Physics Based Critique of the Petrov Method

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Re: Physics Based Critique of the Petrov Method

Unread postby PVstudent » Fri Apr 04, 2014 9:24 am

Once again thank you to ADTF for correcting my error and for his kind offer to put up some critical stills from his video that will confirm

1. That Bubka was out at take-off and
2. Show “beyond reasonable doubt the distance “out” at take-off" was approximately half a metre.

(An approximate gauge that might be used is 1.5 to 2.0 of Lavillenie’s natural foot lengths on the photographic images. My guess estimate of foot length is in the 30 to 35 cms range based on Renaud’s stature. Someone may actually know his actual foot length).

Back to Willrieffer’s primary argument which is that the physics basis of the Petrov -Model Approach to teaching and coaching of flexible pole vaulting is in Will’s words:

“I contend that the Petrov model is full of hyperbole, half truths, and myths, in part because they got results! First of all is that Petrov and Bubka may have preached the "free take off" but Bubka did not practice it, at least as far as its described, in that if you simply look at the vids in vault after vault his pole starts bending about as soon as his take off foot hits the ground.”

Will’s claim that Bubka didn’t practice what he and Petrov preached is an untenable assertion.

Evidence can be presented to shows that Bubka performed during his career, in significant competitions on different occasions a pre-jump, free-take-offs and pre-pole bend take-offs. Applying pole vault jargon the jumps can be considered as “out”, “on” and “under” take-offs.

This demonstrates that Bubka’s performance varied with respect to the “on” rubric. This is but one of the criteria to be met in the performance of a “free take-off”.

A second and more important criterion of a free-take-off as defined (Petrov, 1985) is the extent to which the pole vaulter’s motion is resisted or impeded by the reactive forces on the vaulter induced by the pole striking the rear wall of the planting box.

A third criterion to be met in the performance of a Petrov-Bubka concept of a “Free-takeoff” is the extent to which the completion of the take-off enables the vaulter to achieve minimal resistance to penetration translatory motion due to vaulter take-off inertia whilst simultaneously allowing the vaulter to obtain the optimum swing amplitude and continously provide pole and vaulter rotation about pole tip and wrists.

A fourth criterion to be met in the execution of a free take-off is that the take-off ensures uninterrupted continuity of motion into active swing with minimal losses to the rate of pole chord rotation about the pole tip axis.

There is a fifth criterion that Petrov includes as necessary for a classification of a take-off to be considered “free” that relates to the manner in which the pole bend starts and is continued due to the interaction of the vaulter’s inertial force combining with the vaulter’s weight.

There are some other considerations but these 5 criteria I consider to be the essential ones in relation to just the take-off. The Petrov - Bubka Model is much more comprehensive and deals with the total pole vault sequence from gripping the pole, taking the first step to the culmination of the vault with the vaulter landing on the pads.

These criteria are essentially qualitative and rely upon the informed judgement of the external observer who is operating in an inertial frame of reference. (Is this what Will is asserting? If so I agree that qualitative criteria do not have the objectivity of physical law and principle!)

The vaulter on the other hand may be applying the same criteria with reference to a moving frame of reference and consequently their reports of what occurred during a particular vault may rely on perceived sensations that arrive from their effort and other sensory modalities. It is not surprising therefore that the vaulter’s account and that of the inertial frame observer can and do differ.

Will has yet to offer any clear physics analysis as to why he believes that taking off under creates more efficiency of energy transmission in the take-off.

Perhaps Will does not believe this to be the case and I have not understood his position on the matter.

There is no denial that Bubka on many occasions did not meet all 5 criteria I have listed on every vault.

Each occasion it must be remembered presents the vaulter with a unique set of circumstances, especially when vaulting under non ideal conditions outdoors. Part of the art is therefore modifying and adapting technique to meet the challenges imposed by the changing circumstances under which each vault takes place.

However, it is a very long bow to draw to state that Petrov and Bubka did not did not practice what they preached!

ADTF makes this same point and suggests that discussants are talking past each other because of lack of clarity in regard to their personal interpretations of what Petrov / Bubka actually consider a ”free take-off”.

ADTF goes on to say:

“The free takeoff is a flow or continuation of movements not a position or moment in time. Its a series of events not a photo. It is the ability of the vaulter to move thru the full running and jumping action at takeoff while receiving minimal tension on their body posture by the pole as it begins the stressed sensation of the two fixed points (the box and the top hand).”

I think the dynamic nature of the process of a “free take-off” is captured and functionally defined in ADTF’s statement rather neatly. I fully endorse this statement because it delivers the essence of what the free take-off contributes to a successful vault attempt.

I note Willrieffer has started another thread physics of pole vault Big Energy, perhaps there will be answers there.
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Re: Physics Based Critique of the Petrov Method

Unread postby Tim McMichael » Fri Apr 04, 2014 11:50 am

PVstudent wrote:ADTF goes on to say:

“The free takeoff is a flow or continuation of movements not a position or moment in time. Its a series of events not a photo. It is the ability of the vaulter to move thru the full running and jumping action at takeoff while receiving minimal tension on their body posture by the pole as it begins the stressed sensation of the two fixed points (the box and the top hand).”

