Physics Based Critique of the Petrov Method

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Re: Physics Based Critique of the Petrov Method

Unread postby PVstudent » Tue Mar 25, 2014 7:43 am

To CoachEric

1. I agree with your point fully and add that the “Rocket” analogy illustrates the conceptual need for continuous uninterrupted energy input throughout the pole support phases of the vault for the vaulter to be able to achieve their best launch trajectory at the instant of final pole release. This release point is in fact a second take-off except in this case the net impetus of the vaulter at this instant is the result of the restoration of some of the stored elastic potential energy due to pole bending caused by the continuous muscular efforts (useful work done) of the vaulter throughout the take-off and preceeding pole support phases. The recoil restoration to the maximum chord length possible and the spatial location and orientation of the vaulter’s body segments above the final hand grip release point provide the potential energy above the take-off surface. The linear and rotational kinetic possessed by the vaulter adds to that potential energy exchage. Note that the parabolic trajectory at the instant of pole release is fixed. The success or failure of the flight therefore relies on
A. the angle of projection of the vaulter’s total body centre of mass and total body segmental orientations and arrangements
B. magnitudes of the linear and rotational momenta of the vaulter and
C. the technical ability of the vaulter to manipulate their body part segmental relationships with respect to the trajectory of their total body centre of mass and their respective spatial locations relative to the cross bar.

2. Your request to clarify which of the two distinctly different take-off methods you interpret as part of the Petrov model is also echoed by me.
“In my opinion, the Free Takeoff is the correct result of the points emphasized by the model”. Coach Eric I concur with your opinion.

3. “An issue that this community remains split on, and which introduced this discussion, is the action of the arms post-takeoff. I believe I agree with your position that there is not a row, but a vigorous "realignment" into a hollow-body position, bringing the body in line with the chord of the pole, keeping pressure up on the pole with the bottom arm, accelerating the swing of the hips toward vertical. While I believe this technique to be correct, I don't have much basis to know for sure that this is part of Petrov's model. So, as with this example, please define more clearly what you believe to be part of the model that you are critiqing. For anybody to get smarter on this, we need to agree on what we're talking about.”

I also would like this (all of 3 above) to be clarified because willrieffer, in his latest post, dodges the
question in regard to presenting any actual physics that explains or supports his assertions in regard to
pole bend being induced whilst the take-off foot continues making ground contact in an “under” position
with respect to a vertical line between the surface of the runway and the top hand grip on the pole.

Some physics principles and facts to underpin these claims are necessary and would be
much appreciated!

To willrieffer

You said and I quote:

“The Petrov model got great success. It should be studied and used on that fact alone. It's just that having studied physics and in reading their and their disciples attempts at a physics based explanation, I see either poor efforts at explanation that do not agree with my knowledge of physics, or I'm left with a lot of questions.”

You went on to say:

”But for a bend take off vaulter they do have a chance to possibly add more energy. That's just physics. Is it enough to counter some of the problems in vaulting this way? I can't be sure.”

Just a moment! Let us pause for reflection here.

Your critique is “poor efforts of explanation”… “do not agree with my knowledge of physics” and you then go on to assert “a bend take-off vaulter can possibly put more energy into the vault due to pre take-off bending of the pole.” Your implication being a vaulter in so doing has an equal to or greater chance of a successful vault outcome compared to vaulters attempting to perform a “free take-off!

If the implication I have taken from your statement is correct then please tell us the physics of how and why this is so?

Will, you also stated and here is the quote:

“The free take off and/or its attempt almost in all cases would, I think, provides a higher/better angled force vector at take-off but eschews the possibility to add more energy. So the difference as I see it is in more energy versus better vector. … But again, from what I know of physics and the vault, if the free take off is "crucial", as Bubka claimed, then the method is giving up a possible energy source.”

What is it that you know from physics that convinces you that the ‘Free take-off” method is giving up a possible energy source?

In what way does a Petrov/Bubka free take-off eschew the possibility for the vaulter to add energy to the total pole plus vaulter system? (Great word choice “eschew”… wish I had used it first!).

