Jamaican sprinter Usain Bolt secured his claim as the world's fastest human in August when he ran 100 meters in 9.58 seconds, reaching a top speed of nearly 28 miles per hour. One day, no doubt, someone will sprint faster still. Perhaps by then, scientists may better understand why all speed records made have eventually been broken.
Statisticians have long tried to calculate the upper limits of human speed. One recent estimate, published last year in the Journal of Experimental Biology, put the quickest possible time for 100 meters at 9.48 seconds. That prediction was based largely on past performance and the pace at which current records are falling. But while statistical exercises provide fodder for speculation, no one really knows the limit of human speed--both because scientists still can't fully explain the blend of biology and physics that separates athletes like Bolt from the rest of the world, and because unforeseen technologies can push athletic achievement beyond the merely human.
"The more you understand biomechanics, and the more technologically advanced you become, the more you become capable of intervening," says physiologist and biomechanics expert Peter Weyand of Southern Methodist University in Dallas. Those interventions have become both hailed and dreaded, as they often end up casting a shadow over organized sports. This summer, when little-known German swimmer Paul Biedermann beat Olympic champion Michael Phelps in the 200-meter freestyle, Biedermann seemed unsure whether to credit his swimming or his newfangled polyurethane swimwear: "I hope there will be a time when I can beat Michael Phelps without the suit," Biedermann told sportswriters, some of whom dubbed the new swimsuits "doping by wardrobe."
Technological innovations that confer a competitive edge have paralleled advances in understanding the physiology of human athletic performance, says Rick Neptune, a mechanical engineer at the University of Texas at Austin. "When they intersect, you start to see world records get broken," he says. "We can't say in the future which will matter more, as the rules of competition adjust." In the current issue of Annual Review of Biomedical Engineering, Neptune chronicles how improvements in equipment design have a history of pushing racing past its natural boundaries.
"It's not clear where that boundary is until you've crossed it," he says. For example, in 1997 he witnessed one of the first international speed skating competitions with widespread use of klapskates, which reduce friction and maximize muscle force by allowing the boot of the skate to pivot away from the blade. At a single World Cup competition in Calgary, Canada, he watched 14 world records devoured, one heat after another--all owing to the new skates. The International Skating Union ultimately allowed klapskates to remain, saying they had revolutionized the sport and were widely available to any competitor.
Future conflicts might be avoided as scientists better define the basis for human ability. "It's surprising how little we understand when it comes to tying performance to our physiology and anatomy," says evolutionary biologist Thomas Roberts of Brown University in Providence, R.I. "We don't completely understand the basis for top speed."
Certainly, each separate component of movement has been well studied. Scientists know, for example, that muscle fibers produce force by lengthening and contracting. These fibers come in two basic types, fast-twitch and slow-twitch. Fast-twitch muscles are thick and mighty, producing greater power with each contraction, but they sacrifice endurance for strength. Slow-twitch muscles cannot produce as much power, but they are loaded with mitochondria (the energy factories of a cell) and do not easily tire.
"We know a lot about how muscles work," Roberts says. "I can predict the mechanical output for a single muscle. …