I think the dynamic nature of the process of a “free take-off” is captured and functionally defined in ADTF’s statement rather neatly. I fully endorse this statement because it delivers the essence of what the free take-off contributes to a successful vault attempt.



This is an elegant summation. Beautiful writing as well. I think that the effectiveness and safety of any technical model can be based on these principles. The drive vault that I described a few years ago in my "Manifesto" adheres to these principles in with a different method that I concede may not be as effective, but it was successful and safe.

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Re: Physics Based Critique of the Petrov Method

Unread postby willrieffer » Fri Apr 04, 2014 1:11 pm

Will’s claim that Bubka didn’t practice what he and Petrov preached is an untenable assertion. - PVStudent


Ok. Lets go back to Bubka yet again. In his description, which is quoted in your work on this subject, he says again, as a part of their method, they were trying to get a take off with the foot off the ground AND no pole bend. Or, as I have to take it, a part of their "free" take off model was the inclusion of what is now come to be better known as a "pre-take off" take off. He goes on to say its "crucial", AND that they didn't get it all the time. Do I have to ask, which is it? Who's on first? LOL! No wonder we're still hashing this stuff out 30 years into its history...

Beyond this I have ALWAYS said that the methodology generated a highly functional take off and early swing moment independent of the actual type of take off they achieved. In short you can check off all the other things about their method that you cite.

To me this is the situation that you have. They, and all vaulters have to have an idealized set of parameters and circumstances that form into their goals of initiating and completing a successful vault. Per Bubka, for this group it was a "pre take off" "free take off". LOL. It was an unbent pole. And probably, almost certainly, many other things. That in them, this goal of achieving this form of take off lead them to form a psychology that lead to initiating and generating highly effective take offs in posture and take off vector. The posture and take off vector ideals were also parts of their ultimate idealized goals. But if we now back up to the actuality of the specifics of the take off per their own statements, it mattered little if they actually got all the parameters set forth in their ideal. The combo of their psychological attitude and its inaction in actual vaults let them actually make big vaults from all three common take off ideologies, pole bend, free, or pre take offs. So now we have an ideology that wants to make every element "crucial", and explain it as crucial in terms of physics, backed up against real and actual evidence to the contrary. Video evidence, AND Bubka's own statements about how actually effective they were in reaching their goals. He says, "we get it sometimes".

These types of things go on all through their vault and the assessments of it. Left arm action. Swing speed/moment control where they did not all vault in accord with straight leg model. It's the same thing. The vaulter (and coach) need some idealized goals to set forth to begin a methodology. Yet in the actual I contend that many vaulters, especially masters, form and intuitive sense of what "works" which is, or can be, outside of the training methodology or goals, AND also outside of their conscious reflection on the subject. What's the limit? Well, if they happen to do something that is really bad in terms of physics, they can't either finish the vault, or go high. Happens all the time. Vaulters often know when something is wrong in the vault. That the step is off too much on the approach. That they had a bad take off placement and don't have the energy and momentum to finish and know they better start working fast and hard on a bail. Or, you can't cheat physics, and the vaulters know it. As to the coaches? I'm not so sure about all of them...

So look at the actualities first and work back from them. IF vaulters pull off big vaults from all three take offs, whatever is happening in them that is different probably doesn't matter that much or somehow cancel out, and what happens that they all do is what is important and crucial. Yet, this leads us to a paradoxical situation. Where are you going to get your gains when small increments in outcome are important? Well, that's the trick isn't it!

Thanks for your time. Thanks for helping me asses and reassess the event and my views on it. Thanks for helping me make changes in my outlook. Hopefully I can put it to use with my vaulters.

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Re: Physics Based Critique of the Petrov Method

Unread postby ADTF Academy » Fri Apr 04, 2014 3:00 pm

PVstudent wrote:Once again thank you to ADTF for correcting my error and for his kind offer to put up some critical stills from his video that will confirm

1. That Bubka was out at take-off and
2. Show “beyond reasonable doubt the distance “out” at take-off" was approximately half a metre.

(An approximate gauge that might be used is 1.5 to 2.0 of Lavillenie’s natural foot lengths on the photographic images. My guess estimate of foot length is in the 30 to 35 cms range based on Renaud’s stature. Someone may actually know his actual foot length).



I posted the pictures in the above post. I don't know any exact distances, but that he was outside. The 50 cm was an approximation given by first hand people there. Only people that can confirm are those from his camp who know which mark was Renaud at Mid and Take off. I may have his WR jump too so I can try to compare the two videos, but exact numbers would mean you know exact takeoff spots for said athlete and the range they attempt to be at on each jump. This information I do not know at this time.

Sadly we won't be able to make the training camp at the Olympic Training Center we had hope to attend with their training group. Our own governing body didn't approve us fast enough to go to the Olympic Training Center. The french got permission and our own athlete didn't. Go figure!!!!