However, I dispute that a “free take-off vaulter eschews anything and assert (on the basis of contributions made by me on PVP) but is actually enabled to amplify their capacity and opportunity to add input energy to the complex pendular system during foot contact and immediately after the take-off foot leaves the ground!

In your contribution to critique the Petrov Method so far I believe you have not yet identified the biomechanical and physics principles involved in the technique of how to induce pole bend and minimize input energy wastage that occurs when vaulters have to contend with long pole lengths between the top of the top grip hand and the pole tip and the consequent increase in the horizontal position along the runway to the take-off. The lower pole-ground angle such horizontal take-off distance from the rear wall of the planting box also poses an additional problem to be overcome in effecting a successfully completed vault.

The Petrov/Bubka Method was developed using biomechanical and physics based principles in the attempt to resolve these particular problems and equip the vaulter with techniques to pole vault high successfully, consistently and repeatability with relative safety when using long flexible high stiffness rating composite material constructed poles with long grip length along the pole to the vaulter's top grip hand..

Will, what your contribution highlights is the necessity to weigh up the costs and benefits involved in exploiting modern flexible poles by different take-off and swing techniques in relation to the physical
abilities and acquired capacities of a specific vaulter. This is commendable.

Have you actually gained sufficient familiarity with the total Petrov/Bubka Method to be sufficiently informed and therefore be in a position to critique the physics basis and rationale underpinning the method against your claimed understanding of Engineering Physics? I suspect not.

No matter what Model of the Vault a coach adopts and implements it is incumbent upon the coach to be, so far as they are able, to understand what they are coaching and put themselves in the position to be as sure as they can be that what they profess is “safe” and least likely to put young pole vaulter’s at unjustifiable risk!

What I like about your rather audacious physics critique of the Petrov/Bubka Method is that you have put yourself on the rack but show openness to having your perspective changed!

Since you also are actively engaged in coaching youngsters and providing them with an opportunity to participate in pole vaulting I appreciate the opportunity to respond to your questions.

That said I think you have not yet come through clearly to me in your posts as having really understood or appreciated much about the physics underlying the totality of the Petrov/Bubka method and approach to the coaching of flexible pole vaulting.

Fear not. You are not alone in this regard!

The thread you have established, so far, is covering previously well ploughed and sifted ground on PVP.

Coach Eric is right in stating what PVP coaches want in regard to the Petrov/Bubka approach to vaulting with flexible poles is clarification of the role of the lower arm post take-off and the physical and biomechanical rationale underlying the claimed action roles in the generation and maintenance of swing induced energy input throughout the pole bending/buckle and recoil phases.

Finally, I think that when you have read more and have fully digested the Petrov/Bubka Approach you will find either your knowledge of the physics of pole vault was more limited than you thought or you will become even greater in confidence that your physics understanding is correct and that this approach is as you claimed in your opening to this thread. I do request that you give the physics reasons as you understand them for the claims and assertions you make.

This thread will be all over the internet as “Physics Based Critique of the Petrov Method of Pole Vault!” Will, it is incumbent upon you to put up the physics evidence that definitively falsify or cast serious doubts upon the principles espoused in the Petrov/Bubka Model of Pole Vault. If this thread is simply to be questions and answers concerning the Physics foundations of the Petrov Method then I think the inclusion of “Physics Critique” in the thread title is clearly not justified.

I will stop this post here because in the last posts of yours in regard to pole chord shortening etc., a plethora of issues arise in regard to a human’s double pendular motion, moments of inertia of vaulter and moments of inertia of the total system centre of mass, energy and momenta conservation laws with respect to the timing of momenta directional changes, energy inputs and exchanges that need to be sorted out. That is a big task!
Every new opinion at its starting, is precisely a minority of one!

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Re: Physics Based Critique of the Petrov Method

Unread postby altius » Tue Mar 25, 2014 11:55 am

Agree with Kirk - always happy to hear from folk who are actually coaching. Would be great to see some of your kids on video. Hope to be posting film of a beginner shortly - perhaps we can compare notes.

