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Center of Mass

post #1 of 138
Thread Starter 
Back in the last Millennium, PSIA advocated moving the center of mass and letting the lower body align to it; today instructors talk about the kinetic chain and the feet tipping the center of mass inside the turn. Is this a chicken-or-egg situation? Does the CM tip the feet, do the feet tip the CM, or both? Is one focus superior to another? Why?
post #2 of 138
Symantics. It's whatever creates the right image in your head. It's no different than you asking me to stand on the balls of my feet which, to me, pushes me back because I'm opening my ankle joints and being too stiff in my calf muscles. If I think about closing my ankle joint and trying to be flat footed, I stay forward much better.
post #3 of 138
Thread Starter 
I believe I need to focus more on CM to help my students go from level 8 to level 9. What holds them at level 8 is the inability to create greater edge angles with their CM parked where it is. Most important will be keeping CM just ahead of their ankles and not letting it lag behind even a little. CM has to be to the left or right of the feet to make turns; this can happen by retracting the legs and repositioning feet under the CM or by tipping CM across the feet or a combination of the two. In any case, we want the CM and the feet to separate maximally at the apex of the turn to be long and strong and then to relax the tension and come to neutral again at transition.

RicB said something a while back that rang a bell with me: I teach relationships, not movements. What I really want to get across to my students is the nature of the relationship of CM to the feet. Without that to hang all the myriad movements on, it's just a dog and pony show.
post #4 of 138
I ask three PSIA examiners, "Which comes first, the move (of cm) or the roll (of feet). Two (tall men) said the move, one (woman) said the roll. Personally, I roll better than I move because I can feel my feet apparently can't feel my cm move unless I star the roll.
post #5 of 138
Originally Posted by nolo
. . . I teach relationships, not movements.
Just curious: Were you a banker in a previous life? ("We don't make loans. We develop relationships.")

All kidding aside, it's a great subject for discussion, and a thought provoking remark.
post #6 of 138
In my opinion, it all depends on the speed, that is, if you have a momentum or not. If you are just starting the run, and your CM is over the skis, you have no choice but to start with your ankles. Only later, depending on how fast you can get the CM away from your skis, may be even by the second turn (on a steep hill), or by the third, fourth, fifth, etc. turn , or NEVER (on a shallow slope or a catwalk), the movement of CM will ( will not) come into play. If you cannot use the ankle movement on shallow terran, your first turn, may always be awkward, as the speed will hide the lack of precision.
Some people will say it is not so, but that is my feeling and understanding of the physics of the movement. (Disclaimer: My closest relation to physics is my wife, a Ph.D. in physics)
post #7 of 138
I probably shouldn't chime in here and I anxiously await the arcmeister's rendition of this chicken and egg thing but here goes.

"does the cm tip the feet or do the feet tip the cm, or both"

It seems to me that we must consider inertia here....because once we have established movement down the hill with it comes inertia. Now we are redirecting that inertia by creating and changing resistance from the snow with our skis.

I remember years ago working with a great ski coach and mentor during my teaching years, Gary Berger. He would always talk about "the first movement into the new turn should be with your feet". I always begged to differ with him because I felt in my skiing that the feet were just about the last thing to let go of the old turn and that I began the movement toward the new turn as soon as my cm. began to take a path closer to my feet in the "arcistructure" of the turn (hey I just made that up, huh?) and that my feet did not let go and change edges until my cm was over them. In this scenerio I don't feel that the cm dictates tipping the feet or the feet dictate the tipping of the cm.. It seems that during linked rythmical turns we are carressing the snow with our ski edges in such a way that redirects the flow and trajectory of our cms on the path we desire.

Simply tipping the feet will not in and of itself move the cm. and I don't feel that I can redirect my cm without tightening arc or retracting (replement) my feet and legs during a turn. Certainly once we have passed the fall line and the forces begin to align it becomes possible to more easily manage the path of our cms. by blending the edge angles and wieghting or unweighting the skis.

As nolo stated the challenge at this level is to create a fluid turn transition. It is quite easy to differentiate a more skilled skier from a less skilled skier by observing their edge changes and the fluidity with which they are done. As a clinician my primary focus was always to improve this "flow" of the cm. so as to eliminate the flat spot in the turn. One of the analogies I would try to convey is to begin the move over the skis THEN begin the extension, instead of starting the extension movement first then moving the cm. across the skis. This creates a more accurately directed and functional extension that does not detract from the fluidity of the arc.

