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Balance? Stability? - Page 2

post #31 of 61
But as I said earlier, it does give us all an awareness of how precise we need to be with our wording and descriptions, and how we need to use language that would best clarify our points to a specific audience.

Since many of us teach something or another, that is a valuable lesson!

Be Braver in your body, or your luck will leave you. DH Lawrence
post #32 of 61

You wrote: <<For example, at various points in this thread, when referring to movement of the CM, I was not at all sure if a given writer meant up and down (relative to the earth), across your skis, changing the overall shape of your body into a "C" so that the CM moves from your lower abdomen to some point that may actually be outside your body, etc. etc.<<

Not being a physics man myself, correct me if I'm wrong, but I thought the CM was in a constant position on a person, and that it was the CG (Center of Gravity) that would move as the person moves and forces act on the person? Naaa. I'm probably wrong, because a person can compensate for a move of the abdomen in one direction with a move of the feet and shoulders in the opposite direction.

However, as it pertains to this, I know that I use CM to represent the position person's overall mass (with applied forces) as it relates to the position of the skis and the slope of the hill. In other words, if you want to go from the old turning edges to the new turning edges, your "CM" or "mass" must cross over the skis. And that your "CM" or "mass" is directly over the skis at the point where you are able to have the skis flat against the snow's surface. But there is nothing static about this position, your CM is simply "passing through".
post #33 of 61
John H -

1) Gravity acts independently on every single atom in an object, and can be summarized by a single force acting on the "center of gravity" of the object.

Other centers of force exist. For example, aerodynamic forces act on the surface of the body, and can be summarized by a single force acting on what is called the aerodynamic center of the body.

It turns out that the center of mass and the center of gravity of a body are exactly coincident when the gravitational force is constant in magnitude and direction over the whole body under consideration. To make them not coincident, you would have to be dealing with a body that is about the same size as the size of the source of gravity (ie, the earth) and not very far away, so for skiing discussions there is little reason to distinguish between them.

2) The CM (or CG) of a body can actually lie outside of the body (if the body is oddly enough shaped). For example, the CM of a boomerang lies out in space along the diagonal between the two arms.

If a person bends forward at the waist by 90 deg, from the side, they will be shaped somewhat like a boomerang, and their CM will move from their lower torso to a position in front of, and slightly below their hips. When a person uses hip and knee angulation, when viewed from the appropriate direction, they are also in a "L" shape, and their CM again moves perpendicular to the viewing direction.

3) External forces do indeed make the CG of a body move relative to a fixed reference frame (eg, the CM of the skier goes down the hill).

However, if you have a rigid body, external forces acting on it do not change the position of the CG relative to the body itself (as described in #2, above). Put differently, for a rigid body, the CM/CG DOES NOT MOVE AT ALL in the frame of reference of the body. This is where my previous warning about specifying your reference frame comes into play.

4) Finally, your description of the CM moving across the skis and not being static (in the reference frame of the skier) is exactly correct.

Sorry for all the detail, but this is about as short as you can make such a description.
Hope it helped.

post #34 of 61
Whew - my brain cells are strained. All I meant to say is "its easier and faster to move one leg that two". Should have known better than to assume that legs have any mass
post #35 of 61
Yeah, well I just blew out one of my two brain cells.

Thanks, PM, that's why it's nice to have a local Physics Man around here. I stand corrected (with my CM and CG conveniently located in the same place).

I think.
post #36 of 61
I bet that will teach you guys never to ask for an explanation again (grin). My 8 y.o. daughter has already caught on to this, and if she sees one coming, she warns everybody in the area - "Watch out! An explanation is coming! Duck!!"

Sorry it couldn't be simpler, but the skiing moves you are trying to describe would probably be called something like "time dependent dynamics of a non-rigid extended body" by academics, and it really IS a complicated subject.

Now - I have a question for you guys.

Earlier in this thread, you were comparing the speed at which you could move your CM (ie a narrow vs a wide stance). In the wide stance that you were considering, were both skis edged, say a foot or so apart (in the horizontal plane), or were you envisioning somebody hypercarving with the inside knee strongly bent, the inside ski fairly flat and directly under the body, and the outside ski WAY out like an outrigger and edged almost 90 deg to the snow?