However I have seen too much misunderstanding of the Petrov model put up on this board, where it is clear that the folk concerned have not done even the basic research into what Petrov and Bubka actually did and said. My argument has always been that almost any athlete will benefit if they can begin to master elements of this model - especially the free take off. In fact I believe the athlete PVstudent is using to provide the data in at least one of his posts is a lad I coached from beginner to 5.40m, Patrick Jesser and I certainly have film of him, Tom Lovell, another 5.00m 18 year old that year (Which has already been posted on PVP) and Wendy Young, 4.40m at 19, who all employed a free take off. Will be interesting to see if the young girl I am helping now will be able to use it - and when- and especially how long will it take? These are the issues I am interested in - not any discussion about what happens after take off. My position on that is clear and has been expounded many times. In essence I go with Bubka's AAAAAAAAAAAGH!

I believe the only area of contention is to tuck? to tuck a little? or as Petrov says , not to tuck at all! I happen to believe in the latter approach but as far as I know there is no data available - and there is unlikely to be any data available for a long time because of the sheer complexity of obtaining it.

Finally I believe 80 year olds who have been coaching over 55 years should be cut a bit of slack and allowed to be grumpy old men and especially be allowed to rant against armchair coaches who do not have to put their ideas to the only true test - coaching athletes, especially beginners. :D ;)
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Re: Physics Based Critique of the Petrov Method

Unread postby willrieffer » Tue Mar 25, 2014 1:42 pm

Just some more thoughts.

Once again I have never said or advocated that using this method (or any other for that matter) is bad or a bad idea. In fact, I've done quite the opposite. Really, much of my interest in this comes from Bubka himself and his statements. His claim about the free take off being "crucial", and then also only being made "sometimes" and the part where he says they were worried about something akin to losing energy in the box. I will note that it has been suggested that with the latter, "box" might have been a mistranslation and he meant "pole". That may be so, and with the more energy you put in the pole, the more you will lose do to it's inefficiency. Still, the poles are actually quite efficient and along the way you make a trade with a reduction in grip height going along with putting energy into the pole. So you really worry more about losing control of the energy, its direction, over just plain losing it.

IF the idea that foot contact and pole bend imparted more wanted energy, it would be an advanced technique. And probably one that could not be used or mastered by every vaulter. Still, that event happens all the time, and happened with Bubka and the old Soviet group. I take this from both Bubka's own admissions and from video evidence of these guys missing the free take off. It is here I have to say something about "being under". It would seem this group in missing the free were possibly and probably too far forward at the end of their run, something we know is all too common. That is they missed their mark and were slightly "under". TO me it seems it didn't matter. Why? Well, they were still able to take off with proper posture and a good take off vector. Bad take off posture almost always goes with the wanted take off mark being passed. The hips are forward, the shoulders back, the top hand behind, etc, all things which we know are bad and which leads to early swing progression which can't be corrected and a bad and possibly dangerous vault. It's here I would say that proper posture and a good take off vector will overcome missing what has generally been considered as the ideal take off spot. This, of course, Bubka and the Petrov vaulters had mastery of in their take offs and vault. There are obvious limits to both how far "under" or "out" any vaulter can still initiate a good swing. Those are going to be based on the individual. As PVStudent makes some note, more advanced bend take offs seem to be seen more in shorter vaulters who have shorter limbs as levers. It's also going to have to take a lot of core strength. And its likely that any movement towards this idea as method leads to a greater chance and occurrence of bad and dangerous vaults. The progression will put more stress on the vaulter (and yet a lot of vaulters take off somewhere under over and over again even as they see it as less than idea) and push into even further and further territory when a bad posture take off will be possibly a severe problem.

For some time I had been troubled about how it would be shown what the added mechanical energy would be at take off for a pole bend because it would be very hard to calculate the force in relation to the vaulters CoM slowing down. IN fact I now believe that PVStudent provides the answer in the box force analysis. That mechanical force is the difference between the free take offs measurement and the bend take off measurement. The difference, which again if I remember right, he characterized as "high", also measures the extra energy going into the pole. Is it worthwhile to pursue? I can't say. In part the whole actuality of Bubka and his contemporaries both achieving and missing the free take off as much as anything puts much of this in doubt. Or, both methods might well provide something that cancels out. I remember no reporting on any what my be called radical jumps having occurred from either method. As I asked before would we not have seen something like Bubka saying, "Well here I actually got the free take off and you can see the difference."

