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Re: Fw: Dinosaurs and birds
Note: I assume "stride rate" = stride length?
----- Original Message ----
From: Michael Habib <email@example.com>
Sent: Wednesday, April 11, 2007 11:35:09 AM
Subject: Re: Fw: Dinosaurs and birds
> 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
Whoops, that should read "stride frequency, stride rate, and force
imparted per stride are all at maximum and cannot rise without loss in
other two". Alternatively, try to show, quantitatively, some way in
which one of those factors could rise without loss in one or both of
the others for an animal that cannot actually fly.
1). Thrust from a source other than the hind legs reduces "the force imparted
per (hind limb) stride" required to maintain a given rate of forward motion.
Therefore, the hind limbs are free in that case to increase stride frequency;
because, as you say, increasing stride frequency requires reducing
"...requirements of imparting force to the substrate."
Further: if "running" in bipedal animals is strictly defined as locomotion in
which stride length (and thus airborne phase length) is determined by thrust
from the hind legs only, and if locomotion in which the resultant stride length
includes the effects of aerodynamic thrust from the fore-limbs is defined as
"flying", then I suppose one can say that increasing running speed w/ fore-limb
assistance is "impossible", because then "they're flying". While such
hair-splitting may be highly appropriate to discussions of locomotive
terminology, it is irrelevant in evaluation of 'assisted running' evolutionary
scenarios, and smacks of simple gamesmanship. Need I actually state that the
derived flight condition requires more than exceeding an arbitrary lift/thrust
requirement? Must I type long words like "control mechanisms" and
"flight-related neurological and behavioral phenotypes"?
Most people consider flying to be a state in which touching the ground is not
necessary to maintain the state, even "... the legs just giving a few extra
pushes." (see a). Whoever invented the term 'assisted running' was evaluating
scenarios wherein lift-production is increased by natural selection to the
point where flight in usual sense becomes possible, and manifestly did not
realize a verbal catch-22 was being created that would doom the scenario to
2). You stated earlier that upward trending fore-limb generated aerodynamic
thrust vectors were counter-productive when running, because, in simplest
terms, the feet slip. When I pointed out that reducing the downward vector on
the feet does not necessarily increase slippage, due to claws and/or specific
substrate conditions, you stated that the case of non-slippage was "moot" (see
b). Logically, it follows that upward trending fore-limb generated aerodynamic
thrust vectors are NOT necessarily counter-productive, and "active upstrokes"
"unlikely to occur" in "basal birds" are not relevant to evaluation of the case
of fore-limb-assisted running, _in the context of potential evolutionary
advantage_. Unfortunately, the context of the dialog had been so altered, your
statement regarding the "moot"-ness of the case of non-slippage could easily
have been misinterpreted as a rebuttal of my argument, or even my understanding
of the physics involved. Would you care to expand on the
case wherein reducing the downward vector on the feet by the generation of
directional aerodynamic thrust does NOT increase slippage of the hind feet?
WITHOUT altering the context? --Don
a). "... it's actually propelled and held up by the wings at that point, with
legs just giving a few extra pushes..." --MH
b). " 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