Ground forces create two kinds of motions - horizontally oriented and vertically oriented - as the orientation of forces can be horizontal or vertical as well. The latter are simple forces appearing thanks to altering our weight. It is very useful here to remember that the notion "weight" is always equal to a force:
- "statically" = your true mass x gravitational acceleration;
- "dynamically" = your true mass x gravitational acceleration +/- additional acceleration.
Vertically oriented motions happen because the weight changes, e.g. when you prepare to jump you create additional acceleration acting in the same direction as gravity, therefore, your weight changes which can be observed on the scales (with some delay though if we talk about mechanical spring scales). What always remains unchanged is mass.
Horizontally oriented motions are being induced thanks to shear forces that exists because of created torques in the hard structure of human's organism. The linear part is not being induced by the spine but by human's body response to simple physics ocurring from the ground up. Shear forces are strong enough to create torques that move the pelvis area targetwise. What happens with the spine as e.g. the secondary axis tilt is a consequence of this scenario. We already know very well (vide: the SPC concept) that automated targetwise CoG shift is totally dependent on what happens below pelvis and, consequently, on horizontally oriented forces.Feet should be aware of and prepared to feel both types of forces. The optimal combination of horizontally and vertically oriented motions forces that there exists the optimal distribution of pressure points of feet. We remember from the Diagonal Stance concept that the surface of the feet is relarively very small comparing to the mass of the whole body, especially in a dynamic movement and, therefore, both feet should never be placed in line with each other in 2 basic dimensions (lead foot flared while rear one square to the target, rear foot taken back in relation to the lead one - both rules better visible the longer the club is).
The Diagonal Stance diagramme shows the pressure points of both feet at address. What is not without importance, practically the same allocation of the pressure points occurs at impact. Let us present what happens during the entire motion in this sphere:
The red rectangles shows where the pressure is being allocated, white arrows shows the horizontal orientation of the torques (red arrows shows the same but without interaction between feet and the ground) and the white dots shows the vertical oriented forces.
At setup, we need to ensure the firm rear side from the ground up via presetting the rear knee and ankle joints. The lead side uses only normal (vertically oriented) forces. The relation between the line joining most important pressure points of both feet (point under lead foot ankle joint and rear foot's balls area) and the target line is dependent on how closed is the stance, i.e. how long is the club one actually is using (according to the D-plane principles).
During the backswing phase, the pressure point of the rear foot gradually moves to the heel as the rear leg straightens and rear hip joint goes up and back. The preset done at address still exerts impact of torques that affects the rear side. The lead side being passive starts to correspond to it parallelly.
At the top, overtorques of the rear side joints set the direction of the automatic linear shift to the S-W (assuming we are facing North at setup). The lead heel rolls up and loses contact with the ground preparing to establish the new downswing vertical axis of body rotation. Lead heel will always come off the ground unintentionally if the pelvis works in a slanted way (at 45 degrees up & back with the whole body and arms working together) and there is a proper sequencing (the lead side is totally passive during the backswing letting the lead knee joint bent inside). The lead heel should replace at least the same spot, or better a tad closer to the target because of the vertical axis of rotation linear shift to the front side of the body in a perfect world so that the dynamics of the motion is utilized most efficiently.
After the linear shift has been done and the vertical axis of downswing rotation has been established the main pressure point of the lead foot goes under the lead ankle, i.e. where the lead heels hits the ground. The rear side becomes to be passive and inertial, thus, the pressure point of the rear foot goes entirely near the inner edge. There is a slight impression of targetwise pushing from the rear foot since the overtorques have already been released and the pressure point stretches from the heel towards toes. The lead side uses the vertically oriented forces during the phase of pure rotation since the natural limitation in the lead ankle and knee joints has not been reached yet.
At impact, the horizontally oriented forces start to act, or better said, to accompany the vertically oriented ones, due to the process of releasing the stored energy which can naturally happen thanks to appearing strong torques in the joints. The rear heel rolls up and loses contact with the ground and the entire rear foot starts to spin out.
After the energy has been passed to the ball, the ground forces start to be of lesser importance. The lead foot spins out together with further post-impact body rotation while the rear foot loses contact with the ground almost entirely and often is being dragged - almost, i.e. there is some pressure left near the toe because of balance reasons.
The above visualization is totally in line with the SPC concept. For a more global view please go to the section where and compare coresponding phases of the swing motion.
