Dear all,

Disappointing response to the bird leg quiz. Either the solution is
obvious and I can't see it, or it has completely foxed you all. So let
me throw one possibility into the ring to stimulate discussion.

I wonder whether the reverse flexing joint might be for stability during
landing. My reasoning goes as follows. Our legs are most stable when the
ground reaction vector passes anterior to the knee joint, locking the
joint against the posterior capsule ligaments. It is for this reason
that we have an "L"-shaped foot (rather than "T"-shaped), since the
center of pressure is normally maintained anterior to the ankle.

I hypothesise that a bird's most unstable time is when it is landing,
because there will be a sudden and large force on the undercarriage
(legs). Since birds generally fly forwards, this force will have a large
posterior component, which will tend to flex the leg (ankle). I contend
that a bird evolution has reversed the leg so that it is most stable
when the ground reaction is posterior, so increasing stability for

Am I completely away with the birds?

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

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