I have a hunch what's causing the difference of perception / opinion between you guys, but I've got to ask some questions to be sure.



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[This message has been edited by PhysicsMan (edited July 23, 2001).]</FONT>
post #37 of 61
Not to inject humor into such a serious subject, but there is an instructor at Bretton who calls the center of mass WORCESTER. Get it? Center of Mass.


Be Braver in your body, or your luck will leave you. DH Lawrence
post #38 of 61

Besides, everybody knows that the correct pronounciation is Wor-chest-er.

post #39 of 61
I thought we all understood there is no such thing as centrifugal force...heh heh

When the going gets weird, the weird turn pro...
post #40 of 61
post #41 of 61
Pierre eh! & Rick H -

First, let me make sure I understand exactly what you guys are discussing. If I can paraphrase it down to a couple of sentences, I think the main argument is that:

(a) Rick thinks you can more quickly adjust your body position to stay in balance from a narrow stance, whereas,

(b) Pierre eh! thinks you can more quickly adjust weight distribution between the feet, (as well as body position) to stay in balance from a wide stance.

I think that the main reason for the disagreement is that (a) and (b) involve two different mechanisms to maintain balance. The bottom line is that I have to agree with Pierre eh! that maintaining balance from a wide stance is almost certainly quicker and more certain, but it will involve more energy expenditure.

For the purpose of this argument, lets consider a skier perfectly in balance, in the middle of a constant radius turn going at constant speed.

Case (a): In the limit of a very narrow stance, skiing on one foot, or even a snowboarder, the person must have adjusted his body position so that the combination of gravity and centrifugal force (pushing on him via his edge) goes exactly through his center of mass. If this isn't true, the person would start falling to either the inside or the outside of the turn and would need to correct this.

About the only thing any single-planker or narrow-stancer can do to smoothly get back in balance is to change the radius of his turn. He can do by changing his edge angle or by making changes in fore-aft pressure. As Pierre eh! essentially pointed out, such changes can be made with relatively little effort, but take a while for them to take effect.

Case (b): In a wide stance, the skier that needs to make a balance correction has another option open to him. He can immediately start pressing down with the appropriate leg. Even without making any change whatsoever in the position of the CM, this can get the person back into balance - its exactly what you do when you are standing still on the flats.

Now, after you do this, the weight distribution between your legs will certainly be altered from what you prefer (for example, you may want the outside leg to be carrying 80% of the weight on hardpack), but this is something that can be dealt with separately from the immediate issue of namely instantaneous balance.

For example, if the skier is starting to fall to the inside of the turn, he can almost instantly weight the inside leg more and immediately restore his balance. If he then wants to make sure his outside ski continues to carry the majority of the load, he can then push with his inside leg and raise his CM back to where it needs to be to get the desired L/R weight distribution. Unfortunately, actually moving your CM around like this takes energy, and hence this type of balance correction move, altho fast, will likely tire a person out more quickly than the edge angle (or fore-aft pressure) changes of case (a).

The bottom line is that both for theoretical reasons, and from my own (much lower level) sking experience, I have to agree fully with Pierre eh! and his concise explanation.


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[This message has been edited by PhysicsMan (edited July 23, 2001).]</FONT>
post #42 of 61

The reason why a narrow stance feels contrived is that your technique is wrong.

Gang teachings don't work with a narrow stance.
post #43 of 61
I skied with Lisa, her stance is not narrow AND her technique is not incorrect. She is quite smooth actually. And I didn't notice any gang tattoos, but they could have been hidden!
post #44 of 61
Not to join in on knocking you around, but many gang members ski and teach narrow.

Are you still a builder?
post #45 of 61
I wouldn't show my "gang tatoos" to just anyone!

