I am now in a position to answer Kirk Bryde’s question on the basis of the facts as far as I have been able to determine them.
The journey has been long, very arduous and extremely time consuming for me. I hope I have used the empirical evidence visited along the way to clarify a rationale based on scientifically sound principles in making the analysis of the pole vault take-off.
Here is Kirks question as he posed it to me:
“If a free takeoff is a mere millimetre (or thereabouts) off the ground and a pre-jump is something much more substantive than that (e.g. Fefanova's technique), why does Agapit say that anything more than a mere millimetre is inefficient? Or why does Bubka (via Altius) say that a pre-jump would be even better than a free takeoff? “First some factual data in regard to the first ever officially recorded Pre –Jump by Delko Lesov of Bulgaria in the Junior World Championship in 1986.
Velocity at take-off = 8.6 m/sec
Horizontal Velocity = 8.1 m/sec
Vertical velocity = 2.7 m/sec
Angle of Projection COM = 19.5 degrees above horizontal.
The researchers said that Lesov was in the air for 1/hundredth of a second ie 10 milliseconds.
Vertical Displacement of the COM is 2.7 m/sec x 0.01sec = 27 millimetres (2.7 cms)
Horizontal Displacement of the COM is 8.1 m/sec x 0.01sec = 81 millimetres (8.1 cms)
Using the total pre-jump time of 0.032 second (time from take-off to pole tip impact for PSJ in Jump 1147B) and the same take-off kinematic parameters as determined for Lesov:
Vertical Displacement of the COM is 2.7 m/sec x 0.032sec = 86.40 millimetres (8.64 cms).
Horizontal Displacement of the COM is 8.1 m/sec x 0.032sec = 259.2 millimetres (25.92cms).
I think it is reasonable to suggest that the vertical displacement to be expected in a Pre-Jump is around 2 cms to 10 cms (ie 20 -100 millimetre range: remember 25.4 millimetres is 1 Inch).
The COM horizontal displacement can reasonably be expected to be between 8 cms and 30 cms (80 to 300 millimetres in a pre-jump.
In a pre-jump the vertical displacement of the vaulter’s COM is estimated to be in the range of less than 1 inch to approximately 4 inches (25.4 to just over 100 millimetres).
My conclusion, based on the evidence and the balance of probabilities, is that the minimum unit of displacement in a pre-jump is not less than a centimetre for it to be detectable with reliability even when assisted by visual recording without some means to confidently determine the time relationships and locations of the take-off toe and pole tips throughout the take-off and the initial portion of the first phase of pole support.
For a Petrov / Bubka defined “Free Take-off” where the take-off foot toe-tip breaks ground contact as the pole tip impacts the rear wall of the box the unit is the millimetre. Logic suggests that the time gap is zero but practically it is the minimum time possible that can be discriminated by the capabilities of the observer or measurement system.
Because of the speed with which such an event occurs the vaulter will only know that this event happened a very long time after it has occurred. The data suggests that the vaulter has advances well into the pole support phase of the vault, for at least 70 – 120 milliseconds beyond the toe off point in time, before it is possible to become aware of what occurred at the instant of take – off. Even then the vaulter is highly unlikely to become fully aware until he uses introspection to interrogate his kinaesthetic memory of the vault process after the jump has been completed. The method of introspection is notorious for its unreliability!
In the force time graph below if the impact of the pole tip was to occur within a millisecond of the vaulter’s toe tip leaving the ground and the pole tip had been in contact with the apron of the planting box some milliseconds prior to that instant then there is the minimum interference or interruption of the take-off motion that is feasible in real world practice. In the case of the real world force time graph of the pre-jump PSJ 1137, assuming all else remained the same, the pole tip would have to have contacted the planting box about 64 millisecond before the identified toe off occurred.
- Pre jump final 2.jpg (78.66 KiB) Viewed 35545 times
It is very interesting that this is just about the same time in the take-off ground contact that the amortization phase ends and the propulsion phase begins. It points therefore to a “Free- Take-off” as having to commence about half way through the ground contact time in order for the pole tip to have advanced towards the deepest part of the planting box and strike the rear wall within 1 to 2 milliseconds of toe tip leaving the ground.
