Student Solves Mystery Of What Limits Running Speed

By William J. Cromie

Gazette Staff

A 21-year-old Harvard student has found an answer to a question that has puzzled humans for centuries -- what determines how fast we can run.

"I couldn't believe it," said Deborah Sternlight, a senior from Los Angeles. "People have been thinking about running at least since the Panhellenic Games in the sixth century B.C.; even 17,000 years ago humans painted pictures of running animals on cave walls. Yet no one had answered the question of what limits an individual's running speed."

A biology student, Sternlight was fascinated by what she calls "the explosive speed" of Olympian record-breakers Donovan Bailey and Ben Johnson. "It blew me away. I was enthralled with how fast they ran, but I was even more intrigued by what factors limit the capability to run like that."

While studying animal locomotions at Harvard's Concord Field Station in Bedford, Mass., Sternlight got the opportunity to find out.

"I kept asking my instructor, Peter Weyand, are you sure no one has solved the puzzle of what limits the speed of ordinary and Olympian runners," Sternlight recalled.

"I told her that despite a great deal of interest, speculation, and experimentation, no one had come up with a definite answer," Weyand said.

With the help of Weyand and research assistant Seth Wright, together with a small grant of $600 from Harvard, Sternlight decided to try to resolve the matter.

A Swinging Time

Reading and talking to experts revealed that differences in muscle power, the most logical answer, was probably wrong.

"When you see someone running at top speed, his or her legs and arms are swinging all over the place," Weyand explained. "There is just not enough active muscle power available to account for all the motion you see taking place."

"The minimum power needed to swing your legs at this rate is three times greater than the peak aerobic power of a champion endurance athlete," Sternlight adds. "The amount of muscle in the legs directly available to perform this work isn't sufficient. There's an enormous mismatch between what you see and what you can get from the active muscle power."

Working through e-mail and the Internet, Sternlight recruited 33 volunteers, aged 18-37 years. Fifteen subjects were competitive runners; there were 24 men and 9 women. She had all 33 run at top speed on a level treadmill, and five run at treadmill inclines and declines of 9 degrees and minus 6 degrees.

Sternlight guessed that an upper limit on the frequency of stride might restrict a person's running speed. She measured stride frequency and length, the amount of time a runner's foot is in contact with the ground, and the time each foot is in the air. The latter is called "swing time."

To Sternlight's amazement, whether people ran fast or slow, or whether they ran uphill or downhill, everyone had approximately the same swing time at top speed. Those running 14 miles an hour and those running 27 miles an hour both took between 0.37 and 0.40 second to swing one leg in front of the other.

"What limits top speed, then, is the minimum time you take to swing your leg into position for the next step," Sternlight concludes. "That's evidently a fundamental limit for all humans. What determines how fast you can run is how fast you're going when you reach that limit."

Slower runners in her test group hit the limit at about 14 miles an hour, intermediates at about 19 mph, and the fastest sprinters at about 25 mph.

Sternlight obtained videotapes of runners at the 1996 Olympics in Atlanta from NBC Sports. She measured the swing times of the best sprinters in the 100-meter dash, including the world-record run of Donovan Bailey, who finished in 9.84 seconds. Remarkably, his minimum swing time turned out to be virtually the same as the slowest of Sternlight's 33 runners.

Another way of putting it is that the more a runner throws his or her body in the air, the faster he or she will go.

"To run faster, you have to throw your center of mass higher into the air," explains Sternlight. "That makes your swing time longer at any given speed, so that you will be moving faster when you reach the limit."

These results support the conclusion that running involves little active muscle power. "Much of the work of running is done through passive mechanical processes, in which tendons and muscles act though elastic rebound, much like springs uncoiling," Sternlight comments. "The uncoiling delivers the power to swing your legs."

Comparisons with Animals

Sternlight devoted most of the fall and spring terms to this research, spending more time on it than on her other classes. The project evolved into an undergraduate thesis titled "The Secret of Running Speed: Are Sprinters the Sultans of Swing?"

"The work was well worth it," Weyand comments. "We're extremely excited about the outcome, and will try to get the results published in a scientific journal."

Not only do the findings advance our understanding of human movement, but "they should apply generally to all four-legged land animals," says Weyand. "Minimum swing time may vary with size and anatomy from species to species, but the basic principles should be the same."

It is much more difficult to get animals to run on a treadmill than humans, and, if you do it, there's no way to know if they're running at top speed. So, in this case, humans became the experimental animals who taught researchers about locomotion.

Weyand believes that Sternlight's work could lead to some practical applications in training competitive athletes, building less clumsy robots, and aiding the rehabilitation of those who suffer spinal cord or leg injuries. "By learning that certain muscles don't act the way we thought they did," he notes, "we get a better idea of how to improve the performance of injured people, robots, and jocks."

Sternlight thinks the new knowledge might find use in the design of artificial limbs, particularly for victims of injury who want to be athletes. She may follow through on this idea with a job in industry or research. She has already been offered a job with a company in Boston that specializes in marketing new medical technologies and devices.

"I'm fascinated with how new medical technologies are made available to consumers," Sternlight says. "If I don't take the job, I'd like to work in a research lab or with patients in one of the large teaching hospitals in Boston. My long-term goal, however, is to go to medical school and become a physician."