Rice researcher finds cause of limited animal performance

Rice researcher finds cause of limited animal performance

BY JENNIFER EVANS
Rice News staff

When a racecar, a thoroughbred and a sprinter shoot out of the gate, each can hit top speed in a matter of seconds. As time stretches to minutes, the man-made engine can easily sustain its max, but the engines powered by muscle quickly peter out.

Have the biological engines run out of gas?

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Not according to a study by Rice University researchers, who found that animal performance is limited not by an exhausted fuel supply but rather by exhausted muscle.

“The maximum speeds of runners, cyclists, racing horses, even dogs, all drop off sharply over time, not because their muscles lack fuel for contraction but simply because they become fatigued,” said Peter Weyand, assistant professor in kinesiology and lead author of the study.

“When nature’s most powerful engines go full throttle, they have plenty of gas, but they lose horsepower with every working stroke.”

These findings present a significant challenge to existing views and are published in the March issue of the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology in a paper written by Weyand, Jennifer Lin and Matthew Bundle of Rice University and the Texas Medical Center’s Center for Human Performance.

Scientists have long believed that muscular performance is limited by the availability of the chemical energy that fuels muscular contractions.

A burst effort, such as a sprint, is fueled rapidly by the anaerobic energy reserves immediately available within the muscles doing the work. But these reserves are limited and quickly depleted, so they can support only brief bursts of extreme power and speed. An endurance effort, such as a marathon, is fueled more slowly. It depends on the rates at which oxygen can be delivered to the working muscles to support aerobic metabolism. The metabolic power available for prolonged efforts is much less, and sustainable speeds are considerably slower than burst-effort speeds.

In their study comparing sprint running and sprint cycling, the researchers found that performance drops off twice as rapidly during cycling than running. They also found that the maximum rates of fuel use during brief bursts are roughly two times greater for sprint cycling than sprint running.

They offered a simple explanation for both the two-fold differences observed: “The muscles of cyclists contract for periods that are relatively twice as long,” Weyand said. “During cycling, the muscles of each leg apply appreciable forces to the pedals only during the down stroke, and therefore, for 50 percent of the total revolution time. During high-speed running, the muscles of each leg apply force to the ground for only 25 percent of the total stride time. Thus, both performance drop-offs and the rates of fuel utilization are set by the length of time that the muscles are actively contracting in the respective exercises and not by the rate limits of the chemical reactions that provide the fuel.”

These findings complement what scientists studying muscle fibers and cells have noted for some time: Furiously contracting fibers do not run out of chemical energy. Rather, muscles develop progressively less force with successive contractions when they rely on anaerobic metabolism for chemical energy.

“These new results indicate that the biological equipment provided by evolution involves distinct trade-offs in specialization for speed and endurance,” Weyand said. “The horsepower that can be conferred by some of nature’s fastest muscle motors does allow some animals to reach extreme speeds of 50, 60 and even 70 miles per hour. However, because natural engines are not like car engines, these speeds are not for long.”