It's not a secret to you if you have watched football for the last 40 years;  a guy once almost big enough to be a linebacker can't even be a safety today.   Elite athletes are getting bigger.

Specifically, while the average human has gained about 1.9 inches in
height since 1900, new research showed that the fastest swimmers
have grown 4.5 inches and the swiftest runners have grown 6.4 inches.

In a new analysis, Jordan Charles, an engineering student who
graduated this spring, collected the heights and weights of the fastest
swimmers (100 meters) and sprinters (100 meters) for world record
winners since 1900. He then correlated the size growth of these
athletes with their winning times.

"The trends revealed by our analysis suggest that speed records will
continue to be dominated by heavier and taller athletes," said Charles,
who worked with senior author Adrian Bejan, engineering professor who
came up with the constructal theory 13 years ago. The results of their
analysis were published online in the Journal of Experimental Biology.
"We believe that this is due to the constructal rules of animal
locomotion and not the contemporary increase in the average size of
humans."

The theoretical rules of animal locomotion generally state that
larger animals should move faster than smaller animals. In his
contructal theory, Bejan linked all three forms of animal locomotion --
running, swimming and flying. Bejan argues that the three forms of
locomotion involve two basic forces: lifting weight vertically and
overcoming drag horizontally. Therefore, they can be described by the
same mathematical formulas.

Using these insights, the researchers can predict running speeds
during the Greek or Roman empires, for example. In those days,
obviously, time was not kept.

"In antiquity, body weights were roughly 70 percent less than they
are today," Charles said. "Using our theory, a 100-meter dash that is
won in 13 seconds would have taken about 14 seconds back then."

Charles, a varsity breaststroke swimmer during his time at Duke,
said this new way of looking at locomotion and size validates a
particular practice in swim training, though for a different reason.
Swimmers are urged by their coaches to raise their body as far as they
can out of the water with each stroke as a means of increasing their
speed.

"It was thought that the swimmer would experience less friction drag
in the air than in the water," Charles said. "However, when the body is
higher above the water, it falls faster and more forward when it hits
the water. The larger wave that occurs is faster and propels the body
forward. A larger swimmer would get a heightened effect. Right advice,
wrong reason."

In an almost whimsical corollary, the authors suggest that if
athletes of all sizes are to compete in these kinds of events, weight
classes might be needed.

"In the future, the fastest athletes can be predicted to be heavier
and taller," Bejan said. "If the winners' podium is to include athletes
of all sizes, then speed competitions might have to be divided into
weight categories. Larger athletes lift, push and punch harder than
smaller athletes, and this led to the establishment of weight classes
in certain sports, like boxing, wrestling or weight-lifting.