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Re: Physics Based Critique of the Petrov Method

Unread postby willrieffer » Fri Apr 04, 2014 9:31 pm

Will has yet to offer any clear physics analysis as to why he believes that taking off under creates more efficiency of energy transmission in the take-off.- PVStudent


This is not exactly my position, but I have addressed it several times. My position is that its possible that taking off with a bending pole offers the possibility for more total take off energy in the system. I have stated that proving that is so is going to be very very difficult in measuring a mechanical force addition with a vaulter in pole braking. In as much as I need to show something as proof, I thought your analysis, in that it actually came to a precise conclusion and did not address this possibility left the question open. I have not made a conclusion on whether this is so or not. You, on the other hand, have made a conclusion...

Further it could be possible that there is added energy, but the cost is actually in energy management efficiency. What happens when a vaulter is "under" too far? They lose posture and suffer swing progression regardless of total take off energy. I have supplied a video of a vaulter clearing 5.85m ( correction - this is Dosevi vaulting for the French and clearing 5.75) with pole bend initiated before the plant foot even hits the ground! And he pulls the vault off by maintaining posture through the initial pole brake moment through to take off. It can be done...

One reason to me that this question remains open is the long running debate on the fact that you can find notable Petrov vaulters clearing notable heights from bend take offs, "free" take offs, and pre take offs. So there seems to be no actual evidence to pin down your conclusion, but in fact evidence which makes it dubious. I, on the other hand have only contended that one possible reason that they were able to do so was based on trade offs in the position. That with them, while bent pole take offs were not to their ideal, their ability to maintain posture with the possibility of a bit of added energy evens things out.

There is one other conclusion that I have hinted at but have not exactly put forth. Look, while you can't go out with a new vaulter and say, "well, your take of spot is out there...somewhere", and that you must give them a particular idealized take off spot, its possible that the actual necessity of this spot to an actual vault is unnecessary. There are upper and lower limits for being out and under, and the perpendicular to the top hand fits somewhere in the middle, but too there is a lot of leeway in actualizing the take off spot. A bit "under" and some pole bend, or a slightly out pre take off are things not ultimately detrimental to enacting a good vault.
Last edited by willrieffer on Sat Apr 05, 2014 4:47 pm, edited 1 time in total.

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Re: Physics Based Critique of the Petrov Method

Unread postby KirkB » Fri Apr 04, 2014 9:52 pm

Thanks for the pics, ADTF!

Very blurry, but from the 2nd pic, the takeoff looks like a mighty fine "Goldilocks" takeoff to me. Not too in and not too out - just about right.

But the pole hasn't bent yet in the 2nd pic, so I don't know if it's "just" hit the back of the box, or if it's still approaching it (I'm guessing the latter, so the pic is therefore deceptive). This makes a big diff. As we know, there's an epic fail right after this, so something's amiss. If you're saying he's out by 50 cm, then the pole must still have a ways to go before it hits the box (and then ricochets above the box). What happens in the frame right after this one? Is this the last frame where the pole is straight?

Just curious.

By my standards, his posture is just about right. This is approximately the posture I strove for on takeoff. So IMHO, he has just about the right amount of forward lean in his posture. However, it depends on how close he is to hitting the box. By Renaud's standards, he might feel that he's too far out (apparently by 50cm or so). By Will's standards, he's probably out by a long shot, and could (arguably) be in another 100cm or so and still get a good vault out of if.

Maybe we should be looking at his WR vault re how far THAT ONE was out or in (and by whose standards). It's always more interesting to study WRs - not epic fails. :)

Then we could get back to the main theme of this thread, which is critiquing the Petrov Method (and whether Renaud is doing anything similar or not).

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Re: Physics Based Critique of the Petrov Method

Unread postby dj » Sun Apr 06, 2014 9:03 am

He was out 50cm at the six step mid!!!

Not at the takeoff...

You can't draw your conclusions from the takeoff point alone!!! 100% of what happened at the plant/takeoff was controlled by what happen BEFORE that point...

Coaching is about determining the "cause" and that can only happen correctly if you, as a coach, look in advance of the error..

Most coaches are not doing that... Here in these threads or on the field.

In golf if you slice it's how the ball came off the club... How it comes of the club is determined by the swing, the swing is determined by the "drawback" which is determined by the grip... So forth and so on....

What distance you are at, and the speed you are moving in relationship to your ability, determines how you can function at and during the plant and swing....

If we all used the, cause and effect and application of force process in our thinking/analyzing I think there would be fewer arguments on here and a better sharing of positive info...

Dj

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Re: Physics Based Critique of the Petrov Method

Unread postby willrieffer » Mon Apr 07, 2014 2:20 pm

dj wrote:He was out 50cm at the six step mid!!!

Not at the takeoff...

You can't draw your conclusions from the takeoff point alone!!! 100% of what happened at the plant/takeoff was controlled by what happen BEFORE that point...

Coaching is about determining the "cause" and that can only happen correctly if you, as a coach, look in advance of the error..

Most coaches are not doing that... Here in these threads or on the field.

In golf if you slice it's how the ball came off the club... How it comes of the club is determined by the swing, the swing is determined by the "drawback" which is determined by the grip... So forth and so on....

What distance you are at, and the speed you are moving in relationship to your ability, determines how you can function at and during the plant and swing....

If we all used the, cause and effect and application of force process in our thinking/analyzing I think there would be fewer arguments on here and a better sharing of positive info...

