# Re: Fw: Dinosaurs and birds

Here we go again . Are you now trying to cast me as arguing that hind limbs are NOT limiting? Obviously they are, w/ out any question, in any terrestrial animal, and I have never stated or implied otherwise. Yet here I am, morphing into a moron who thinks that stride "oscillation and length" increase into infinity....

I have no intention of casting you in any particular way at all. I am merely trying to sort out exactly how you are working out the physics of the problem. In any case, I wasn't sure what you felt that the hind limb limitations really were. I specifically related the limitations to power; am I correct in assuming that you feel the same? --MH

You contend that velocity at the hind limb fail point minus velocity at max running speed = zero, which strikes me as an overly dramatic and non-sensical statement, on both on the dreaded "intuitive" and the more esteemed non-intuitive levels. Safety factors are built into every aspect of locomotion, for one thing.

That depends on what you mean by "fail point". There certainly are safety factors built in, from a structural perspective. On the other hand, there don't appear to be hard fail safe mechanisms to keep an animal from exerting its maximum power. I am assuming that a running animal is using its maximum power at maximum running speed. Do you know of any data to suggest otherwise? --MH

There is a downhill grade such that a sprinter's maximum speed is greater than on flat ground, for another. My guess would be about > 5%...

Well, that's easy enough to test. I rather doubt it, but I'd be fine being proven wrong. We can get a large sample of level sprints and downhill sprints and see if any consistent difference exists. --MH

I say and have said that velocity at the hind limb fail point minus velocity at max running speed is not zero, even in _large bipeds_ like humans, and that "close" may convey advantage. Sheesh. --Don

Again, easy enough to test, if you have a firm definition of "fail point". I'd imagine the the fail point in this context is maximum available power (though "fail point" normally suggests structural failure). --MH

Who said anything about "cruising"? MAXIMUM SPEED is what my comments apply to, have applied to, and will CONTINUE TO APPLY TO. What terrestrial animal maintains maximum speed for more than a few > seconds?

Fair enough; do we see any cursorial birds flap at maximum running speed? I cannot think of any examples.
--MH

1). When volants maximize output they FLY, Mike. They can't help it.

Not true. They can flap without launching if desired. If they maximize flapping output, with a lift-producing angle of attack, and a partially or fully minimized upstroke, then yes they would start to launch. That doesn't mean that a volant bird cannot add thrust to running kinematics without launching. If a volant bird is running at top speed, and then adds aerodynamic force then it will not automatically take off. I do, of course, assert that the bird will tend to fall over (if it isn't launching), which is presumably why they don't try it. --MH

The non-birds that populate the ground-up scenario have a top gear, but can't fly yet. 2). It's a completely different evolutionary path and dynamic, especially from the perspective of the competitive cohort, and beginning anatomy. This applies to your comment 3, below, including the neo-flightless. --Don

Can you be specific about how the dynamic differs, and how the anatomy differs in such a way as to change the mechanics of the scenario? In particular, I am wondering why the neo-flightless situation is so different. Is there a particular reason that something like a wood rail cannot derive the running benefits that you suggest for an incipient flyer (and thus has weaker forelimbs)? --MH

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Oh-ho! so you finally tried it? About time...--Don       }:D
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Yeah, I was "inspired", I suppose. Had a spare moment with a friend. --MH

Stresses equal... Interesting idea. In total, probably, is my uneducated guess. Not so power mass, which controls speed, does it not? Back stride: action/reaction. Front stride: rotation at hip...

By power mass, do you mean hind limb muscle mass? --MH

Hind limbs definitely limiting, at some point. I say-- velocity at the hind limb fail point minus velocity at max running speed is not zero. --Don

Again, we need to come to a consensus on what this "fail point" is. To the best of my knowledge, it is maximum power output. To put it another way, is there a reason that you think the hind limbs cannot exert maximum power at maximum speed? Is there something that limits their ability to exert available power that is somehow lifted by having forelimb-imparted thrust? --MH

We also need to keep the footfall time in mind: if our near-bird
increases its travel speed during aerial phase with the forelimbs, then
it will be going beyond the velocity sustainable by the hind limbs when
it makes footfall at the end of that aerial phase.

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You say "beyond velocity sustainable." ??? That is the _question_. Again! Wow, man.
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Not from my perspective, though perhaps there is a misunderstanding. I felt that the question was "can forelimb imparted aerodynamic thrust increase maximum speed, even if the hind limbs are already acting at maximum power?". If the relationship between maximum power usage and maximum speed is in question, then we have a new nut to crack: Do running birds use maximum available hind limb power when at maximum running speed? I have been assuming that they do, which I mentioned in my last couple of posts. Have you seen any data to suggest otherwise? --MH

It's not a law, but it is a result of laws (physical ones), and thus is
going to be a pretty firm rule most of the time. Reduced foot force
against the ground can be advantageous, but not for running speed. --MH

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Depends on substrate.

How does that depend on the substrate? --MH

Cheers,

--Mike H.