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What's wrong with rotation? - Page 3

post #61 of 83
Thanks Razie. As far as two feet on the snow, the two contact points can be used as anchors, both doing that task for the other. Elimimating much of the need for the torso to act as that anchor. That being said using some core tension and holding the pelvis rotationally stable is a very common thing. IT bands, groin muscles, lower abs all contribute to that. But if like in the pic of the boy, a 90° knee bend isolates almost all of the feet twisting to the lower leg. Which makes what Bob suggested about avoiding absolute statements even more important. It is all situational.
post #62 of 83
Quote:
Originally Posted by razie View Post
 
ignorant question: is a pivoting movement centered in the upper body? does it come from the upper body?

 

I was just thinking about one of the differences between edging and pivoting and edging is an action of the lower legs, regardless of the upper body (even decoupled by flexing the knees/hips), but when you try to pivot i.e. twist the feet, isn't that centered in the upper body? i.e. you can't twist the feet without an upper body to rotate against?

 

Razie--yours is a very good question, and an important one to explore for anyone who really seeks to unravel the technical mysteries of skiing. The simple answer is no--pivoting movements do not have to involve the "upper body" (by which I mean any or all of the torso, arms, head, and abdomen--any parts of the body above your center of mass). But this simple answer hardly gets to the heart of the understanding.

 

I started to compose a detailed description and explanation of the mechanisms of "rotary," but I gave up for now and instead dredged up a very old article on the subject that I completed twenty years ago (yes--pre-shaped-ski). I've put it into an article in the Performance Articles section. When you find time (and you'll need some), please read Rotary Mechanisms Demystified (The Saga of Sir Isaac and the Snow Bunnies) (click on the link, or find it in the Performance Articles section). 

 

In the meantime, I'll agree that your question hints at a very important truth--that every action has an equal and opposite reaction (Newton's Third Law of Motion). So it makes sense, on the surface, to assume that if the lower body and skis turn one way, the upper body must therefore be involved to counter that motion. And it can--that's what "Counter-Rotation" is all about. But it does not have to. The opposite action--the reaction force (note: not "reactionary force"--that's something rebel forces deal with, not skiers!) must occur, but it can occur elsewhere. All of the other mechanisms besides Counter-Rotation, including Upper Body Rotation ("Rotation"), Blocking Pole Plants, and Independent Leg Rotation (sometimes still called the "Fulcrum Mechanism"), involve external forces--forces originating outside the body. Ultimately, the planet earth itself provides the force (or the counter-force, depending on your frame of reference). The upper body may be involved as a link (Rotation, Blocking Pole Plant) or not (Independent Leg Rotation), but in all cases, the ultimate source of the force is the earth.

 

For more explanation, please read about Newton's adventures with the Snow Bunnies, in my article! There is more in my Complete Encyclopedia of Skiing, Third Edition (written only shortly after the article) if you can find a copy (PM me if you'd like a PDF version), and brief explanations and illustrations in The Complete Encyclopedia of Skiing—EpicSki Skiing Glossary, also in the Performance Articles section of this site.

 

Best regards,

Bob

post #63 of 83
Quote:
Originally Posted by Bob Barnes View Post
 

I started to compose a detailed description and explanation of the mechanisms of "rotary," but I gave up for now and instead dredged up a very old article on the subject that I completed twenty years ago (yes--pre-shaped-ski). I've put it into an article in the Performance Articles section. When you find time (and you'll need some), please read Rotary Mechanisms Demystified (The Saga of Sir Isaac and the Snow Bunnies) (click on the link, or find it in the Performance Articles section). 

 

Thumbs Up

nice article. long...

 

Right - it makes sense: when not using poles or ILS, the upper body is the "source" of leverage. In "anticipatory release' or 'uncoiling' or unwinding, there is a build up of twist in the body leading to the 'release'.

 

I think in the ILS/fulcrum mechanism, some of the leverage is against the chairs and their anchors in the horizontal plane?

 

cheers,

razie

post #64 of 83
Quote:
 I think in the ILS/fulcrum mechanism, some of the leverage is against the chairs and their anchors in the horizontal plane?

 

Yes--all of it, really, when this is the only mechanism involved. And you've found the complexity of it too. With skis on snow, at least in the fore-aft plane, there is very little resistance available to "lever" against. This is why, despite the protests of many instructors, this mechanism is often not sufficient all by itself. It seems almost infinitely powerful on those barstools (that twist easily, but do not move forward or back), or when twisting your shoes on a carpeted floor--limited only by your own strength--but on the lower-friction surface of snow, the mechanism is less powerful. Of course, we don't often need a ton of power, either. Sometimes we don't need any. And we do have other ways to supplement when needed....