Arc! where are you? me now! Help me mister wizard......
post #8 of 138
Just to venture a guess, I wonder if Arc would say something like retracting the old outside ski takes away the support base of the old turn, thusly causing your inertia and the pull of gravity to naturally move your CM across the skis as you tip the new inside ski onto its little toe edge.

I too look forward to his explanation.
post #9 of 138
Since Arc is taking his time I 'll tell you. The extension of the inside leg starts to move the CoM into the new turn. There is no chicken and egg.
post #10 of 138

You have mistaken "extension" with tip/roll (inside leg tips and outside leg extends), but you also assume that inside leg action HAS to get the CM going. The plain fact is (as Bud Heishman indicated already) that I can stand there and tip my leg without moving the CM.

So I would say that the tip/roll could be a trigger, but you have to be willing to let you CM "fall" to the inside of the new turn.

I would further say that the progression to advanced or expert skiing is to learn to use tipping/rolling to initiate the new turn and then to eventually graduate to a level where you CM's position's is always where it should be and tipping/rolling is more of an effect of the CM being in the right place at the right time.
post #11 of 138
Okay, here's something to discuss. I can make railroad track turns on flats by making small hip movements that redirect pressure from one foot to the other. These hips movements consist of lifting up of one side's hip and then the other. this works because of the mechanism that lifts one hip is the opposite abductor, so along with the changing pressure under the feet, there is a coresponding slight lateral move. No direct tipping of the feet. I can do this with the feet also but I find myself in better balance control with movements originating in the hip.

I can also point my skis straight down the falline and initiate a turn by this same mechanism and continue it to a stop, or reverse the movement and turn the other way. I can combine this with inside leg extention to initiate positive foot mechanics early in the turn tipping the feet and make the turn more dynamic. I can add in femur rotation from both legs as the turn progresses and shape the turn by increasing the edge angle and pressure.

understanding the hip gives the ability to move the center into the turn balanced and connected instead of letting it fall into the turn looking for a connection. Maintaing the relationship.

For me it is the relationships of these different movements and parts, how they work together, and how they affect the pressure under our feet and the ski's relationship with the snow that we (I) need to learn to ski better. Okay, have at it. ;>D Later, RicB.
post #12 of 138

Modified thinking

When one pours cement around their ideas, one is stuck in cement. I am a voracious learner and creative thinker. Whenever my ideas are questioned, I sit back and examine my ideas instead of pour cement around them.

In the Phantom edging thread Arc said:
Maybe more on base than what our society has taught us to think. This is in concert with reasearch that suggests that contradicts our "western view" that all thinking is done in the brain only. As an example our imune system generates the same neuro-peptides as the brain does when we have a thought, indication that is courses through our body as a thinking system interelated to, but dependant on, consious thought of the mind. I accept the body genius as a "thinking body" as well.

The more "eastern" concepts of mind-body-spirit interelationships precludes the western thinking in brain only concept. While my spirit motivates me to ski, my body does the actual skiing, while my mind enjoys the ride and wonders about how it all works, suggesting adaptations inspired by the curiousoty of my spirit to seek discovery, all netting out toward becoming "one" with my skis, gravity and the mountain. I ski my best when I allow the most by causing just enough.
There were also discussions by nolo and RicB concerning this idea. nolo also introduced the idea of "smart hips" that, at the time, I did not agree with. Some discussions have also gone on in the "two skis" thread concerning this.

In this thread nolo is once again proposing the same type of thing by concentrating on CM movement vs feet and movement patterns.

Friday and Saturday I was at the ski hill. The ski instructors have a dorm that we can stay in overnight if we don't want to drive home. I did foot beds for another intructor whom also stayed over night. He is a level II instructor and we started to talking about this Tia Chi stuff. He had studied Tia Chi in the past. Per our previous discussions I was very willing to explore this with him. It was great because the exchange was two way. I filled in many blanks for him.

Once I found the Chi or the zone of energy I was able to get into and athletic stance and connect one foot to the other through this zone with suprising results. Exploring around and connecting movement patterns through this zone I could in fact connect the hips, both feet and the center of mass together. The result was a coordinated flow.
Yesterday we went out on the snow first thing to try this connection out. I'll be damned if I couldn't connect the hips, both feet and the CM together without concentrating on any one of them by simply connecting them through the chi.

My turns have never been better and I was able to finish them much more without losing the connection between two active feet and the CM.

I have modified my thinking and given merrit to the idea of moving through the chi (CM/energy zone) and gaining connection. I think this worked for me right off the bat because I already own movement patterns that do not block the inside hip movement into the new turn.

