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mdlussanet56
05-22-2001, 06:16 AM
Dear all,

Indeed, I must say that this third question is one that I have very
often broken thoughts about, and I was in fact still wandering about
it.

Certainly the stability hypothesis does not seem attractive at all I
must say. The reson for example why our centre of pressure is
maintained before our ankle is because it is an "L" shaped joint. Not
the other way around! (Had it been "T" shaped, we would keep a
different centre of pressure).

But let me try:

If, for simplicity we define the "knee" and "elbow" joint as the one
between the two longest limb segments, then the distinction does not
seem to be birds versus mammals, because according to that
definition, most mammals (horses, cat mice kangaroos, mention it)
like birds do have a backward "knee" in the hind leg. In fact many
animals have a forward "elbow" in their fore limb. Humans seem to be
in the "odd" group rather than belonging to the regular ones.

The hindlegs' most important and general function is forward
propulsion of the animal. The problem of having a "forward knee" in
the hindleg is that it has to extend infinitely fast just before
take-off to keep ground contact let alone to keep on accelerating the
animal. This problem is not present in a backward "knee".
Why then do some silly human-like creatures have such a "wrong"
forward knee? Maybe just a case of unfinished evolution? Fact is that
we are not terribly good at running, whereas long-legged but
short-feeted birds, such as ostriches, do not seem to have a problem
with balance

Marc de Lussanet

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