 

Best regards,

Bob

post #65 of 83

Also, a point worth noting about the 1-2 action of "Rotation." The upper body is, as you suggest, the immediate source of the rotational (angular) momentum that is then transferred to the lower body and skis by slowing or stopping the rotation of the upper body. But there had to be some external force involved prior to that to generate the upper body momentum in the first place. And that, again, is the earth. Rotation only works when our feet are somehow and at least somewhat locked to the snow when the rotation begins--typically with an edgeset at the end of the previous turn, what we often call a "platform." If there were no resistance from the snow, the feet and skis would simply counter-rotate the other direction as the upper body rotates into the turn, and no angular momentum would be generated. So, while the upper body is certainly involved in upper body Rotation, the actual origin of the force that turns the skier is still the earth.

 

"Rotation" is also often described as "rotary pushoff," which highlights the fact that the rotational motion originates from a pushoff from the snow. It's easy to demonstrate or observe in a freestyle park, where rotation is the key to all spins off jumps and in the halfpipe. The spin--upper body rotation--must begin when you are still on the snow, as you ski up the kicker. When you leave the snow, and the skis have no more resistance, that's when the spin of the skis and whole body begins. Conversely, it is not possible to go off a jump with no rotation and then decide, once you're in the air, to do a spin. You could do a "twister," which is just an airborne counter-rotation of the upper and lower body, in which there is no external force required (or spin generated). But you cannot decide to throw a heli once you're in the air, if you haven't already committed to it before you left the ground--because the twisting force that throws you into a spin comes from that ground.

 

Best regards,

Bob

post #66 of 83
Good stuff Bob. I am curious how to explain the cat rotating in the air after being dropped thing Larsson and Majors mention in their book WC Ski Technique.
post #67 of 83

 

  zenny

post #68 of 83

well, that explains it all. the fabled Ninja technique of the tip lead, first time caught on tape:

 

post #69 of 83
Thanks Zenny, the idea from that book I mentioned talks about experts using agonist / antagonist muscle groups rather than pushing against something external. A bit of both internal and external seems reasonable for most of us on skis and in most situations. There is where it gets interesting. A double with a half twist after the first flip is not uncommon in freestyle jumping. So somewhere in that the skier does like the cat and figures out how to work one half against the other half and thus create angular acceleration. So it is possible as razie pointed out for angular acceleration to be internally started. Considering most skiing is done in contact with the ground that additional reaction force is added to the sum total of forces and Bob's wonderful explanation makes total sense.
post #70 of 83

Yes, it's amazing what cats can do--seems to defy the laws of physics--in particular, the law of conservation of angular momentum, which states that--like the freestyler who cannot initiate a spin after leaving the kicker--we can neither increase nor decrease angular momentum without an external force applying. You can't just "start spinning" in the air. And yet, starting with zero angular momentum (not spinning or rolling), cats can right themselves to land on their feet, even when starting upside down. That would appear to involve rolling over without the help of external forces. A paradox?

 

I've seen that video clip you posted, Zentune--it's awesome (although I'm not sure the cat would share in my enthusiasm). Even with cats' unique anatomical design, extraordinary sense of up and down, and their innate reflex to make the right moves (the "righting reflex"), they still have to obey the laws of physics. As I understand it, there are two keys to the physics.

 

First is the understanding that for angular (rotational) motion, unlike linear motion, it is not just the mass of the object that determines its inertia (resistance to changes of motion), but also the distribution of that mass--the "moment of inertia." The farther from the axis of rotation the mass of an object is, the greater the moment of inertia (also known as "swing weight"). It's the spinning figure skater who slows down the spin by spreading her arms or speeds it up by pulling her arms in close to her sides. Despite the change of rotational velocity, the angular momentum remains constant, as The Law says it must. 

 

Second is that flexible spine that allows a cat to twist its body around like a slinky, and arch its back like a horseshoe, both up and down. 

 

Combining these two things allows the cat to do the seemingly impossible. With their flexible spines, they essentially separate themselves into a forward section and a hind section that move independently. Sensing immediately which way is down when they fall, they extend their hind legs, increasing the moment of inertia of the aft section, and counter-rotate their head and forebody against that aft section with their forelegs tucked close to the axis of rotation (tucked under, or extended forward) to minimize its moment of inertia. Then they extend their front legs out, increasing the moment of inertia and slowing the rotation of the forward half, and tuck their hind legs to reduce moment of the aft half, allowing the hind section to rotate more quickly and align with the front half. 