Dj


I tend to agree. IF he was 50cm out at mid, now he's going to have to do some things outside of his normal approach to "fix" it, i.e. overstride, possibly leave "low" from overstriding, lose position at take off, etc. Occam's Razor and previous analysis probably return the answer. He was out, he did not particularly realize he was out (thus his take off looks "normal"), but dropped the pole tip short in the box, and it bounced out. That's pretty simple. OR, the explanation is he contorted himself to make the vault. I find that highly suspect in such an accomplished vaulter. Not impossible, but suspect. Had he realized he was 50 out at mid, he probably would have stopped.

There are kids on here all the time asking for invert help. And you can trace the hips getting slowed all the way back up the runway. Their run is off so they are under. They force their hips up early in the swing. They "hug" the pole. They short leg the swing. They pull with the arms. It's elements of many things being off that keeps them from the end. In fact it's only in watching this stuff go on and on over the last half decade with the young vaulter's I'm around which has honed my insight and beliefs. It's only the rare young vaulter that can take off in position and keep and maintain their swing with proper hip placement.

If I mention the Dosevi vault again one thing stands out. Early on he avoids having the hips come forward under pole breaking. This is something that the Petrovians were worried about as well, possibly THE thing and had an outlook toward with their take off ideology to achieve keeping the hips back and were successful with it, and I might say because of it. Yet, Dosevi accomplishes the same thing, again keeping the hips back in a radically different way. Which is to say, overadvanced early hip progression is the #1 killer of vaults and leads to the vault ending dangerously short. This isn't particularly a secret and has been known for some time. But it can get mired in more or less trivial concerns. Other minutia of the take off. Left arm action. etc.

I still contend Lavillenie was able to get out of that situation because he works longer and harder to stay back and down than most. About the time it's entering the catastrophe phase he's dropping his right down into the double which gives him added time to figure out he needs to bail and control that action. He hangs both legs and works to get and keep the hips back, fighting with the arms, and down to keep from winding up both short and at altitude. In some ways I find this more a testament to my ideas than any rather routine normal vault.

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Re: Physics Based Critique of the Petrov Method

Unread postby PVstudent » Tue Apr 08, 2014 10:38 am

Thank you ADTF Academy for the pictures.

On the basis of this evidence in regard to the take-off in question I remain skeptical that the fail was due to an “out” take-off.

To be fair, the public domain evidence is perhaps lacking sufficient quality and hence unable to unequivocally resolve the issue.

By way of “mea culpa” for my approach run error and as penance for my heinous sin I provide some frames from a Utube video of Renaud Lavillenie’s 6.16m World Record.

[img]
Renaud%20Lavillenie%20take-off%20for%20Indoor%20World%20Record%206.16m.jpg
[/img]

Readers can judge for themselves as to whether it was “under” or “on” and whether Lavillenie because of his longer grip length and stiffer pole comes closer in technical form to a Bubka type take-off. The dropped right leg that occurs following the initial pole bend is a post take-off adaptation to develop:

Some Potential Benefits:

1 Greater swing amplitude of the take –off leg
2 Longer swing time to drive the bending pole forwards before it “snap” buckles to the left
3 Maintain a larger moment of inertia about the top hand axis whilst maintaining pole chord rotational speed (due to minimization of total system COM moment of inertia about the pole tip axis)
4 Optimizes the forward – upward translatory total system COM motion as the pole chord shortens

Some Potential Costs:

1 The pole chord angular displacement required to rotate the total system COM to the vertical can be large from its position at maximum pole bend to pole release
2 There is very little time to complete the swing into the inversion to continue vaulter effort to move the COM vertically as maximum pole bend is reached and being prepared to be accelerated upward as pole recoil begins
3 Timing of the “tuck –semi-pike” may slow or stall pole chord rotation if this occurs at or immediately following maximum pole bend

The lowered right leg also plays a role in “dynamical balancing” the imbalances due to the vaulter torso and limbs asymmetry of the preceding take-off actions.

http://youtu.be/QQedifF_KzU
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Renaud Lavillenie take-off for Indoor World Record 6.16m.jpg
Renaud Lavillenie take-off for Indoor World Record 6.16m.jpg (61.47 KiB) Viewed 9509 times
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Re: Physics Based Critique of the Petrov Method

Unread postby PVstudent » Tue Apr 08, 2014 1:34 pm

willrieffer wrote:This is not exactly my position, but I have addressed it several times. My position is that its possible that taking off with a bending pole offers the possibility for more total take off energy in the system. I have stated that proving that is so is going to be very very difficult in measuring a mechanical force addition with a vaulter in pole braking. In as much as I need to show something as proof, I thought your analysis, in that it actually came to a precise conclusion and did not address this possibility left the question open.


Will, my precise conclusion was with respect to identification of "an ideal location for the take-off" with respect to the vaulter optimising the take-off in such a way that the pole offered minimal resistance to being rotated about is tip located in the box. In so doing I was careful to use force tranducers under the pole to help identify and differentiate the clear distinction between a Pre-Jump and a Petrov defined "Free Take-Off. I did not attempt to substantiate or disprove any theory with respect to the ideal mechanisms of pole bending as you appear to be implying. The mathematical and physics basic facts I used in arriving at my conclusion in making the distinction is still open to being corrected.