Lastly, the vaulter I feel most compelling to look at right now is Tradenkov, who was in vaulter in this system, and my intuitive sense based on my physics knowledge was one of the most efficient vaulters of all time. And further that difference comes from his swing speed variances and characteristics over other vaulters using the same method. I have a further guess that an extrapolation of this is going to lead right to Lavillenie...

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Re: Physics Based Critique of the Petrov Method

Unread postby willrieffer » Tue Mar 25, 2014 2:29 pm

Well, gentlemen, I hope my last post helps some as I wrote and posted it before the latest two comments. LOL.

I just have to reiterate. Foot on ground and pole bending leads to the possibility of the energy input being the kinetic of the vaulters initiation speed plus the mechanical they can generate through the foot on the ground. Any free take off has only the kinetic energy at plant. As I posted somewhere before, imagine that a vaulter is magically suspended off the runway with the pole tip in the box. Now they have very little means to put energy into the pole over an action reaction based on their mass. Now if their foot is on the ground we know they can create significant bend based mostly on the use of their generally powerful leg muscles pushing on the unmoving ground. I've now said the difference is that measured in the box.

Continuous energy and input. Ok. My initial views on this were, LOL, poorly stated.* OF course a vaulter can effect energy transmission to the pole back and forth, that is the essence of the vault. It is necessarily action/reaction. For any vaulter they have to be careful for any input for it effects their CoM, once again, the essence of the vault. Now, what is more important, volume of energy transmission or direction of the CoM. Well in some sense both. In the compression phase the direction of the CoM, the swing moment, is paramount. The vaulter does not, in fact cannot worry much about simply generating energy into the pole as it will force their CoM off, most often they will "row" themselves into a progressed swing. Decompression. While they can put energy into the system, they cannot put energy into a pole that is releasing energy, something also leading me off base. Still, any force will regulate the output. Again, as is noted above, direction of the CoM is actually paramount to the volume of input, but the nature of the system allows for very good gains from the sheer amount of force and volume of energy....as Bubka demonstrated. Still, I was blinded by a comparison of the input energy of the vault, kinetic energy being equal to mass times velocity squared, with a possible addition of mechanical energy already talked about, over the amount of energy the vaulter can impart after take off. My bad. Still, I might point out that in the vault post take off I find force vector far far far more important than energy volume.

*An anecdote here to close for now. While at Ohio State I was in an upper level physics class. We had the first test and on the day we got the results in class he had all the scores posted up to be read by all in the room. He also had posted the average and mean which were right around 12 of 100. The scores went 98, 97, 47, 35, 34, 18 and then on with something like 35 or so students. There were lots of single digit grades and a few zeros. And it was tough enough just to get into this class! The test was graded on a curve. I waited very apprehensively only to find I had gotten the 47 and an A on the curve. Yet, I did not know whether to be happy or sad considering the two scores in the upper 90s! In other words, physics is hard, and its very very easy for even good student of it to make mistakes. Drastic mistakes.

And thanks for your generous and charitable replies.

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Re: Physics Based Critique of the Petrov Method

Unread postby willrieffer » Tue Mar 25, 2014 3:15 pm

PVstudent wrote: 3. “An issue that this community remains split on, and which introduced this discussion, is the action of the arms post-takeoff. I believe I agree with your position that there is not a row, but a vigorous "realignment" into a hollow-body position, bringing the body in line with the chord of the pole, keeping pressure up on the pole with the bottom arm, accelerating the swing of the hips toward vertical. While I believe this technique to be correct, I don't have much basis to know for sure that this is part of Petrov's model. So, as with this example, please define more clearly what you believe to be part of the model that you are critiqing. For anybody to get smarter on this, we need to agree on what we're talking about.”


First, if there are more questions or something I missed addressing, please ask again.

What is my intuition on this? Well, first, that a completely free swing is good, but less than ideal. It will want to tend to progress away and forward of the most efficient swing. Again this comes from my knowledge of physics and of watching a lot of vaulters, kids with good swings, but which are progressed, but the force vectors are better to move the pole forward and down. IN fact I believe that the most efficient swing is actually somewhere slightly behind the free swing and even the chord. To "Get back and down" takes force and leverage. Again we are talking about a trade off. IF you get too far behind you can't get out. To get first behind a bit and then catch back up takes a lot of effort and sense of timing. SO you have a very very important period around the time of the transition from compression of the pole to its rebound and release. "Tuck"? In some sense most vaulters who tuck do so to late and it does nothing but thwart their upward progress.