You must hit neutral or the inside hip is blocked and will drop back/lateral and to the inside. I lose the connection through the chi between both feet when this happens. I am then forced to use a ride ski and a guide ski, one foot passive and one foot active scenario.
post #13 of 138
What a loaded question this is ! I think part of the thinking here should be the old "on vs. against" arguement.

If you think about it you can eaily move the CM by tipping the feet if you are standing "on" the ski, that is to say there is more reliance on our movements (internally) to make something happen rather than externally (as in laws of nature and physics).

Standing "against" the ski (see big honkin GS turn) I can not even begin to imagine I could tip my feet as a first move to get the CM to relocate (I'm talking sucessfully here as in w/o dieing).

I suspect the answer has to be both work depending on whether you are relying on the ski to hold you up or your natural balance and skills (moving fast vs moving slow). I agree with what Bud said (and his mentor) if I read it right. I think in a high speed turn the feet (well maybe legs) do make the first move in order to redirect the CM, but I don't think it is tipping, it is a pressure change of some sort. (stand in the center of a see-saw and you can pressure the right leg to get the right side to drop or reduce pressure on the left leg to get the right side to drop---lets save what unweighting we do for another time--OK ?)

So my .02 is tip all you want to move the CM at slow speed and when you get to the point where the laws of physics take over, better be using those feet to effect a pressure change that allows movement of the CM, thereby tipping the feet.

About the edit----In the last sentence the word "weight" was replaced by the word "pressure", and the words "allows movement of" replaced "moves".
post #14 of 138
Uncle louie and RicB seem to be using a wide stance where the cm is between the feet and therefore very easily redirected. The real challenge is linking turns that finish more across the fall line and in a functionally narrower stance where the cm is outside the base of support.

I tend to agree more with Onyxjl. I tend to think of releasing or taking away the point of resistance that is holding my cm in a turn to allow it to be pulled into the next.

This concept also works to initiate a turn from a straight run, by taking away the support of, and tipping the inside ski we will redirect the cm.

Unfortunately, for many skiers misinterperet this move by making a quote "early weight transfer" which indeed releases the supporting edge of the downhill ski and inappropriately relocates it to the uphill edge of the uphill ski and stalls the flow of the cm into the new turn.

The challenge is to release the supporting edge and allow the cm to move unaffected by any input from the skier and pulled only by the forces acting upon us for a brief moment as it moves over to the other side, then extending against the new inside edge once the cm has passed to the other side far enough to make this possible. (Wow, that was a run on sentence my english teacher would love)
post #15 of 138
Good post Bud----I think we are on the same vein. LOL-- anyone who has ever skied with me knows a wide stance is the last thing evident in my skiing ! I think my see-saw example may have given the impression I was thinking transfer, but that's not the case.
I think your idea of "taking away the support" equates to my "pressure change of some sort" and it allows something to happen. That other something is the CM is "allowed" to go somewhere and tipping allows the new edge(s) to engage.
post #16 of 138
Same for me. No one has ever accused me of having a wide stance. For me Bud the idea of releasing is fine but the idea of no input from the skier is not. Now before anyone gets confused thinking I mean big moves and lots of pressure, it may be only a few onces of effort needs put into the release but wihtout this effort to maintain contact we give up control of movement momentarily, and the ability to read what is hapening under our feet, and the relationship between our Com and feet, or more importantly, the outside foot.

For me it is the old "four onces of effort redirecting 1000 lbs. of force". The connection of our center through hips down to our feet is critical for this to happen. I feel that if I want to manage the forces efficiently I have to have a connection with some input from the skier.

The hardest thing for me to learn is and was the subtlty of the connection some of the time. Whether I'm on a bridger double black and pivoting from the center of the ski or on a blue run laying trenches, the connection runs from subtlte to managing big forces, and the feet move under and out to the side depending on where the relationship needs to be. My take anyway. Later, RicB.
post #17 of 138
Back in the day when PSIA was really pushing CoM movement I was a pain in the butt to many decls because everytime one of them mentioned moving the CoM I asked how to move it. The usual answer was extention of the uphill leg to move the CoM across the skis and into the new turn. I would point out that that move would only work if the ski had already changed edges which would mean that the CoM had already moved to the other side of the skis and that the extention before the movement of the CoM would move the CoM up the hill in the direction of the old turn. None of them ever really answered this point they would just say that the crossing of the CoM was necessary to tip the feet and skis and then move on to something else. I would also point out that the CoM was already moving at a good rate and maybe all we had to do was allow this motion to move the CoM across the skis through movements of my feet. Again, none of them really seemed to understand what I was saying they were so stuck on the idea that the CoM drove the actions of the feet and that to try to use the feet to direct the actions of the CoM was totally backward. Some have changed their thinking but unfortunately many cling to the idea that we must somehow move the CoM through physical actions.