 

That's one part of it--manipulating the relative moments of inertia of their forward and hind sections so they can rotate each one quickly against the greater moment of the other. But I think the real key is the other part, that occurs when they bend their hyperflexible spines like a "U." This move changes the relative orientation of the axes of rotation of the fore and aft sections, allowing them essentially to counter-rotate against each other, even as they both rotate the same direction. So both the problem and the solution involve an apparent paradox! When they straighten out, they're lined up upright with landing gear down, rotated 180 degrees from their starting inverted position, all without violating the Law of Conservation of Angular Momentum. 

 

The magic is in the curve of the spine, that pulls the axes of rotation of the front and back halves of the cat almost 180out of their original alignment (in all except the 4th frame), allowing counter-rotation to rotate both halves in what will end up being the same direction! In this U shape, the halves rotate in nearly opposite directions, each against the other. "Equal and opposite reactions" remain internal, no external forces are involved, and no angular momentum is generated. 

 

That's my understanding, anyway. Anyone care to debunk it, and propose an alternate explanation? I do think we see similar principles with many of today's extreme tricks in the freestyle park. As skiers and boarders bend and twist and fold and otherwise contort their bodies, they are able to manipulate the relative moments of inertia and axes of rotation of different body parts to play with the Law of Conservation of Angular Momentum in creative and seemingly paradoxical ways. Like cats, it may happen subconsciously, but from a physics perspective, there's a lot to think about!

 

Best regards,

Bob

post #71 of 83

There is no independent part.  The cat is showing STNR reflex...

 

https://en.wikipedia.org/wiki/Symmetrical_tonic_neck_reflex

 

The righting response begins in the inner ear, it is the fastest sensor, responsible for head position information and acceleration sensation.  The flexors of the neck fire, flexing the upper extremities, the STNR reflex is called upon to simultaneously extend the lower extremities.  These are the fastest synergies to call upon.  

 

Once head control is established, the eyes oriented to landing, the balancing not he flexor/extensor groups aligns the rest of the body.  There is no rotation against one another IMO, it is a propagation of flexor/extensor control from the head down.

 

Regards,

 

Chad

post #72 of 83
post #73 of 83

I love the disclaimer about simple tasks being oh so complex was offered right out of the gate. Thanks Chad for the additional links. Seems moving is far more complex than I previously thought.

post #74 of 83
I think its also important to point out that the cat does not continue to keep spinning once he's right side up.  He gets turned around right side up and then falls straight down onto his feet, he (or she) doesn't actually create any new angular momentum in the overall system.  He just gets "twisted around".  
post #75 of 83

From the last sentence of the articles cgelb just posted:

 

Quote:
Thus the answer to the question "can a man twist in free fall?" is the rather qualified one that, if untrained, he can-but only up to a 90 degree turn.  Whatever the maneuver used a further rotation means starting another set of movements-and considerable gymnastic training is necessary to do so.

 

Its evident from these articles that the U shaped articulation BB talked about is quite relevant having more to do with the way the body parts twisting against each other, as opposed to overall system angular momentum, which remains unchanged.

 

Now how does any of that relate to ski turns by the way?  Perhaps some oscillating angulation, reverse angulation could accomplish the world's strangest looking on snow pivot? 

post #76 of 83

Look at the cats feet(1:05-1:20).  They stay relatively passive, very little effort until righted and prepared to land.  The ability to use the largest muscles of the body efficiently will allow for less reliance on the muscle of the extremities.  The coordination of the trunk, the flexor/extensor groups, is what generates the dynamic range of movement in the spine.

 

A better balance between these muscle improves the effectiveness of the limbs.  In this case, the ability to absorb a landing.  It is the same for us and our ability to work with gravity.

post #77 of 83
Quote:

 

Good articles, C. I'm glad to see that the first one very much supports my description of the cat above, including both the manipulation of the relative moments of inertia (by spreading legs wide or tucking them in close to the axis of rotation) of the fore and hind halves of the cat, and the changing of the angles of the two axes of rotation by flexing the spine into a "U." Also interesting that he presents evidence that it is primarily visual cues that the cat uses to determine up and down, and only secondarily (and quite limited) the inner ear balancing apparatus (the vestibular systen, semi-circular canals and all). He notes accurately near the end of the article that the vestibular system only functions properly as a "gravity sensor" when the cat is not falling. It would defy the laws of physics for this organ to be able to sense gravity's direction when in free fall (ie., "weightlessness").