Be Braver in your body, or your luck will leave you. DH Lawrence
post #46 of 61
This topic is great! However, my time is very limited for some thoughtful replies. Currently, I am doing a stream (creek) restoration project in the very northwest corner of Colorado. Internet is non-existant. We got about 250 cows, yesterday, into the pasture that we are working. We have the stream enclosed with electric fence. It looks like there will be about another 300 tomorrow. Once after these critters get trained (10,000 to 11,000 volts train them quite quickly!), I will start another thread with some observations. This will probably be next week. BTW, the creek has narrowed over the three years of the project from five feet average to three feet average and has gained a foot in depth.

post #47 of 61
10,000 Volts? Hey maybe Harold can use that to keep people doing the EVIL WEDGE!
post #48 of 61
Thread Starter 
"narrow" is a pretty relative concept. If a narrow stance feels contrived it probably is. Try skiing with a natural stance, one which suits your body and the way it was put together. It may be narrow for some and wide for others. Then there is the option of adjusting your stance to suit the terrain and or application. You may find your stance changes quite a bit from bumps to crud to corduroy. Don't be limited to the same contrived stance all the time and you might find more stability.
post #49 of 61
First of all, thank you Camel for posting this topic. It has allowed me to work and play at the same time.

I notice that you are a snowboard instructor. At the last ski fitness workshop I went to, the instructor was both a ski and snowboard instructor. She was a strong advocate of a wider stance for skiers, and had us do a # of balance exercises in both a wide and narrow stance to prove her point.

However, she had a major gripe about the rental shops for snowboard equipment. She said that if a woman who is in her late 30s, early 40s is a first time snowboarder (sorry, but the thought of this makes me shudder, given how ridiculous I look learning skiing in my 40s} the shop will sometimes set the bindings in a ridiculously WIDE stance, and she often has to send her students back to have them adjusted.
Have you found this to be true?

Be Braver in your body, or your luck will leave you. DH Lawrence
post #50 of 61

I teach a lot of snowboarding, have been training SB staff for awhile too. Snow board students come out of the shop with MORE gear problems than ski students. It may be because we have so many more first time boarders than skiers coming through our shop. Mostly I believe it is due to the turnover-factor in the shop. Or it may be my impression, as I am so much more experienced dealing with the ski problems.

First time skiers can get away with boots too big, and they buckle their boots instead of lace them up, so that is easier too. Most shops have gone to step-in bindings, so boot fit is imperative and causes severe lack of edge-control, severe foot, ankle and calf fatigue if they are too big or too loose.

Stance width, Stance direction(tho many do not know until they try), bindings reversed from each other, board size, boot fit, missing leashes, broken laces, missing buckle/strap parts. Missing binding parts...that is a pretty good list, but I will remember more after the first week of the next season.

There is a lot going on with snowboard teaching. Companies (ROSSI, BURTON) are making specialized first-timer boards which are much softer, twist easily, and are extremely beveled to reduce edge catch. The national team is working throughout the country to teach directional slipping/traversing before sideslipping to minimize exposure to edge catch. Some guys at Schweitzer actually patented a device(don't know the name) that attaches to the downhill edge of a board and totally negates any edge catching at all. Burton is implementing an equipment/methodology called the Learn to Ride(LTR) Program that areas can 'buy.' That is just off the top of my head. (My directors are both on the National Team).

Give it a try. You'll probably fall down more the first day than skiing, but the learning curve is generally steeper afterward. If you take a private, as opposed to a group you'll be exposed to falling less(probably). The way I see it some people are natural skiers, some are natural boarders, and noone knows which until they try both.

My bro in law tried skiing with my sis for 5 years and never was able to get it at all. One day on a board and he was on the intermediate slopes. Not as controlled as I would have liked, but he is kinda like that everywhere and I couldn't stop him from going.
post #51 of 61
Thread Starter 
Stance problems out of the rental shop will never end and are almost infinite in their possibilities. I think it is just a lack of knowledge or interest in the guest on behalf of the rental staff. I try to recccomend people to snowboard specific shops where they can get a performance rental for very little more than the cost of a regular rental. They usually have better service standards and are more knowledgable.
Stance is also a very personal thing. Park riders like wider. Quite often the kid in the rental shop is sending people out on boards with his favorite stance and not what is best for beginners. We as instructors just accept and expect that these problems will arise and deal with them. That's why almost all instructors carry at least one tool in their uniform.
As for which stance width is right for a women in her forties... Again I would recommend a stance that is close to her natural stance. Something that leaves weight evenly distibuted across th width of the foot. Too wide and you can feel it in your arches. To narrow and you feel your wight mostly on the outside of your feet.
I am a firm believer that individuals are built differently and while the parts all work pretty much the same way no one stance is right for everybody. There are rules of thumb but for the most part you figure out what feels right and stay with it. The most important thing is that you don't get so wide that it disrupts the way the body was designed to function.
Hope I didn't ramble.
post #52 of 61

We have the Rossi LTR stuff at my area. It does help.
post #53 of 61
Just a bit of insight about women and stance width. I'm not talking about "Q" angles, since all of you are serious professionals, and know about that stuff anyway.