Here is my present thinking on the subject. The position of the CoM is responsible for most of the tipping of the outside ski but it is tipping, guiding, flexing and extending of the feet and legs that direct the CoM to where we want it to be. So to answer one of Nolo's questions the feet must direct the path of the CoM in order for the CoM to perform its job of tipping the feet and skis so the first actions to move into a new turn are movements of my feet and legs.

More later on why I find focus on the feet and legs to be the way to go,

post #18 of 138
Thread Starter 
Does anyone remember Mr. Natural? Mr. Natural moved his feet and CM followed.
One can see that the intention to release the CM to start into the next turn should occur some time before the edge release. This could be way up in the arc coinciding with the point of maximum leg extension (as in SL with CM traveling a more direct line with skis arcing out/around) or well after the skis arc has deflected the CM around the corner. (as in a GS turn).

Analysis of the release of the CM in many advanced skiers reflects that they release their CM too late. When we hold on so long that the skis edging decreases and the arc has begin to lengthen, the potential energy of the CM to release into the new turn dissipates and the momentum of the CM is now in the direction of the skis travel. Simply put, the window of opportunity to “let go and flow” is long gone!

This creates, as Bob points out above: “the need to aggressively redirect the CM with an active weight transfer during the transition”. In that so many skiers ski this way it is no surprise that extensive analysis is out there of the best way to make these various compensating movements that are complex, inefficient and hard work. Especially so when compared with the simply efficiency of releasing the CM sooner so as to dance with gravity instead wrestling with it.

Part of the learning to discover this and integrate it in ones skiing lies in developing increased awareness of the separate and unique, yet wholly interrelated, paths of both the skis and the CM. One aspect of efficient skiing might be described as managing the harmonious relationship of both paths (skis and CM) and their interacting energies with the mountain.
I concur with Arc's analysis from the Release Timing thread of a few weeks ago.
post #19 of 138

cm and the feet

I spent a couple of hours saturday and all day sunday with John Clendenon. His whole theme is that your CM and your feet need to be connected for great skiing. The saturday session is all done on one of his 2 ski decks (and he has some large ones.)

He premise is that the feet are loaded with nerves and is your feedback as to where your CM is. Now, the discussion in this thread so far seems to focus on lateral positioning of the CM. To Mr. Ski Doc (Clendenon) fore/aft awareness and control is just as important. As you tip or change your fore aft, you have immediate feedback in your feet as to where you are moving your CM. The CM by itself has no feedback system except where you connect with a surface.

To the lateral part of the discussion, if you tip your inside foot, unless you counterbalance with the upper body to reisit it, you'll fall to that side. So tipping of the feet in a low energy turn is a great fine control point for shifting balance laterally.

In railroad turns I've played with them 3 ways. Dip the shoulder will create edging for a railroad turn or point the knees in or the way Arc taught me in Novemeber - tip the inside foot while standing on the outside foot. The tipping Arc had us do for railroad turns was not like tipping I had experienced before. He had us first stand in a line and held the front of the ski to the snow. He wanted us tipping so strong that the ski was heavily bent while he held the front flat. The mental cue was no tip to the LTE but to feel the inside top of your arch press strongly against the boot. This type of tipping does a lot of things to the kinetic chain and gives you a railroad turn you can slowly, smoothly expand to a full gs turn.

Clendenon's use of fore/aft in the bumps was fascinating, but that's a bit off topic. I'm a pretty new skier but he had me going down the black bump runs on Ajax by mid lesson.

(and I got to shake the hand of the guy that owns Aspen while we at lunch at the club on top of the mountain where you get slippers in the slipper room to use while you "dine")

In discussions such as these, in my own skiing there is a distinction. I can race along in high speed race turns. Or I can finess stuff with fine-edge control to create turns with much shorter radius than the skis carving would allow. The feedback in the feet, at least from what I've experienced so far, is more critical when using "soft edges" and finessing (like bump skiing) then high speed more pure carved skiing (like GS turns). In the GS turns, pressure control between the feet become more important for controlling CM placement than tipping, yet, tipping of the inside ski never goes away as the main "steering wheel" in what I've experienced from my 3 major different instructor sources.

(the slipper room on the top of Ajax in the private club)
post #20 of 138
Quote Nolo----"Mr. Natural moved his feet and the CM followed."