IN part this comes from some recent observations I've made of Bubka, Tradenkov, and Lavillenie. Bubka did not tuck, but in transition did manipulate his rotation speed by adjusting leg extension in both legs. Tradenkov was even more active in this area, although I need to watch more of him. Lavillenie? Well he drops the right for a quasi double leg and then to get out of that has to radically alter his rotation length mid vault to get back into position to go up. He uses it in as short a time as possible and extends asap. I had the intuitive idea that this was the superior method (sans the double, but it too makes sense in terms of the physics) before I ever saw him vault.

I tell my kids a few things. Most are familiar with ice skating and tell them to think about how figure skaters induce fast spins by starting out with their arms and legs out and then pulling them in to spin very fast. So it is with what Lavillenie does. At take off I tell them to reach down and back and imagine they are trying to ride forward as far as possible under a big wheel (that the pole is bending into).

I'm with a new group of kids this year. My sis is a teacher and coach at a nearby school and asked me to come and work with them. We're just getting the plants straightened out! Got one guy with a nice take off position, but then he throws his head back and rows while pulling both legs up. Got another one that has just a great natural swing and swing extension, but puts his forehead on the pole every time. Went to a meet yesterday and it started snowing. I advised our HC and boys not to vault...

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Re: Physics Based Critique of the Petrov Method

Unread postby KirkB » Tue Mar 25, 2014 3:58 pm

I don't have much time to post today, but I wanted to give you a couple thoughts to ponder about ...

1. Re the tuck: I don't think tucking is a sin. I think PAUSING in a tuck is the sin. So if the vaulter needs to shorten his radius in order to speed up his inversion, then I see that as a good thing - so long as he's not hesitating, waiting for the roll of the poll to catch up. I think tucking is a good way to recover from a bad vault, but ideally, a vaulters BEST technique should not be targeted towards tucking.

2. Re what you call the bend takeoff - intentionally bending the pole before takeoff: From personal experience, if I was under then I would most likely miss that attempt - I could not jump well with an under takeoff. I attribute this to my slow runway speed. I have always been amazed at vaulters that could recover from a bad vault (an under takeoff, which implies a bend before takeoff) when they're under. I knew several vaulters in my day that could do this, even some that PURPOSELY took off under. They were typically very fast down the runway, and relied on their horizontal speed more than their jumping ability to put energy into the pole (and convert their horizontal momentum to vertical momentum). Bruce Simpson and Jan Johnson are 2 of my buddies that come to mind re this - although I believe Jan's intent was a free takeoff (he can correct me if I'm wrong).

Fast forwarding to the 1980s and 90s, clearly Bubka was able to do this too - but he was not only very fast down the runway, he ALSO had a very good vertical on takeoff. What I'm getting at here (without explaining it very well) is that your mileage will vary according to your style, jumping ability, and what you're used to.

BUT ... and here's my punch line: Don't you think that bending the pole before takeoff is akin to putting the brakes on - which would be hugely inefficient?! Especially if you're flat-footed when you do this. This is when (in my understanding of physics) the energy would truly go into the BOX, and dissipate into the ground. If you're still on your toes, then not so much. Okert Brits cleared 6.03 this way - how I don't know, but it does go to show you that there's more than one way to jump 6 meters!

Kirk
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Re: Physics Based Critique of the Petrov Method

Unread postby grandevaulter » Tue Mar 25, 2014 9:43 pm

I'd say the late or neutral takeoff misapplies energy to the back of the box and back through the pole shocking and momentarily stalling the carrier. From a practical stand point, it retards the efficiency of the forward rotation of the bending pole. (As Kirk says; "puts on the brakes")

Also, a free take off that is out also adds a few degrees of pendulum motion, making it a longer swing that once again from a practical stand point lends it more energy into the swing. One cannot start a pendulum to swing from the bottom. Give it a try.

