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Re: Fw: Dinosaurs and birds



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1). Why in the world would it immediately fall over? The implication, as pointed out in my last post, but edited out of this one, is that a biped running in place (ie, relieved of the necessity of imparting thrust w/ the hind feet) cannot exceed the stride frequency attained at full unassisted sprint.

I know that seems implied by my statement, but it isn't. The stride frequency can rise in that case because you loosened the requirements of imparting force to the substrate. The running speed is constrained by a combination of stride frequency, stride length, and force imparted by each foot fall. If the animal is at maximum velocity for a given gait, then increasing one of the those factors will require a reduction in one or both of the others.


It's actually worthwhile to sit down and work out the calculations. See if you can find a quantitative result that will support higher maximum speed when the stride frequency, stride rate, and force imparted per stride are all at maximum and cannot rise. --MH

Maximum (unassisted) running speed is not constrained by simple stride frequency in humans. A man running w/ a 20 mph tail wind is going faster than his "normal running speed". Will he fall over? No! A man running in still air at top speed that catches a wind gust strong enough to increase his speed an increment above "normal running speed" doesn't "fall over"!

Actually, someone who gets a large gust from behind probably will fall over if it actually pushes him (or her) beyond maximum running speed. I'd be happy to clock some individuals and make sure. In any case, the reason why the gust can be advantageous (and the reason that gust-assisted records are not valid for running humans) is that the individual need not work as hard to keep at max speed while the gust is helping. So, if I run 200 meters and had help for the first 100, then I'll have more endurance in the second half and get a better time. It can also help in acceleration, though only in a relatively small window. Now, the decrease in work load possibility seems great for our basal bird, until you recognize that it doesn't get the forward push for free: it has to produce it and push against drag to do so *unless* it is actually milking a wind gust itself. So, your 20 mph gust doesn't help much in terms of maximum speed, but it will help reduce work required for the near-avian (as well as allow any number of additional burst maneuvers).


The regulations in sports regarding assistance, swamping goal-lines, etc. are not in place because the individuals hit higher sprinting speeds. There are plenty of other reasons why winds etc. affect performance (either negatively or positively) and hence the regulations. --MH

I am entirely serious on this one. Think of the money you can win on
bar bets w/ professional athletes if you are correct!
"Hey, Mr. (insert name of favorite wide receiver), bet you a million dollars
you can wear any rocket pack, designed and built by any team of scientists you chose, and you can't
increase your maximum speed." You'd have every skill guy in the league
pounding on your door, and you'd be rich beyond your wildest dreams. Actually, ANY athlete. If
you are right, which of course, you are not.

Actually, if their feet have to keep up, they probably won't increase maximum instantaneous speed. Of course, they could just let their feet more or less drag behind or barely touch down, while the rocket just shoots them across the finish line. This image is what makes my statement seem so silly, but the legs aren't keeping up, and they're not really running (they're basically flying).


Here's a much easier experiment, actually: Sprint as hard as you can over level ground. Record your speed over a very short interval so that you have your maximum instantaneous speed. Now run downhill and try to exceed it. I recommend wearing a helmet. --MH

There is no solution
possible where stride length increases without reducing stride
frequency or imparted force, if you don't let the hind limbs work any
harder and the animal cannot actually fly.

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2). Top running speed has no intrinsic relationship to minimum flight speed. There is NO connection WHATSOEVER..
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Well, there is a correlation produced by selection as a result of launch dynamics, but you are correct that there is no intrinsic connection. But, I did not say that there was a connection to minimum flight speed, so I am a little confused where you are drawing that from. The reason for the "animal cannot actually fly" caveat is that if the animal can fly then it can use a combination of forelimb and hind limb mechanics to reach a high instantaneous speed at the end of a running launch cycle because the lift keeps it from falling over (it's actually propelled and held up by the wings at that point, with the legs just giving a few extra pushes). However, this falls outside the bounds we were discussing, because we assume that the animal is and incipient flyer only (ie. cannot actually launch and sustain flight). --MH



This solution only becomes possible at the point where the animal is actually holding itself up with the forelimbs, and helping to push with the hind limbs, which is launching. --MH

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3). In animals that can ALREADY fly it is called 'launching'. In flight wanna-bes, and mallards w/ clipped wings, it is called "wing-assisted" running.
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In animals that cannot fly it is called impossible. If the animal could launch, then it would be a flyer (even if it didn't stay up very long). In the scenario I mentioned, the animal is opposing its full weight with lift and producing enough thrust to overcome drag: it's a flyer. A mallard with clipped wings might be able to manage something along those lines over water because the inboard wings still produce a lot of lift and the animal can hold itself up enough to run on the water. Short-winged grebes (which are flightless, but only barely) can do this as an escape mechanism. If we want to move the origin of flight to semi-aquatic animals, then we could actually make an argument for maximum speed increases in incipient flyers. --MH


In a sense, but if the animal pushes itself forward with the forelimbs
while the hind limbs are slipping, it will probably fall over.

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4). Why? The thrust foot slips, the front foot is forward, in position to begin the next stride...
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Maybe it'll recover in time; but barely catching yourself on every stride is not a great way of running. Having your feet slip out on you is generally a detriment to a runner; getting pulled forward while slipping is generally worse. Improving traction requires an increase in the y axis vectors; increasing the x axis forces will not improve traction so the animal keeps slipping. --MH


In addition, we are assuming that the upstroke is powerful for the
traction increase model; that's probably not a good assumption for
basal birds. --MH
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5). Huh? Mike, what happened to the claws? I have to remind you, I am really skeptical of the benefits of increasing foot friction through down \ward force vectors unless you are wearing running shoes and are on pavement. I am a big fan of claws, however. Roots and such aren't unusual at all. No need for increased downward force vectors.
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Claws are very helpful indeed, and I haven't forgotten about them. I was discussing a situation wherein the animal is slipping despite its claws. To increase traction for the foot (claws included), the animal would need to increase vertical forces on the foot, and it can do so with an active upstroke, assuming the critter in question can produce an active upstroke with sufficient power. If the claws are maintaining traction already, then the situation is moot. --MH



Cheers,

--Mike H.