What is it that makes Michael Johnson, world's fastest human, swifter than a typical man or woman plucked at random from the street? Isn't it absurd even to compare them? Actually, in some respects, no, according to a study by Deborah Sternlight '98, HMS '04, Harvard research physiologist Peter Weyand, and others published recently in the Journal of Applied Physiology. In the course of analyzing human runners of varying abilities at top speed, they made a startling discovery. The very fastest sprinters reposition their legs in the air from back to front during a stride no faster than the slowest runners. "There was only a 360- to 400-millisecond difference among subjects," Sternlight says. Yet in timed sprints, the fastest runners covered almost twice as much distance.
Sternlight and Weyand made the discovery while working at Harvard's Concord Field Station, where the National Institutes of Health has funded studies of animal locomotion for years and Weyand has "run" everything from emus and prong-horned antelope to wallabies and kangaroos on treadmills. ("Getting kicked by a wallaby isn't too bad," he says, "but the kangaroos really pack a wallop.") In fact, Sternlight's decision to study human beings for her senior thesis (partially funded by a Harvard College grant) was largely based on Weyand's advice that, given the time constraints on an undergraduate researcher, "the most compliant subjects are human."
Using a treadmill connected to a computer and equipped with a force plate under the moving rubber mat, the researchers were able to measure variables like foot-to-ground contact time, total stride time, and the total pressure exerted by a runner during a foot strike. In order for Sternlight to get accurate readings, her subjects had to maintain their position over the force plate to within 10 centimeters for at least eight strides. The investigators gradually increased the speed until runners reached a "failure rate": the speed at which they could no longer maintain their position on the treadmill.
It turns out that what separates the swift from the slow is the amount of vertical force applied to the ground with each stride. The speedy are also strong, and hitting the ground harder allows them to increase both stride length and frequency, the two variables that, multiplied together, determine speed.
Here's how it works. Stride length measures one footfall to the next, and runners who push harder against the ground at any given speed spend more time in the air--and therefore travel farther during each stride. Hence, high-speed running is actually a form of bounding, a series of one-footed jumps. Those who bounce higher and longer go faster.
Runners can up the other variable, stride frequency, by completing each stride in less time. To measure this, Sternlight broke total stride time down into two components: the time it takes a runner to reposition limbs in the air--called "swing time"--and the time each foot spends in contact with the ground. As already noted, Sternlight found that swing time at top speed is virtually the same for everyone. "There seems to be an intrinsic limit to how fast the limb can move at its maximum velocity," says Weyand. What allows the fastest runners to achieve higher stride frequencies, therefore, are "wingéd feet" that actually spend less time on the ground.
Using measurements from their treadmill's force plate, the researchers found that as the amount of force applied to the ground increases, ground-contact time diminishes. "It is not intuitive," says Weyand, "but basically, pressing harder shortens the ground-contact time"-- thereby increasing stride frequency.
Is this research likely to change the way coaches train their sprinters? Probably not, says Paul Turner, Harvard's assistant track and field coach, who holds a doctorate in human performance and a master's in kinesiology. "This work quantifies and reinforces what coaches, whether through experience or intuition, already know." Indeed, as head track and field coach Frank Haggerty notes, research from the same lab helped in designing Harvard's indoor running track, which produces high-energy return that minimizes foot- contact time.
Interestingly, Sternlight found that sprinters apply greater force to the ground at all speeds, and therefore don't need to hit the minimum "swing time" except when competing against top rivals. If you've ever watched the spring-like steps of a top sprinter like Michael Johnson as he walks around during a meet, you may have attributed the extra bounce to a healthy dose of ego. Instead that may simply have been high ground forces in action. "Sprinters can be very casual about repositioning their limbs because they are slapping such high forces down on the ground," says Weyand. "That's why running fast is easy for them, and even looks casual."
