Speed and Endurance Are Doled Out By the Pound

The conspicuous size difference between lithesome distance runners and more muscular sprinters is dictated by simple rules of form and function, according to researchers from Rice and the Texas Medical Center’s National Center for Human Performance. Specifically, the greater bulk of sprinters is explained by their need to hit the running surface harder to attain faster speeds.

“We found that, regardless of the runner’s race specialization or gender, we could link an ideal body mass for running performance to how hard a runner needed to hit the ground,” explains Peter Weyand, assistant professor of kinesiology and lead author of the study. “The mechanical requirements of running and racing at different speeds are related to the notable differences in body types long observed among specialized track athletes—and even among animal runners in nature.”

Previously, scientists and others considered massiveness in any form to be disadvantageous for running performance. This idea was based on studies of distance runners and on the limited running abilities of elephants and big dinosaurs. However, Weyand and recent Rice graduate Adam Davis found the trade-offs involved in specialization for speed versus endurance conform to precise body-mass rules.

Their research provides a new twist on the body mass index, or BMI, which measures body size based on a person’s height and weight. BMI has long been used as a guide on body sizes to be avoided. However, Weyand and Davis unexpectedly discovered that this same index can guide some athletes toward body sizes that will optimize their performance.

In the study, Weyand and Davis found that the ideal massiveness for running performance is provided by a constant relationship between the BMI and the force runners need to apply to the ground at their racing speeds. A practical implication of this finding for runners is that, for the amount of ground force required for any race, an ideal BMI exists.

To establish how much ground force was required for different running and racing speeds, the researchers measured how hard runners hit the surface of a treadmill at different running speeds. Men and women alike hit with forces of about one-and-a-half times their body weight at slower speeds and with as much as two-and-a-half times their body weight at a sprint.

The pair established an ideal body size for each race distance by compiling the average heights and weights of the 45 fastest male and female runners in each of the eight competitive racing distances, from 100 meters to 10,000 meters, during the past 14 years.

As they related the force required for the runners’ racing speeds to the ideal size of the different runners, the researchers found a consistent relationship across all groups. Whether an athlete is male or female or a sprinter, a middle-distance, or an endurance runner, the ideal massiveness for running was the same function of how much force the runners needed to apply to the ground at their racing speeds. The specific amount of body mass needed to regulate ground forces and racing speeds is relatively small: only 2.5 kg of mass per one meter per second of racing speed for males and 1.8 kg per one meter per second for females.

“The results provided powerful support for a basic conclusion,” Weyand notes. “Sprinters have more muscle and body bulk because they need to hit the track harder to attain their blazing speeds. In contrast, endurance specialists do not want or need bulky muscles because the ground forces required at their slower speeds are so modest.”

Weyand says further testing is needed to determine how appropriate BMI guidelines might be for individuals or any given athlete. Details of the researchers’ findings appeared in the July 15, 2005, issue of the Journal of Experimental Biology.