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should the cm or the butt travel in a same horizontal plane?

post #1 of 13
Thread Starter 
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.
post #2 of 13
Good qestion!

Yes, there is a difference. The Center of Mass (CM) is the point about which all mass of the object is evenly distributed. It does not have to be inside the object -- a tire has CM at the center of the hole.

Holding your CM above your feet is not the same as holding your hips above your feet. That is easy to show -- bend forward at the waist, your hips move back to keep your CM above the feet so you remain "in balance" -- you don't fall.

When you move an arm outward, the CM moves in it's direction (something to think about when poling).

When you stand up from a low chair, you fling both arms forwards to get the CM above the feet well before the hips have even left the chair.

So no, the CM is not the butt.

I don't have the book here, but you need to check a few details as to why the butt may be raised:

Is there enough room under the hips at their lowest point above the snow for the legs to get under and into the new turn?

Will getting the legs into the new turn put you very far into the back seat if you don't raise the hips?

Is there any energy in the turn that could be launching the skier up and into the new turn?

Is the skier moving very slowly, so that keeping the hips low would be artificial, require a huge bend forwards at the waist and be energy inefficient?
post #3 of 13
Quote:
Originally Posted by carver_hk View Post
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.
You are looking at the wrong part of HH. Instead focus on the legs. Note how the outside leg is straight and then shortens (flexes) as he releases the turn. The key is to flex the outside leg to release the turn. How much flex will depend on the forces of the turn, how quickly you want to get out of the turn, etc...
post #4 of 13
If the angle at the knee, especially the outside knee, and at the waist increase, then the skier is down-unweighting, or flexing (different in detail) as HH skis. If those angles straighten, then the skier is up-unweighting as taught in most ski instruction. The center of mass usually can't stay in one plane.
post #5 of 13
Thread Starter 
BigE - I think the sample turn is somewhat looks like this, so your last consideration might be it:
http://www.harbskisystems.com/olk1.htm
Max - I think you are in a very good position to tell what's going on with HH's sample. did you mean the cm in a plane is not an issue or in fact HH is wrong?
post #6 of 13
Animation of HH shows turns made by ILE. Both legs remain extended at the same time (during transition). This is not so hard on your muscles since they get a moment of rest during that moment. OLF is a more advanced technique and requires more strength. That puts you in the driving seat and your skiing looks a lot more athletic and skillfull. Think HH new book skiing.

Anyway, even if we talk about our head remaining vertically in the same plane and our shoulders staying horisontal the truth is that our head moves up and down and our shoulders lean one way or the other depending on situation and they should, othervise we would be limiting natural movement. The same applies to our butt. Try to stand up right were you are right now. Stand up tall, fully extended. Now flex your leggs by bending at your knees. If you do it properly your knees point forwards, your butt backwards and your sholulders forwards. You brake a bit at your waist. CM is kept in same place horisontally allthough it moved down vertically. To prevent your "butt pointing", curve your back. Try curving it and try the opposite. See how that affects your flexing and extending. Curving your back puts your body in a better position to flex and lessens the butt pointing syndrome. Actually, I allways tell my students that if they want to point their butts they should point them uphill and not backwards. Big difference. Curving the back slightly is offcourse a highly individual thing but is part of the austrian way of teaching. HH points that out as positive as well.
post #7 of 13
Quote:
Originally Posted by carver_hk View Post
Max - I think you are in a very good position to tell what's going on with HH's sample. did you mean the cm in a plane is not an issue or in fact HH is wrong?
HH is correct. I'm saying that focusing on CM and what its doing (up or down) isn't the issue. Instead look at the outside leg. If the outside leg flexes (even just 1mm) then that is the movement that releases the turn.
post #8 of 13
Max, your posts here are nonsense. The OP is asking about CM plane, and you're spouting off about leg extension and turn initiation.
post #9 of 13
Quote:
Originally Posted by carver_hk View Post
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.
post #10 of 13
Thread Starter 
tdk6 - very good picture and your advise about pointing butt sideway did work. sending you my new ma. Please advise.

Quote:
Originally Posted by RicB View Post
...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.
does it mean the butt is a reasonably good indicator of cm then? btw. can i invite you to give me a MA? thanks in advance.
post #11 of 13
Quote:
Originally Posted by carver_hk View Post
tdk6 - very good picture and your advise about pointing butt sideway did work. sending you my new ma. Please advise.


does it mean the butt is a reasonably good indicator of cm then? btw. can i invite you to give me a MA? thanks in advance.
Only for a general reference for height of the CoM above the snow. Like others have said very well, side to side and front to back, the CoM frequently moves outside of the body/hip area. Like I said though, I find the rate and timing movements of the hips up and down to be a good general indicator of lower body movements.

Sure, I'll give them a look.
post #12 of 13
Quote:
Originally Posted by Harry_Morgan View Post
Max, your posts here are nonsense.
Harry, I just reread the original post and looked at the sample turn he references. I stand by my previous posts.

Quote:
Originally Posted by Harry_Morgan View Post
The OP is asking about CM plane, and you're spouting off about leg extension and turn initiation.

The OP specifically references the release phase of the turn which is what I addressed. I didn't post anything about leg extension or turn initiation.
post #13 of 13
Thread Starter 
Quote:
Originally Posted by Max_501 View Post
..Note how the outside leg is straight and then shortens (flexes) as he releases the turn. The key is to flex the outside leg to release the turn. How much flex will depend on the forces of the turn, how quickly you want to get out of the turn, etc...
At the very moment of both skis being flat. should we have to be careful to make sure that there are no pressure on both skis? I feel this help. Just not sure if I m on the right track.

Quote:
Originally Posted by RicB View Post
.. Like I said though, I find the rate and timing movements of the hips up and down to be a good general indicator of lower body movements.
Sure, I'll give them a look.
I am guessing by checking the rate and timing movements of the butt can give an idication of its dynamic relationship to the cm.
I send you my vid to your pm. Thanks for offering help.
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