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STATIC & ROTATIONAL EQUIBRIUM
Dr. John P. Cise, Professor of Physics, Austin Co=
m.
College, 1212 Rio Grande St.,Austin
Tx 78701 =
jpcise@austincc.edu
& New York Times Dec. 13,2007 John Schwartz
Spines, Made Extra Curvy for Women=
Pregnant
women do not tip over, and the reason has a lot to do with an evolutionary
curve, researchers say.
Skip to next paragraph =
p>
Question: From the third female vertebrae to the=
axis
of rotation(shown as center of gravity in above graphic)the distance is 0.4
feet. This is the same distance before pregnancy(graphic on left) and after
pregnancy(graphic on right). In the none pregnant situation(left graphic) t=
he
force the vertebrae exerts is 70 pounds. In the pregnant situation the forc=
e is
still 70 pounds but at an angle 25 degrees from the none-pregnant case. If
while pregnant her belly moves
out(to the right in above right graphic) 4 inches, find the additional weig=
ht
she can hold with the 70 lb. force the vertebrae is providing at the 25 deg=
ree
angle more than the none pregnant line of action? Ans: 35 lb.
Anthropologists st=
udying
the human spine have found that women’s lower vertebrae evolved in wa=
ys
that reduce back pressure during pregnancy, when the mass of the abdomen gr=
ows
by nearly a third and the center of mass shifts forward considerably. Even
without the benefit of advanced study in biomechanics, women tend to deal w=
ith
the shift — and avoid tumbling over like a bowling pin — by lea=
ning
back. But the solution to one problem creates another, since leaning puts e=
ven
more pressure on the spine and muscles.
Anthropologists ha=
ve long
known that the lower spine in humans developed a uniq=
ue
forward curve to help compensate for the strains that arose =
when
the primate ancestors began walking upright. Researchers looked for a mecha=
nism
that compensated for pregnancy’s additional burden as well.
What they found, s=
aid
Katherine K. Whitcome, a post-doctoral fellow at Harvard and the lead autho=
r of
the paper, was evidence that evolution had produced a
stronger and more flexible lower spine for women.
After studying 19 =
pregnant
subjects, Dr. Whitcome found that =
the lumbar, or lower back,=
curve
in women extends across three vertebrae, as opposed to just two in men. And=
the
connecting points between vertebrae are relatively larger in women and shap=
ed
differently in ways that make the stack more stable and less prone to shift=
ing
or breaking. Since the engine of evolution runs on the passa=
ge
of genes from one generation to the next, pregnancy is a critical moment.
Without that adaptation, Dr. Whitcome said, females would have been in
considerably greater pain during pregnancy and might not have been able to
forage effectively or escape predators, ending the pregnancy and the genetic
line as well. Working at the University of Texas with Dr. Liza Shapiro, an
associate professor of anthropology, Dr. Whitcome found that the differences
between male and female spines do not show up in chimpanzees. That suggested
that the changes occurred in response to the pressures of walking upright. =
Of the two samples, she fou=
nd the
three-vertebra arrangement in one sample and not in the other. Separate
evidence suggested that the extra-curvy spine belonged to a female and the
other to a male. “It was very exciting” to have the fossilized
puzzle fall into place, Dr. Whitcome said.
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