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Greiner, Thomas M. Ph.d.
05-21-2001, 02:12 AM
Ho boy, another classical problem.

Yes, the fact the "joint" is the ankle in birds and the knee in humans does
answer the question. In fact, the knee and ankle both bend in the same
direction in both animals. Again, we need to re-phrase the question. Why
does the bird have such a long foot, and why does it walk with such a small
portion of the foot in contact with the ground? (I should point out that
from, a comparative perspective, the human plantigrade posture is the more
unusual adaptation. I should also point out that since both birds and
mammals (humans) evolved independently from a reptilian ancestor the
orientation of their knees and ankles is an example of convergent evolution
-- ie., both evolutionary lineages came up with the same biomechanical
solution).

My solution: the foot (the bony link between the ankle and the toes) is the
primary locomotor lever. Animals with long feet, relative to the rest of the
limb, are adapted more for speed than for power. Thus, a bird runs or hops
(a hop may be useful in providing the initial rapid airborne thrust,
although I am just guessing here since I don't know a lot about birds) while
a human strides slowly, but with greater bipedal stability. Humans with
"short" feet are evolutionary hold overs from the necessity of powerful
locomotion in their arboreal (tree climbing) ancestors.

Thomas M. Greiner, Ph.D.
Assistant Professor
Department of Anatomy
New York Chiropractic College
Seneca Falls, NY 13148-0800 USA

Phone: (315) 568-3183
Fax: (315) 568-3017
Email: tgreiner@nycc.edu

> -----Original Message-----
> From: Dr. Chris Kirtley [SMTP:kirtley@CUA.EDU]
> Sent: Monday, May 21, 2001 11:12 AM
> To: BIOMCH-L@NIC.SURFNET.NL
> Subject: Summer science quiz #3
>
> Dear all,
>
> I'm glad to see that the feedback to these quizes remains encouraging,
> so I'll keep going until you beg me to stop!
>
> This week's question is another Aristotelian one, and also from his "On
> the Gait of Animals". I will give to you straight (via Farquharson's
> translation):
>
> "why do man and bird, though both bipeds, have an opposite curvature of
> the legs. For man bends his legs convexly, a bird has his bent
> concavely"
>
> Now, before an early bird chirps up, I will note immediately that the
> joint in question is the knee in man but the ankle in birds. However, I
> don't think that this fact alone answers the question ... does it?
>
> I look forward to your musings.
>
> Chris
>
> --
> Dr. Chris Kirtley MD PhD
> Associate Professor
> HomeCare Technologies for the 21st Century (Whitaker Foundation)
> NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
> Dept. of Biomedical Engineering, Pangborn 105B
> Catholic University of America
> 620 Michigan Ave NE
> Washington, DC 20064
> Tel. 202-319-6247, fax 202-319-4287
> Email: kirtley@cua.edu
> http://engineering.cua.edu/biomedical
>
> Clinical Gait Analysis: http://guardian.curtin.edu.au/cga
> Send subscribe/unsubscribe to listproc@info.curtin.edu.au
>
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