May I suggest that Mr. Natural used an active or passive form of retraction of the legs in a subtle mannor (just enough) to allow the CM to be redirected.

I fully agree with the quote you posted (#18), but note nowhere in that quote that the feet moved anywhere. The feet eventually have to tip/roll over but to think that this can happen before the early movement of the CM as discribed in the quote is a very unnatural move, if at all possible.
post #21 of 138
Thread Starter 
My point exactly, Uncle. Thanks for driving it home.
post #22 of 138
What about Psiman? In his case the movement of the CM causes the "feet" to tip, release, and engage the skis. This reinforces that the CM movement comes first.
post #23 of 138
Hi T-Square----you and Psiman are on board here---We are just on what causes the CM to start to go somewhere. There has to be a pressure change (active or passive action of the legs) to begin the process so the CM has a place to go to !

(By the way, how was the Jackson Trip---and the surgery ?---PM ?)
post #24 of 138
I'm going on what I thought Nolo meant by active and passive in the Two Skis Thread. Quoting Nolo, "By active and passive I am making a distinction between human-driven and force-driven."

I would think that any change in muscle tension is an active process. Those changes in muscle tension will change pressure distribution on the skiis.

Therefore, softening the outside leg so you move towards the new turn is an active process. Just because you "give in" to the forces does not mean you are "passive." The passive part of this will be the movement of the CM. The CM is reacting to the centrifigal force and gravity. Although there will be a slight active movement in response to movement of the skier's joints.

As I think about this it appears that most movements are a blend of active and passive. Any thoughts?
post #25 of 138
The active/passive that I refered to here was specifically directed to the means by which we can use muscle structure in the legs (mostly), to "change pressure distribution on the skis", as you coined it above. I don't know if the present "lingo" used today still includes "replement" and "avalement" (spelling ?), but these are the French terms for the actions of the legs I spoke of.

Simply put---the muscle structure can be "active" and used to "pull" the feet toward the CM, or passive, as in just "allowing " an actively engaged muscle to relax. In both cases the resulting change in muscle tension will change the pressure distirbution on (or against) the skis and the CM will react.
post #26 of 138
It looks like we all agree for the most part on what happens when we want to BEGIN TO REDIRECT the cm.. and I agree with randyydnar post #17

This point is different than the point where our cm's are perpendicular to our feet and slope for a fleeting second, the skis are flat, and we are in dynamic balance (freeze frame) now.... Let's examine this exact point, that nolo and others mentioned earlier. Now let's talk "chicken and egg". This is the point where it could be the chicken OR the egg. My point is from this split second one could actively tip the feet and extend or passively extend on a path closer to the ski tips in order to maintain dynamic balance, (or heaven forbid, release some stored rotary energy) In either situation the cm is going to pass over the skis and there is nothing we can do about it (inertia) but, we can to some extent control it's path with the input we apply and how we apply from this frame onward.

Until the cm has at least aligned with the feet perpendicular to the slope tipping the feet seems downright impossible and if it could be done would look very awkward don't you think?

I sure am looking forward to meeting and skiing with you guys in Big Sky!
post #27 of 138
Originally Posted by bud heishman
Until the cm has at least aligned with the feet perpendicular to the slope tipping the feet seems downright impossible and if it could be done would look very awkward don't you think?

but I would swear that is what I do in fast/long turns.... get outside edge of old inside ski to engage (while it is still inside ski) then start to remove pressure from old outside ski so that old inside foot goes from supinate to pronate(inside edge of NOW new outside ski) & the old outside foot goes from pronate to supinate(now the new inside ski) .....

the new inside foot dislikes being in supination while there is weight on it - so it is necessary that all happens together so that feet can hold their pronated/supinated state easily...
post #28 of 138
Thread Starter 
I agree with Terry Barbour that the key here is that the relationship between CM and feet be harmonious (this is paraphrased third hand). I think as a profession we have over simplified ski technique in our enthusiasm for the shaped ski.

Today I will ask my class: what's the first muscle group you relax when you release a turn?
post #29 of 138

yep !

OH---I like that question nolo !

Yeah Bud---in total agreement, I guess at your "freeze frame point" it's the chicken sitting on the egg, both together !
post #30 of 138
Originally Posted by denyadog
I ask three PSIA examiners, "Which comes first, the move (of cm) or the roll (of feet). Two (tall men) said the move, one (woman) said the roll. Personally, I roll better than I move because I can feel my feet apparently can't feel my cm move unless I star the roll.
IMHO, one can't roll their ankles all the way over to where they need to be unless one moves their CM just ahead or along with the ankles.-----Wigs
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