 

Best regards,

Bob

post #78 of 83
Quote:
Originally Posted by borntoski683 View Post
 
... its also important to point out that the cat does not continue to keep spinning once he's right side up.  He gets turned around right side up and then falls straight down onto his feet, he (or she) doesn't actually create any new angular momentum in the overall system.

 

(Bolding added for emphasis.)

 

Yes, BTS--as they say, like gravity, "it's not just a good idea, it's the law."   :cool

 

That no change in angular (rotational) momentum occurs is a given, since once he's falling, no external forces apply to the cat (other than wind resistance, which is negligible). The question was never, "does the cat violate the Law of Conservation of Angular Momentum?" It was only, "how does he do it--without breaking The Law?"

 

I think there is strong relevance to ski turns. The more we angulate at the hips, flex the hips, or fold at the waist, the more we move the axes of rotation of the upper and lower bodies out of alignment with each other. When they're not aligned, the whole action-reaction equation for rotation and counter-rotation between the two parts changes. To use an extreme example, if you were to bend forward while standing such that your torso and spine were horizontal, and then vigorously twist your torso and arms about their (horizontal) axis, there would be no counter-rotation effect transmitted to twist the skis. If anything, the movement of the upper body would produce lateral forces on the feet, and if you were to "unweight" your feet to eliminate the resistance of the ground, they would move sideways.

 

Best regards,

Bob

post #79 of 83

https://en.wikipedia.org/wiki/Vestibulo%E2%80%93ocular_reflex

 

"The VOR is ultimately driven by signals from the vestibular apparatus in the inner ear. The semicircular canals detect head rotation and drive the rotational VOR, whereas the otoliths detect head translation and drive the translational VOR. The main "direct path" neural circuit for the horizontal rotational VOR is fairly simple. It starts in the vestibular system, where semicircular canals get activated by head rotation and send their impulses via the vestibular nerve (cranial nerve VIII) through Scarpa's ganglion and end in the vestibular nuclei in the brainstem. From these nuclei, fibers cross to the contralateral cranial nerve VI nucleus (abducens nucleus). There they synapse with 2 additional pathways. One pathway projects directly to the lateral rectus of eye via the abducens nerve. Another nerve tract projects from the abducens nucleus by the medial longitudinal fasciculus to the contralateral oculomotor nucleus, which contains motorneurons that drive eye muscle activity, specifically activating the medial rectus muscle of the eye through the oculomotor nerve."

 

 

"The vestibulo-ocular reflex needs to be fast: for clear vision, head movement must be compensated almost immediately; otherwise, vision corresponds to a photograph taken with a shaky hand. To achieve clear vision, signals from the semicircular canals are sent as directly as possible to the eye muscles: the connection involves only three neurons, and is correspondingly called the three neuron arc. Using these direct connections, eye movements lag the head movements by less than 10 ms,[9] and thus the vestibulo-ocular reflex is one of the fastest reflexes in the human body."

post #80 of 83

Nice, it segues quite well into the concept I mentioned earlier. Experts tend to do the cat thing far more than beginners who struggle to lean against something with their feet. When the feet slide that sensation elicits a response to try it again before moving on to a second option. That second option might be cat like, or might be counter balancing moves like swinging the arms forward to avoid falling backwards. A lot depends on the skier and their life experiences. Our job as I see it is to condition the cat like responses but that needs to be done in stages.


Edited by justanotherskipro - 10/6/15 at 2:04pm
post #81 of 83
Quote:
Originally Posted by Bob Barnes View Post
 

 

(Bolding added for emphasis.)

 

Yes, BTS--as they say, like gravity, "it's not just a good idea, it's the law."   :cool

 

That no change in angular (rotational) momentum occurs is a given, since once he's falling, no external forces apply to the cat (other than wind resistance, which is negligible). The question was never, "does the cat violate the Law of Conservation of Angular Momentum?" It was only, "how does he do it--without breaking The Law?"