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Re: Physics Based Critique of the Petrov Method

Unread postby KirkB » Tue Mar 25, 2014 11:23 pm

grandevaulter wrote:One cannot start a pendulum to swing from the bottom.
:star:

That's why I thrust my trail leg BACKWARDS before I swung it DOWN and FORWARDS! :idea:

This was an innovation of mine that does NOT cater to the Petrov Model. And that's probably why I couldn't clear a bar if I was under. Try THAT, GrandeVaulter! :)

My technique depended on a free takeoff!

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Re: Physics Based Critique of the Petrov Method

Unread postby altius » Wed Mar 26, 2014 5:47 am

I beg you not to go back to the notion that you SHOULD bend the pole before you leave the ground - I suspect it was that fault that lead to four vaulters being killed in one month some years ago and lead me to watch and listen carefully in Reno what coaches were telling athletes. Dangerous stuff. Read what Bubka says about it on Page 119 BTB2 - I really do suggest you - and anyone else who is unsure about this issue - read some of that stuff before you take this further -and certainly before you teach your athletes to do it.

But in simple terms if the pole is bent early the energy must come from somewhere -the only place it can come from is the vaulters speed as they leave the ground. They will have already lost a great deal of speed by taking off under -something they will have to do if they want to bend the pole before they leave the ground. Then if 'the spring' is compressed early it will recoil early - at worst leaving the vaulter going vertically up over the box, at best needing the stands at 3 or 4 for a clearance . diagrams in Pages 31/32 in BTb shows this notion of early compression and early rejection diagrammatically. A recipe for disaster.

However if folk are still unclear about these issues now it is probably too late for me to do much but watch as more athletes are killed in the USA. I have done my best over the past few years to spread the Petrov Bubka message - but clearly I have not done a good job. Nor it seems have the other Petrovians amongst us -they will have to carry on the fight -I can't go there again. Instead I will endeavour to show film of how our kids do it - and even better how they learn to do it.
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Re: Physics Based Critique of the Petrov Method

Unread postby IAmTheWalrus » Thu Mar 27, 2014 8:28 am

Yay physics!!

I think what willrieffer is talking about is the additional mechanical work (which is equivalent to the change in mechanical energy) which results from the vaulter compressing the pole during an inside takeoff. I would surmise (granted I was recently told that I don't stand a chance of getting into a physics graduate program so take this with a grain of salt) that this work would not be "lost to the box," but would instead be converted to elastic potential energy (that's a thin, right?) which may be vaguely approximated as 1/2 kx^2, where k is the stiffness of the pole approximated as a spring constant (given in kg/s^2), and x is the distance that the pole is compressed (e.g., a 15' pole which is compressed to an 14' chord length would have an "x" value of 1'. The amount of compression that occurs while the vaulter is on the ground is what is being considered here. If the energy stored in the pole as a result is greater than the loss in kinetic energy due to the braking effect of the pole, then there will be a net gain. This does not necessarily constitute an overall net gain, as the energy returned to the vaulter is less than the energy stored in the pole. A more complex analysis would be necessary to determine if the energy returned as a result of the additional pole compression would be greater than if they vaulter had taken off freely.

Limitations of this analysis
- It does not factor in kinematics or biomechanics
- It does not factor in the effect of an inside takeoff on body posture, and how this will effect the alignment of the body with the direction of force provided by the pole at tht top of the jump
- It does not factor in the effect of an inside takeoff on the timing and effectiveness of a swing
- It does not factor in the effect of an inside takeoff on the body, and what energy is lost through the bodies reaction to this additional force
- It assumes that the pole can be modeled as a spring and the stored energy obeys Hooke's law
- It assumes that the "k" value the pole is independent of the direction of force applied by the vaulter, which is obviously not the case. I would imagine that the k value, would actually be a function of the position and direction of force
- It's a really really really big oversimplification

All that being said, I don't disagree with the Petrov/640 model or it's advocacy of the free takeoff. I still feel that the free takeoff is the best way to vault (and coach). I also think it is less dependant on the physical qualities of the vaulter, which is convenient for coaches. It's also a bit more consistent and less likely to lead to a case where a vaulter "blows" through on one pole and is stood up on the next (I said less likely, not impossible).