But in the fitness industry, there have been some studies on women and body image. If you ask a group of women to place their feet hip width apart, many of them will exaggerate that distance, simply because many woman think they are wider than they actually are.

And since that same study found female fitness instructors to be, ironically, the worst offenders, from time to time, I catch myself in a stance that is in fact TOO wide.
I think that when Todd and others refer to a wide stance , they do in fact mean a natural, not boot locked stance.

Back to balance and stability. A weight lifter performing a bicep curl in excellent form, staying stable at the elbow joint, and not exhibiting any swaying of the upper body is demonstrating Stability.
If they were standing on a wobble board, performing the same or a different exercise, they would be exhibiting both Balance and Stability.
Snowsports require both.

Be Braver in your body, or your luck will leave you. DH Lawrence
post #54 of 61
I agree with you 100% about stance Lisa. Morever - I think you are a total stud (studess?) for even knowing what the "Q" Angle is!
post #55 of 61
Todd: !!
post #56 of 61
Okay, here's another article brought to you by the "Link Lady". http://gambetta.com/articles/a97002.html

I would like to hear everyone's thoughts about 2 things:
First, what do you think of the concept of problems with speed, flexibility, and skill sometimes being balance problems?

Second, what do you think of the idea of the Balance Threshold?


Be Braver in your body, or your luck will leave you. DH Lawrence
post #57 of 61
LisaMarie, my sister started snowboarding (never skied before) at 38, and she didn't look ridiculous. Of course, she is one of those people who ALWAYS seems to look cool.
post #58 of 61
Obviously, coming to this thread eleven days late, what I'm going to say will seem out of context.

Everyone has heard me say often that for me the hardest part of a ski day is the walk from the parking lot to the lift, and I'm not kidding. Walking from the red lot at Keystone to the gondola totally exhausts me if I carry my skis, so does the walk back.

But I can ski all day at my advanced age and not get tired. I believe that is because of the years of practice and the finesse developed during that time.

Folks often wonder what makes me turn. Control and smoothness are my secret. If I never get out of balance there is no need to make movements to get back into blalance.

Just like standing for an hour or two in the back of a theater or church, you may find yourself swaying and be concious of corrective movements to keep balance. But if you never let yourself get out of balance enough that you have to make a large adjustment to regain it, the perception by others may be that you are standing still, though you make many small movements all the time to keep your balance.

Besides, skiing is easy, no matter how complicated we try to make it (Really!).

post #59 of 61
Miles, when I took that snow sports conditioning workshop, we did an exercise to find out if our stance was "Goofy" or regular.

Turns out I'm Goofy. Some of you already know that!

But being older and goofy and wearing those god-darn baggy and androdgenous clothes.... I think, not!

Ott, this is not out of context at all. What you are saying is in accordance with Gambetta's concept of being out of balance meaning lost balance. I thought the idea of using energy to regain balance being wasted energy an interesting thought.

Be Braver in your body, or your luck will leave you. DH Lawrence
post #60 of 61
I agree that skill, speed, and flexibility are intertwined with balance. It all relates to Motor Control and how the body works. It is the "readiness" of the nervous system in the body. The body receives input from the eyes, inner ear centers, and input from the joints and so forth....
Balance and core stability are bandwagons right now so everone has their buzz words. I think balance threshold is a real thing. It sounds as though it is your limit to balance.
If you are really interested in the topic, I would pick up some good Motor Control and Motor learning books from the library. They would be an invaluable tool. <FONT size="1">

[This message has been edited by GeorgiaPT (edited July 29, 2001).]</FONT>
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