 

I think there is strong relevance to ski turns. The more we angulate at the hips, flex the hips, or fold at the waist, the more we move the axes of rotation of the upper and lower bodies out of alignment with each other. When they're not aligned, the whole action-reaction equation for rotation and counter-rotation between the two parts changes. To use an extreme example, if you were to bend forward while standing such that your torso and spine were horizontal, and then vigorously twist your torso and arms about their (horizontal) axis, there would be no counter-rotation effect transmitted to twist the skis. If anything, the movement of the upper body would produce lateral forces on the feet, and if you were to "unweight" your feet to eliminate the resistance of the ground, they would move sideways.

 

Best regards,

Bob

 

I'm pretty sure...but not completely sure...that the cat trick works a bit different.  I don't see this as being much at all related to angular momentum.  I think its simply a matter of twisting the upper half one way and the lower half the other way; creating bio mechanical tension and no net angular momentum...and then unwinding that tension back to zero again.  Except that because of the way the cat arches its back as it goes to release that tension, this is what causes it to twist to face the other direction.  The back arch U switch, in combination with releasing the bio mechanical twist tension.  Yes when the cat does the twist, it uses body parts to push against other body parts to have the angular action-reaction.  That results in net-zero angular momentum.  

 

The cat's back is kind of like a bow being released in a way, from a U facing up to a U facing down, but with the net-zero twist happening at the same time.  This causes it to land on its feet with no angular momentum whatsoever.  The cat actually has to move its back twice.  It has to first arch its back and then reverse the arch back again for the landing.  So basically it start with a bowed back, does a twist while beginning to arch its back, after reaching full twist tension, releases the twist..still arching its back a bit and finally un-arching its back again.

 

I do not think that while skiing, angulating some how creates angular rotation stuff to happen as some are imaging.  Its a zero sum game.  In the cat's case, its the unwinding of the torso tension and that back arching and unarching at the same time that causes him to twist to face the other way, in a way that does not create any new angular momentum, but does reorient his body to face down.  To relate the cat to ski turns you'd have to say that somehow the skier does some radical move where they do the twist at the same time as some radical back arching.  Might be related to ariels, but to skiing on the snow, I don't think so.

post #82 of 83

I started this by asking about a statement made way back when Stenmark ruled the race world. In context it goes something like this, "the beginner relies on contact with an object (Earth) for balance and experts while they still use some of that they also incorporate the internal push / pull of body parts against other body parts to maintain balance. Much like how a cat will right themselves in the air when dropped on their back". I appreciate all the research that got posted but feel a little guilty for steering the thread away from the right / wrong question. So in that spirit I am hoping to move the discussion back to rotary and if any real reason exists for labeling it either way.

 

For me and I hope most skiers, the object lesson here is over time we all develop familiarity and thus can predict with a certain level of accuracy how a particular movement will effect how a turn will turn out. The variables like snow, terrain, fatigue, emotional state are always present and will modify that to some degree but if like most folks once we discover a workable and repeatable solution we tend to rely on that solution and pretty much stop exploring other options. Which on a practical level is not a bad thing. It reminds me of the age old saying "if it ain't broke why fix it?" Not bad thinking unless of course you have a personality that thrives on exploring new stuff. There are a lot of names for that personality type where performing just one type of turn would be for you, the equivalent of a mind numbing and spirit robbing, manufacturing job. Other see comfort in repetition though and so they seek familiarity above the uncomfortable feelings we get when we explore new movements and maneuvers. It's my feeling that skiing is about expressing ourselves through performance and how we choose to perform has a ton to do with our basic personalities (yes we all have more than one). Which also means if we aren't careful and we go out exploring we likely will end up learn something about ourselves in that process. So learning to ski is in my opinion it's a personal growth thing and who is to say what is really right and wrong for anyone. Especially when it comes to a skill like rotary, or any of the other skill pools.

Oddly enough as instructors we sell change and in spite of paying a pretty penny for our advice many folks cannot change much about their skiing until they change something about themselves. It's more than anything else what keeps me in the game after so many years, I get excited because I get to see people grow right in front of me. How cool is that! In any case hopefully rotary becomes seen as just one of many options available and over time we see it lose the age old and rather dogmatic negative labeling. I won't hold my breath waiting for that day because even the Title of this thread hints at how that negative label is still prevalent.


Edited by justanotherskipro - 10/6/15 at 10:58pm
post #83 of 83

Originally Posted by cgeib View Post
Interesting articles in The New Scientist
How Does A Cat Fall On Its Feet? by Dr Donald McDonald

 

 

Here's a screenshot of the relevant passage from McDonald's book.  

It is much the same thing Bob Barnes said.  This explanation of how

angular momentum is conserved makes good sense.

 

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