But what about shorter-distance runners, who exert themselves more at the start of a race?

New research suggests that they slow down, too, but in a distinctly different way.

In today's issue of Nature, American and Italian researchers report that, in running events of more than 1,000 meters, the average speed of world-class athletes falls gently over the course of the race. In shorter races, however, the drop in speed is much sharper.

And the phenomenon may not be confined to the track. In the swimming pool, the same patterns occur at around the 400-meter mark, the study claims.

While the findings deal with human physiology, the lead author of the study is a professional astronomer at the Space Telescope Science Institute in Baltimore. A few years ago, while working at the European Southern Observatory in Munich, Germany, Sandra Savaglio joined a swim team training for an amateur competition. To monitor their progress, team members timed their performances, and Savaglio compared their results to world records.

Immediately, she says, she noticed the pattern of deceleration among world-class swimmers. She turned her attention to track records and found the same trends. Together with physicist Vincenzo Carbone at the University of Calabria in Rende, Italy, Savaglio analyzed the data.

Both swimmers and runners switch from anaerobic metabolism to aerobic metabolism -- and begin to slow down -- after about 150 to 170 seconds, she says.

"When you run or swim short distances, you're basically using your anaerobic metabolism," says Savaglio. "You don't breathe too much. You just use your energy at the maximum. If you want to run or swim longer distances, then you need energy from oxygen." This requires breathing, and doesn't produce the same speeds as anaerobic metabolism, she says.

Men and women showed the same pattern of decline in their average speeds, she says, debunking the belief some have that women make better long-distance runners than men.

"It's very curious to see how universal this behavior is," Savaglio says. "It looks to be a characteristic of the human body and not something related to the sport that you're performing or your sex."

But George Brooks, a professor of integrative biology at the University of California, Berkeley, says there's nothing new about these findings. In fact, says Brooks, who has studied this field for the last 30 years, "It's really surprising that this got published because people have been making these plots for years."

Brooks also believes that Savaglio's conclusions about swimming are flawed because of how swimming has changed over time.

"This stuff on swimming is all misconstrued," he says. "It reflects the changes in technique and opportunity for women to train. The current women swimmers are faster than many or most Olympic champions of a generation ago."

What To Do

To read about the benefits of aerobic exercise, check out this earlier HealthSCOUT article that describes how exercise helps the aging brain.

For some examples of anaerobic and aerobic exercise, visit the Web site of the CareGroup Healthcare System. If you feel up to a fairly technical explanation of anaerobic vs. aerobic metabolism, try the Web site of the Muscle Physiology Lab at the University of California, San Diego.