In regard to your clearly stated position
willrieffer wrote:My position is that its possible that taking off with a bending pole offers the possibility for more total take off energy in the system.


I don't disagree with or deny this possibility.

My questions to you is, Yes, whilst this is possible what are the costs to the success / failure for the subsequent phases in the remaining vaulting sequence? What does a pole vault coach teach beginners in regard to the take-off?

Your argument so far as I am able to understand it actually supports the teaching of the "idealised Free Take-off" as being just about the right model for all levels of pole vaulters.

I can only agree with you that the "Art" of pole vaulting lies in making the appropriate and necessary adaptations of the vault to the circumstances in which the vault takes place and the individual making the jump.

What is the jumping technique at take-off that gives the most efficient and safe transfers of energy / momenta from the preceding approach run and plant? This is the critical nub of the issues you raise.

It is worthwhile to think about which of these two (amongst many possibilities) conditions offer the least deleterious effects on conserving the energy and momenta of the run up and plant during the take-off:

1. Vaulter at take-off has only to overcome the rotational inertia of the relatively straight pole in the direction of the vertical plane of the cross bar and experience the minimal shock of the rotating pole making an oblique impact with the rear wall of the box whilst executing the jump from the runway!

2. Vaulter at take-off has to overcome the rotational inertia of the pole as well do work against the "resisting stiffness" in bending it whilst also dealing with the instantaneous shock transmission along the pole and brace their body core such that the hips do not advance too far forward before the take-off jump is completed !

Petrov does not insist that the pole has to be completely bend free but rather the vaulter should be attempting to minimize the resistance of the pole to the vaulter's upspring actions in effecting the take-off jump. He places the focus and emphasis on pole rotation at this stage because it gives his vaulters better opportunity to bend the pole by swinging continuously in coordination with body weight whilst suspended below the grips on the pole. The take-of is a dynamic process and involves changing body configuration arrangement and is not about adopting specific postural position maintenance.

Will, I think there is more agreement than disagreement in our views. Physics specificity is one thing and the art of pole vaulting quite another. I agree vaulters adopting "bad physics" in the techniques they learn are less likely to be both safe and successful.

The clear articulation of ideas is difficult at the best of times. Because you cover so much ground in your commentary it is not possible to fully resolve the questions you raise. I still believe that you would not be an advocate of teaching pole vaulters to deliberately take-off under and thereby deliberately place the take-off emphasis on pre-bending the pole.

Dispelling myths suffers the high risk of simply replacing one myth for another. Can we be more specific in the criticism of the underlying physics principles and facts you think Petrov got wrong?

Borrelli many years ago was well aware of the effects of the eccentricity of force application in the bending deflection of beams. Application of vaulter generated tangential force and the moments they can create lies at the heart of the art of well executed vaults and the off centre application of forces compel both the pole and the vaulter pendulums to rotate in a reciprocal relationship provided their timing interplay is in harmony!
Bubka free take-off moments instant before take-off.jpg
Bubka free take-off moments instant before take-off.jpg (83.51 KiB) Viewed 9498 times

Bubka slightly under take-off.jpg
Bubka slightly under take-off.jpg (92.67 KiB) Viewed 9498 times

Petrov Model what he shows as a free take-off.jpg
Petrov Model what he shows as a free take-off.jpg (95.15 KiB) Viewed 9498 times
Last edited by PVstudent on Wed Apr 09, 2014 11:31 am, edited 1 time in total.
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Re: Physics Based Critique of the Petrov Method

Unread postby PVstudent » Wed Apr 09, 2014 6:10 am

Clearly a coach other than Petrov found some advantages to having the take-off “intentional focus “ of the vaulter on elevating the total system COM and rotating the minimally loaded pole at the instant of take-off (see attachments below).

Note the location of the vaulter’s centre of mass (COMvaulter) is well forward of the take-off toe tips as well as raised above its initial height at the flat foot contact prior to the upspringing jump action from take-off contact mid support to toes off.

The total energy of the vaulter’s COM at the instant the pole tip strikes the planting box rear wall is the net summation of:
Total Energy of the Vaulter = PEcom + KEcom + Rotation KEcom – Pole Impact energy instantaneously transmitted (radially through the pole longitudinal axis and/or the chord of the pole to vaulter via both hand grips).

Where
1 PEcom is the potential energy due to vertical height of the vaulter’s COM above the runway surface at take-off
2. KEcom is the linear translatory kinetic energy of the vaulter’s COM at take-off
3. Rotational KEcom is the rotational kinetic energy possessed by the vaulter due to their rotational inertia and angular velocity about their COM at take-off resulting from their body semental configuration at that instant.

Pole Impact Energy is the energy of the pole tip impacting the rear wall of the planting box that does work against the inertial motion of the vaulter and is work that retards vaulter translatory progression. Depending on the radial location of the vaulter’s COM with respect to the vaulter’s top hand axis the pole impact energy transmission can accelerate the vaulter’s COM rotation about this axis and be used to advantage or disadvantage depending upon the joint-muscle actions made by the vaulter (the swing push-pull an as yet unresolved issue).

