIG. 23 A and FIG. 23 B from my US Patent No. 5,265,350. The feet in FIG. 23 A are in neutral whereas the foot in FIG. 23 B is pronated. Note the angular relationship of the leg with the foot in FIG. 23B. It has enabled the alignment of the force W from the Centre of Mass and W of ground reaction force in opposition with each other. The force vectors are at 90 degrees to plane of the supporting surface. Note that the forces pass through innermost aspect of the pronated foot in FIG. 23B whereas in FIG. 23 A the forces are acting at the proximate transverse centre of each foot.
The sketch below is from THE EFFECT OF LIFT PLATES ON SKI MANEUVERS. When the foot is in a neutral it cannot pronate and the foot cannot evert. So it is impossible to align resultant and ground reaction forces so they are perpendicular to the transverse aspect of the base of the ski. In the sketch below the base of the ski is at an angle that is greater than 90 degrees to the vector of the resultant force R and ground reaction force GRF. In this relationship of forces a shear component of force will exist that will tend to make the ski slip out of the turn.
The sketch below shows the same mechanical constraints as the foot in neutral above except that the foot has pronated. The foot has everted sufficiently for the resultant force R and ground reaction force GRF to align in opposition to each other at right angles to the transverse aspect of the base of the ski. The force vector from the mechanical line is exiting the lift plate above the intersection of R and GRF.
The sketch below compares the effect of lift plates on pronated foot (black) and neutral feet (light grey).
FIS regulations permit a maximum combined height of the sole of the foot above the base of the ski of 100 mm. This is about the average width of a US men’s size 9 foot. The sketch below shows a pronated foot with an approximated maximum allowable lift height.
Things get even more interesting when sidecut and vertical axial rotation of the leg (femur) into the turn are added to the discussion. When the outside foot of a turn can pronate, enabling the the use of internally generated vertical axial rotation of the leg (femur) into the turn to counter externally generated torque, skiing becomes ridiculously easy.