I conclude that Agapit
is once again correct in his statement with respect to an “ideal” free take-off” as advocated in the Petrov/Bubka concept.
Altius, on the other hand is being pragmatic, based his suggestion on empirical experience of how to achieve the “ideal free take-off”.
A little thinking about the empirical evidence I have provided will assure readers that if the “planting of the pole in the box” is completed before, about half way through the ground contact phase of the take-off, then the pole will be offering considerable resistance to the forward progression of the vaulter+ pole total system before the take-off point in time is reached.
Altius, suggests that in a pre-jump the height created at take-off is a matter of centimetres not millimetres.
This addresses the critical coaching issue of how to get the vaulter to execute a free take-off by timing the completion of the plant to coincide more closely with toe tip take-off. Practical experience shows that “taking –off under” actually encourages pre -bend pole loading, such that it resists the progress of the take-off and significantly lowers the pole ground angle.
Logically the point in time for completion of the plant to be coincident with the toe tip take-off requires the initiation of the pole tip planting box initial (and sliding) contact to occur in the propulsive phase of the take-off ground contact.
The time before toe tip take-off from the start of the propulsive phase of the take-off is in the range 50 to 60 milliseconds. This extremely short interval of time is beyond conscious control at any time by the vaulter during the entire ground contact of the take-off because the fastest known kinaesthetic reaction times to initiate an error correction response are in the range 70-120 milliseconds.
The time it then takes to complete the error corrective response must be added to this reaction time to determine the total corrective time involved in amending a motor act execution error. When the whole body is involved and in motion in which total body momentum is involved error amendment takes even longer again.
- Pre jump 25.jpg (81.68 KiB) Viewed 35545 times
For the timing of the pole tip impact to coincide ( within plus or minus 1 to 2 milliseconds) with the take-off toe tip breaking ground contact the vaulter (a) must be operating in feed forward mode of motor action control and (b) make the decision to complete the plant well in advance of the take-off foot making initial ground contact. When operating in feed forward (motor skill open loop control does not use sensory feed back to guide and control the motor action sequence) mode of control it would, at best, take the vaulter somewhere between 300 to 700 milliseconds to recognise an error has been made and to amend that error.
This information makes it imperative for vaulters and coaches to recognise that any intentional focus on the take – off has to occur at a time well before the initiation of the plant is started.
It also implies that taking off with the take-off foot closer to the box than that required for the mathematically and biomechanically optimum location of the take-off foot on the runway beneath the top hand grip location will create greater amounts of effective energy transmission losses during the take-off.
Altius is pragmatically and practically correct. By encouraging the vaulter to execute a pre-jump, the take-off foot placement on the runway being
exactly that required in a “free-take-off,” but with the difference that the vaulter attempts to be airborne slightly before the pole tip strikes against the rear wall of the planting box.
Evidence in the case study showed support for this to be the case.
The empirical evidence I brought into the discussion clearly demonstrates that when the observer has to rely on the evidence of their own eyes, even when assisted by normal speed video footage of pole vault take-offs, it is an uncertain and very unreliable means by which to attempt precise differentiation between a free or a pre-jump take-off.
Much higher video recording speed can only go part of the way to resolving the evidentiary problem for the reason that the planting box is usually obscured. It is also extremely difficult to film with just a single camera both pole tip and take-off toe tip and show them both in each frame captured during the take-off.
As promised Kirk you have my answer!
The question as to whether it is possible to execute a Petrov / Bubka type "FreeTake-off" by being even slightly "Under" is shown not to be possible in real world vaulting because there is always some resistance from the pole and greater energy losses incurred. Thus being a little further out from the ideal take-off location is a precursor to learning an effective "Free Take-off". The range in distance "out" from ideal take-off foot location is going to be very small. Perhaps in the order of 1-5cms at most !
Over to you and interested readers of PVP to hopefully bring any errors of fact or misinterpretation of the evidence I may have made.