What occurs post take-off must meet the continuous on going challenge imposed by non conservative forces acting on the pole plus vaulter total system (see attachment below).

The Bubka performances in meeting these challenges has been recorded and studied using scientifically valid and reliable methods. An example from competition shows that Bubka was superior to other competitors on this occasion at doing work during the pole suport bending and recoil phases.

The graph of the energy exchanges shows indisputably that Bubka added energy to the total system and was able to use the total kinetic energy gain to enhance his flight height, rotation and flight trajectory following pole release (attachment below).

In this graph observing the changing course of the vaulter’s potential energy curve the greatest rate of increase in potential energy, due to height above the ground, occurs when the pole is straight and just prior to the total kinetic energy peak.

Note also that the the pole suport phase kinetic energy minimum occurs a very short interval of time after the vaulter’s total energy is at its minimum.

Throughout the pole support phases (pre maximimum pole bend to pole release) the potential energy of the vaulter is smoothly increasing. This is happening despite the vaulter losing total energy with peak loss occuring coincident with maximum pole bend! Of course this energy loss is that which produces the bend in the pole to overcoming the resisting “stiffness” of the pole deflection. Yet the the vaulter’s centre of mass inexorably continues to gain height! Why?

I assert, without proof, that Bubka most probably worked on the pole to direct the swing created energy (a vaulter is not a rigid body!) to move the total system COM continuously upwards and forward.

If Bubka’s COM is continuously rising upwards (as shown on the graph) he MUST have been actively doing work whilst suspended from and supported on the pole. If he was not doing so the potential energy would peak and decrease due to the continuously acting downward acceleration effect of gravity.

It is generally accepted that gravitational force in an inertial frame of reference is CONSTANT in MAGNITUDE and DIRECTION (-ve direction vertically downward) and cannot vary in any practically measurable, or significantly influential manner due to the increase in separation distance apart of the mass centres of the vaulter and the earth.


"Flexible poles, able to store energy, have made gravity less of an enemy, at least throughout the full vault. That is to say that during compression vaulters can orient themselves so that gravity is more of a friend and less of an enemy. They might "mine" energy from gravity in the early vault and then "escape" its clutches at a later point! And since gravity is such a large and constant factor in the vault, the vaulters constant relation to it is of paramount importance. This relation is neither obscured from most coaches, nor well understood." Quote from Willrieffer.

I agree that gravitational force is a large and a constant factor in the vault. But surely the vaulter’s ability to change the arrangements of body segmental mass distribution relative to the vaulter’s own centre of gravity means that the effects of that constantly directed gravitational force acting on these body segments can cause their rotation about that centre.

At the same time these body segments are subjected continuously to a vertically downward acceleration. Since the vaulter is not a rigid body and possesses muscle energy to move these body segments the orientation of the segments and the nature of their linkages are such that the vaulter can manipulate segmental mass distribution to exploit the rotational effects of the constantly sized and directed downward force of gravity.

The vaulter most certainly does not have a constant rigid body shape throughout the vault!

The generalisation that gravity at varying stages in the post take-off pole support phases can “resist” and “assist” the vaulter holds provided that the vaulter has “learned” through practice how to efficiently utilize this invariant force to their advantage. Is this practice effective if the vaulter ingrains an "under" take-off? I remain to be convinced that this is helpful in the long run. In the short term it may be expedient and get the vaulter taking off but at what long term cost?

Readers please show us where, how and why the Petrov-Bubka technical model has been wrong from a physics perspective in regard to how the vaulter can exploit long pole grip lengths with stiff poles and still be able to maintain continuous gravitational potential energy gain post take-off in pole support.

I have not found the Petrov rationale “mythical, full of hyperbole and half truths” from a physics perspective. However I always remain open to being shown the errors upon which my belief rests.

As a coach I want to know the how to do a better job! To achieve this goal I remain an on going student of the event and its master coaches.

If readers have identified any half truths, myths, etc in regard to the physics basis of the Petrov Model as empirically expressed in the performances of Bubka I would especially appreciate their contributions to this discussion.

So far, if the correspondence interpretation I make is correct, no convincing physics based rationale has been posited that refutes the notion of the “Free Take-off’ concept as actually advocated by Petrov.

It is a mistake to accept at face value that Petrov simply advocates that the pole must at all times be straight at take-off and that the toes of the take-off foot are located on the run way exactly perpendicular to and precisely below the top of the top grip on the pole.

By way of analogy “Petrov’s ideal technical model concepts” can be considered as “blue-prints” or “a framework” from which successful vaulters and vaults can be constructed.

As with all processes that operate with human performance there are critical band widths of possibilities within which fully functional outcomes can be obtained and beyond which failure and catastrophic failure become increasingly inevitable. Petrov, to my personal knowledge, is very cognisant of these limitations and is insistent that his vaulters practice within these quite small bandwidths. He is too good a coach practitioner to not recognise that learning to operate this way is a career long proposition!

What the Free Take-Off part of the Petrov ideal technical model does is direct the vaulter and the coach to recognise that there is a continuum from maximizing success to maximizing failure of the vault consequent upon the take-off.

