# Mechanics of the bullet, medium, elongated start?

For a physics project at school, I am investigating how the different types of starts affects the athlete’s acceleration.

I’ve read that the medium start allows for the fastest acceleration due to the large impulse exerted on the blocks…but why is this?

Could anyone help me with this?

Well, I’ve come up with some explanations involving the moment of inertia of the backleg, and angular distance of the joints in the different starting positions.

I was also wondering…Would the center of gravity shift significantly in the different types of starts? The body is in unstable equilibrium in all 3 different types of starts anyway, so would the line of gravity in this case make a difference to the athlete’s acceleration? Is it possible in be in ‘even more’ unstable equilibrium, that it allows the athlete to accelerate faster?

Am I missing any other physical/mechanical theories?

Any help, advice, sources, articles, websites, etc. would be much appreciated.

Thanks

www.coachesinfo.com

They have one article about starts.

the moye start.

The distance of the Moye start is greater yes… but what about speed? If you look at the picture you can see that (imo) the normal start can be executed much faster. Because of the fast short ground contact, the 2nd and 3th stride will come faster. Never saw a start like this before but this was the first thing thing i thought about it when I saw it. Imo its not about covering the largest distance but about executing everything in the fastest possible way?

I should have looked first.

The rear heel is to be off the block and on the gun it is forced back with speed causing an overbalance and a supposedly faster start.

found a book modern athlete & coach april 1993 where canadian national sprint coach Brent McFarlane comes to the conclusion that the systen is guaranteed to improve the start and the pure acceleration phase in sprinting.

From memory Charley Moye was a professor at some uni and it was supposedly going to revolutionalise starting.

study around this moye start

http://www.sportmedab.ca/pdf/Gervais%20SSAA%2004-05.pdf

Conclusions
The faster RT recorded on the Moye block starts
did not seem to translate into faster sprint times to
any of the interval distances. It may be that the
faster RT were caused by some artefact of the
interaction of the Moye block and the ReacTime
units. In fact, only 3 out of 94 recorded trials were
deemed to be false starts by the ReacTime units
(and subsequently not included in calculations of
average RT), and all three of these were during
trials on the Moye blocks. It may be that
something in the block configuration makes it
result in faster recorded RT – this may have
implications for athletes using the Moye block in
competition since major competitions typically
make use of false start detection equipment, like
the ReacTime units used in this study. Further
research will be required to clarify this point.

It is the impulse which acts upon the runner - impulse is the force exerted / the time taken to produce this force.

Hence greater impulse = faster starting times.

The three different starts you are talking about don’t neccesarily change the COG - they refer simply to the arrangement of the feet in relation to one another.

The COG will move foward toward the hands as the distance between the feet & hands increases and hip height in the set position decreases. COG will never be infront of the hands, or else the runner would fall flat on his face.

In theory, the closer the COG is to the hands the faster the athlete will exit the blocks due to the falling effect once the hands leave the ground and the athlete becomes unstable. But there are a number of other factors which means this is not always the case in a practical situation - such as start specific strength (the ability to extend the leg under load to avoid stumbling), rocking in the blocks, etc.

The Moye starting blocks are total crap. If the foot is ahead of the hip in set, there is no way to push directly out of the blocks. with the Moye blocks the front foot os on the line. Endorsed by Tom Tellez- who never had any of his own athletes use them

Reading a bit more in the article it appears the Moy Blocks are being praised by Lauren Seagrave & Kevin Odonnell of Speed Dynamics. I have a video I took at a coaching clinic in Newcastle where Lauren is teaching the Speed Dynamics technique.

To quote article ““This block puts athletes into a sounder biomechanical sprint position sooner than any conventional starting block””

Wonder if the blocks were designed for those athletes than use speed dynamics as their model???

Yes Charlie total crap

I don’t know enough about start technique, but i can give you some advice on the analysis. There are two common mistakes with dynamic analysis. The first is to forget the angular components (e.g. torques, angular momentum). The second is to apply a quasi-static analysis to a dynamic problem. A potential solution for both of these is to build a “stick figure” model with an energy-method like lagrange’s equation. This usually takes the bite out of systems involving joints. Another thing you could do is to take video-tapes of the common starts and obtain the joint angles and angular velocities through time. (Reduce the problem to 2D and only analyze the set-position to first or second step to reduce the amount of data to process). Then using your stick figure model, back out the forces around each joint (or at least estimates as i think this problem is underconstrained). I would use some type of ODE solver to do this (as this would take forever by hand). This would be my first approach to this, it might have some computational problems.The above might be a bit more elaborate that you wanted…

What is your physics background? (Probably should have asked this before mentionning the above )

Has anyone access to a computer program that can do this include the muscles and tendons, I have been looking and found nothing.

Bullshit again. Seagrave never had any top athlete start that way. Sounder biomechanical position?? If your front foot is ahead of your hip in the set position, you’re fucked- plain and simple.

I don’t know any already developed program that would do all of this, off the top of my head. However, there are a few programs that can do pieces. Some good motion-capture equipment (e.g. Vicon) should be able to get the joint angles. Some simple manipulation could get this into 2D. Or you (or a computer) could manually go frame by frame and select the joint locations. The simulation side of things, could be made simpler with a basic mathematics package or physics engine.

However, it is probably a good idea to most of the “footwork” into building it oneself (especially the derivation of the model), as the limitations of the approach becomes more obvious. This is also important to avoid the “worshiping the magic black box” syndrome that these tools have a tendancy to create. If you are really interested, the basic theory isn’t too bad (2nd year undergrad physics).

I’m a high school student, and wow, all that stuff appears very advanced to me… :eek:

I just googled ‘lagrange’s equation’, and it looks very complicated. Much as I want to read up and learn new concepts such as this one, I’m not sure I can…

Oops. I would wait on lagrange’s eq until you have been through a bit more math. A simpler approach would be to use more of a quasi-static analysis. (e.g. assume that at a particular joint angle, the accellerations are constant). Then you draw your free body diagrams and sum the forces. This is a pretty big approximation though, and could give you some odd results (that might not deal well with reality).

Some more simple would be to look at body lean out of the blocks. (e.g. How greater extension requires the athlete to push harder or fall over).

What exactly is quasi-static analysis? Know any good sites to read this up?

So basically looking at things like how the center of gravity shifts in different types of starts? I can probably handle this…

btw, you said before to ‘forget about the angular components’…What’s wrong with them?

Or should I just settle for angular kinetics anyway (since it should be much easier for me to handle)?

Essentually you assume that the object is not accellerating (which is an ironically bad assumption in this case, but it does simplify the math). In this case the sum of the forces/torques on the object are zero. This avoids mosts of the problems with the differential components (the ones that vary with time). There is a book by Beer and Johnson called Vector Mechanics for Engineers that describes this approach pretty well (although there are probably better books). If i remember a better book i will post it.

Yeah, sort of. More like looking at the forces required to maintain stability when the center of gravity is extended. You might have to make some simplifying assumptions, but that is ok for what you are doing (besides, if you really enjoy this stuff you will be learning the math to do it the right way soon enough )

In that statement i was refering to what you should NOT forget. A lot of people who are new to physics (and some who aren’t ) forget about the torques and the rotation aspects of dynamics. It was more of a caution.

When it comes down to it, try your best and have fun with it. It is always nice to hear from someone who is exploring physics. Let me know if any of the above helped at all…