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.