What is not clear is that this is not a linearly scaled continuum!

The potential to successfully take-off from grossly “under” and successfully complete a vault is relatively favorable when using short grip lengths combined with close in towards the rear wall take-off points.

As the grip lengths and take-off distances increase a law of diminishing returns operates reducing both the safety and effectiveness of the take-off.

The band width tolerance in the distance “under “ to “out” with respect to the ideal take-off point on the runway also diminishes as pole grip lengths increase.

There is an inverse relationship between grip length increase and the bandwith of tolerance within which successful vault outcomes can be obtained.

The proportions of the take-off location error bandwith are such that in practice the vast majority of vaulters prefer, or are permitted, to take – off “under”. Also, the further “out” the take-off the lower the pole ground angle which at longer grip lengths poses disproportionally increases of the challenge and risk to the vaulter.

The challenge to the vaulter is to find the ”perfect” take-off to maximize their chances of achieving a personal height record.

In this sense the notion of the ideal take-off spot on the runway may be considered mythical because “perfection” is always possible but almost impossible to achieve in practice.

That it is physically possible, according to laws of physics, mathematics and biomechanics to perform a successfully completed vault using a Petrov “Free take-off” and achieve at least a 6.00m clearance, has been demonstrated by Bubka under the contraints of outdoor competition!

So no myth here but an empirical fact!

Again enlighten me please as to the what, where, when, how and why the principles of the physics used in this vault are wrong?

There are recognisable inefficiencies in many of Bubka’s performances. For example in the energy graph of his 1987 performance he is clearly shown to be unable to maintain the peak in total energy through the push off to pole release.
His kinetic energy transfer to potential energy gain diminishes well before pole release.

(Note: As it should, his total energy remains constant upon pole release onwards whilst his kinetic energy diminishes and he gains potential energy in exchange. The Laws of Conservation of Energy being confirmed!
Within the limitations of the methodology of the study which produced this data it cannot be fully or correctly explained as to the reasons for this reduction in effective transfer from kinetic into potential energy of the vaulter).

Feofanova arguable pre - jump and potential plus kinetic energy at take-off.jpg
Feofanova arguable pre - jump and potential plus kinetic energy at take-off.jpg (90.63 KiB) Viewed 9474 times

Non conservative forces that have to be overcome by vaulter.jpg
Non conservative forces that have to be overcome by vaulter.jpg (106.53 KiB) Viewed 9474 times

Bubka energy exchange in pole vault pole support sequence.jpg
Bubka energy exchange in pole vault pole support sequence.jpg (81.16 KiB) Viewed 9474 times
Every new opinion at its starting, is precisely a minority of one!

willrieffer
PV Whiz
Posts: 164
Joined: Fri Mar 21, 2014 12:00 pm
Expertise: Former College Vaulter, Current High School Coach
Lifetime Best: 15'
World Record Holder?: Renaud Lavillenie
Favorite Vaulter: All of them...

Re: Physics Based Critique of the Petrov Method

Unread postby willrieffer » Thu Apr 10, 2014 1:20 am

PVstudent wrote:Dispelling myths suffers the high risk of simply replacing one myth for another. Can we be more specific in the criticism of the underlying physics principles and facts you think Petrov got wrong?


I can just say the same things I've been saying. First, I've advocated the success of the system, and that you can't, well, "cheat" physics. And so from the beginning I've advocated as far as the take off that their attention to hip placement and attitude in the early post take off and swing were of major influence on their success. Can CoM and hip control be achieved by other means? Look at just about any French vaulter of note in the recent past. Dosevi, Galfione, and finally Lavillenie "lock out" the forward arm creating a moment to keep the hips back and down and ultimately press the swing further back and down. Readers can just look above at the sequence of Lavillenie you have provided. That relation has positive effects. It also goes against another more American school that sees a necessity to not "block" the hips early on with the forward arm and "allow" swing initiation in progressed hips.

Every time I bring up Bubka's noted comment about the "crucial" necessity of a pre-take off as an element of their "free" take off we wind up going around in a circle. That he admits they didn't get it but, "sometimes" and we have video evidence of them making big vaults both from "pre" take off and "pole bend", tells much of the story here. If I draw particular attention to his points, and I have to think he knew the system, I see something amiss. We wind up in a dialog that goes in a circle where you diverge from the actuality of the take off and its importance and back to a point of the early swing where I acknowledge they were highly efficient despite whatever "take off" specifics that preceded. Or, the pre take off was not "crucial" or even necessary. "Petrov does not insist that the pole has to be completely bend free... - PVStudent". Bubka says its crucial, and you say Petrov says, that isn't so? So which is it? At this point, I don't know, and that is a big point that this OP was addressing...and again one of my conclusions is that one part of their ideology, their attention to hip placement severely trumps their attention to ideality of the take off to it's actuality. And that success over the actuality spread from pole bend to pre take off shows this both inside and outside of their system.

Further, there is his "box energy" statements. That they were worried about energy loss in the box.

