Originally Posted by carver_hk
cm or the butt, any difference ?
i ask because i noticed in HH's essential book the 'sample turn' have the butt rise up somewhat at changeover.
There is a difference between the CoM and the hips. This is most noticeable front to back and side to side, and least noticeable with respect to the distance of the hips and CoM off the snow. This is because the skiing movements that move the CoM outside of the physical body laterally and fore and aft, also will generally control the height of the CoM and the hips at a similar rate. This can vary of course, but in general I think the hips and Com height off the snow track closer together than they do in the side to side and front to back plane.
Generally we can figure that the hips rise and fall is generated by the movements in the legs and also the hips. the legs effect the rise and fall through extension and flexion, and the hips effect the rise and fall as well through extension and flexion. As others have pointed out the hips/CoM the the butt can still rise as the legs flex to release. Because of the movement of the hips inside the turn the hips may still need to rise up in some turns to cross over the skis, even though there is a movement of flexing to release.
If you choose to key in on the rise and fall of the hips/CoM, I would suggest you key in to the rate, or how fast, the hips rise in a turn and when they rise in a turn. These two held in contrast to how far the hips/CoM rises will tell a lot about what the skier is doing with their lower body. Ultimately, it is the joints from the ankles to the hips that control the height of the hips/CoM. Sure, spine flexion and arm movement come into the equation, but I think this generally effects the lateral and fore aft position of the CoM more than the height of the hips/CoM combo when it comes to skiing movements.