I hope this facilitates more productive discussion. My apologies to any physicists who may be offended by my overly simple (and possibly dead wrong) analysis. It's been a while...
-Nick

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Re: Physics Based Critique of the Petrov Method

Unread postby grandevaulter » Thu Mar 27, 2014 12:18 pm

[quote="willrieffer"]First...

So I also contend vaulters can adjust their swing moment and swing speed in a way that surpasses an explanation, at least in a way that can easily be condensed into a teaching point or method.

They can also come down on their head as stated by the wise.

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Tim McMichael
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Re: Physics Based Critique of the Petrov Method

Unread postby Tim McMichael » Fri Mar 28, 2014 2:26 pm

altius wrote:
But in simple terms if the pole is bent early the energy must come from somewhere -the only place it can come from is the vaulters speed as they leave the ground. They will have already lost a great deal of speed by taking off under -something they will have to do if they want to bend the pole before they leave the ground. Then if 'the spring' is compressed early it will recoil early - at worst leaving the vaulter going vertically up over the box, at best needing the stands at 3 or 4 for a clearance . diagrams in Pages 31/32 in BTb shows this notion of early compression and early rejection diagrammatically. A recipe for disaster.


This is about as good as this can be said. I think that willreiffer's argument is that energy can be added if the vaulter is still on the ground while the pole is bending because they can use the strength of their leg pushing against the pole. But that strength is not lost if the vaulter is in the right place for the pole to start bending just as the foot leaves the ground. The instant when the toe is just about to clear the track is the most powerful part of anyone's stride. The illustration of the vaulter suspended off the ground and, therefore, unable to add energy to the pole is a bad analogy. It ignores velocity. While it is true that no more power from the run can be added to the system once the athlete leaves the ground, it is also true that they have already put all the energy into the pole that they are ever going to get by the time that happens. (excluding the swing) It also ignores the advantage gained by being so much taller at takeoff at the end of the stride instead of in the middle. I don't know the math, but a physics professor at OU once told us that he had calculated that for every inch higher the plant is when the pole starts to bend there is the potential for a three inch higher jump. The difference between the pole already bending with the takeoff foot flat on the ground and the knee flexed and bending just as the toe leaves the ground with the leg extended can be as much as four inches.

I would also like to second Altius' comments on safety. The truly deadly vault is one that is completed yet does not have the depth to reach the pit. It is true that you can make it to the middle of the pit while taking off under if your technique has the right compensations and you are on a small enough pole. Thousands of vaulters do it every day. It is, however, also possible to complete a vault and land in the box. And this become more and more likely as the vaulter moves up poles. On the other hand, the fundamentals that govern a free takeoff make this almost impossible. You can come up short, but you are also forced to bail out. Because the ability to swing is directly related to the quality of the takeoff, a poor takeoff means an aborted swing. If the vaulter is under, or the pole is too big, the timing that allows the swing to happen is ruined and the vault cannot progress. Landing in the box from a bail out is infinitely better than doing it from a full vault.

This goes to the fact that willreiffer is right about. From a purely physics standpoint, is is impossible for energy to go into the box. For that to happen the box would have to move. The reason that athletes and coaches use that expression is that an under and low plant has a jarring feeling. It feels like the pole is stabbing into the box and there is certainly too little energy to vault well after that happens. Thus, it feels like the energy went into the box. But the fact is that all that energy is still there, and it has to go somewhere. Where it goes is the vaulter's hips, which get yanked forward. An athlete with good body control can use this forward thrust of the hips to go ahead and get inverted (usually by tucking, which is also why tucking has such a bad if not entirely deserved reputation). But they have no pole speed, and the result is potentially deadly.

Any method that allows a vaulter to finish the vault and still land in the box is unsound. Not just because it us unsafe, though that is the main reason, but also because it will be a lower vault, even if it is successful.

That said, I do appreciate willreiffer's ideas and comments on this board. His was the majority point of view not too long ago and was advocated by no less that Dick Ganslen who was also coming from a physics background. It's good to rehash this debate, I think. If only because the arguments for an under takeoff can be surprisingly compelling.


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