PVstudent wrote:Will, my precise conclusion was with respect to identification of "an ideal location for the take-off" with respect to the vaulter optimising the take-off in such a way that the pole offered minimal resistance to being rotated about is tip located in the box. In so doing I was careful to use force tranducers under the pole to help identify and differentiate the clear distinction between a Pre-Jump and a Petrov defined "Free Take-Off. I did not attempt to substantiate or disprove any theory with respect to the ideal mechanisms of pole bending as you appear to be implying. The mathematical and physics basic facts I used in arriving at my conclusion in making the distinction is still open to being corrected.


So do I really have to ask, what was the point? You seem to say that your outcome is relegated to a certain small set of effects boxed out from the rest of the vault. "Ideal" is very good word choice addition for it. Of what real importance is that? The increase in force is a necessity to bend the pole so it alone has no value. Since the relationship of Force = Mass X Accleration to Energy, Kinetic = 1/2 Mass X Velocity (squared) and where box acceleration and velocity are imperceptibly small, fractionally small in relation to vaulter velocity at take off, the resultant relation is going to approach zero. It's the summation of a squared fractional relation. Or, again, box energy loss is effectively zero, especially in consideration of other losses i.e. those possible versus gravity.

Many things we do agree on. And many things you go over here I either agree with or I have addressed before. If there is something again I missed that you find of particular import, please ask again. Bend take off potentiality? I've been over it. Just put a vaulter at their plant still on the runway and have them press their legs into the runway and lean into a pole and bend it. That is the mechanical energy I'm talking about placed in storage in the pole that I'm interested in, which is also not possible from a pre take off. Are there a LOT of questions about that? Without a doubt, but I find the idea of it precludes me ever thinking at this point the the Petrovian method and model are ideally superior. In a competition environment more actually reliable? Almost certainly so...

PVstudent wrote:That it is physically possible, according to laws of physics, mathematics and biomechanics to perform a successfully completed vault using a Petrov "Free take-off" and achieve at least a 6.00m clearance, has been demonstrated by Bubka under the contraints of outdoor competition!

So no myth here but an empirical fact!


Really? This is sort of like me saying, 'Bubka was able to vault 6m because at one point he wore striped sox! Prove me wrong!'. The question isn't whether they could do it, but whether if it was done for the reasons, all the physics based reasons, and in proper relation they and their students claim. That their ideas on important specific aspects are in accord with physics ideas AND are stressed in a proper relation. As a counter I have provided their attitude per Bubka on both the necessity of a pre take off in their free take off AND their ideas on energy loss in and out of the box are misguided. And to jump the gun again, that their attitude on early hip and swing relation was paramount. Just look at the Bubka pics you provide. That take off looks pretty magnificent to me! In fact I find the Petrovian's aesthetically very pleasing to watch. Their vaults typically are smooth and elegant. The practitioners around Bubka were generally a long lithe bunch with practiced and elegant vaults. And then, of course Bubka would top it off with jaw dropping explosions off the top.

The Graph

My intuitive physics sense is going bonkers. Or, you are going to have to tell me how these numbers were derived. Kinetic Energy is easy enough, measure his velocity and mass. I take it potential energy is his CoM over ground height. Total Energy then would seem to be the difference in the loss in Kinetic energy (measured forward velocity reduction) versus the gain in Potential Energy derived from the change in height. It is of no surprise that CoM goes up throughout the vault. All vaulters go up continuously, and so change of rate in it is of greater importance. IF that IS the case, I must say that is not enough parameters, notably because energy goes into the pole and so in this case the Total Energy calculation seems to be ignoring this effect. That its quite possible energy is maximized at max pole bend and actually declines by the completion of the vault. Further, my point in total is that the end energy increase comes at least in part, and I might guess in magnitude of importance mostly from the vaulters early relation to gravity being added to pole energy and thus is present in the outcome increase*. Is Bubka's added mechanical energy in there? No doubt it is, but I find it incredulous that he could add something on the order of 2000 to 3000 joules to the vault in the very short time indicated with only his handgrips as a contact point to the pole and system and especially when we consider that his legendary speed at take off only yields a little over 3000j (I calculate a max around 3600j) in KE generated over a much longer time by his powerful legs working over the full length of his run. If I took your outlook on this graph to a literal extreme it might well be telling me not worry when my young vaulters "pull" up early on the pole because they are just adding increased potential energy through their mechanical work! There are many many interesting things here which are in need of illumination. The rabbit hole deepens...

* IF you want to think about gravities presence in the vault in relation to the pole and its energy just think of this. Imagine you take the tip end and somehow affix it so that the pole is held parallel to the ground. Now imagine that the vaulter grabs the pole with his top hand about where he places it when he vaults and hangs down on the parallel pole. It is of course going to deflect and store energy in its elasticity. This effect occurs to some degree throughout the compression phase in a very complex relation between the vaulter, the pole, and gravity, and since the pole ends straight, it must come out redirected into the vaulter in the end. How and to what degree this effect happens in relation to the vaulter, their take off and swing, are of great importance to me. And its my contention that Lavillenie uses the gravity vector in his vault, especially early in his vault ultimately better than even the Petrovian's, which is why he can be smaller and slower than Bubka while also breaking his indoor world record.


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