Breaking the Speed Limit

Studies examine physiology and technology to better foresee the ultimate edge of human performance

Sidebar: Strife in the fast lane – Click to view

BREAKING THE SPEED LIMIT No one has run 100 meters faster than sprinter Usain Bolt, pictured on the track in Berlin in August 2009. But even he has not yet reached the predicted maximum velocity for the human body. Hannibal/EPA/Corbis
PROSTHETICS’ EDGE Peter Weyand has studied whether sprinter Oscar Pistorius’ artificial limbs confer a biomechanical advantage. Rice University

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.” He can even predict the power output for any given weight of muscle. But just as you can identify individual notes and still not read music, you can know the intricacies of each muscle, bone and tendon and not fully comprehend their harmony. For example, Usain Bolt cannot run 28 miles per hour backward, because his ability lies in the mechanics of his whole body, not just the power in his legs.

Born to run

In fact, much about speed defies intuition. In 2000, while at Harvard University, Weyand reported the surprising finding that fast runners don’t win by moving their legs more quickly. He and his colleagues conducted a series of experiments with 33 runners, placing the volunteers on treadmills moving at faster and faster speeds. The treadmill could measure leg movement and also the force at which each foot hit the ground. In the Journal of Applied Physiology, the researchers reported that faster runners’ feet hit the ground harder, and spring up quicker, than slower runners’, giving each thrust of the leg more forward motion.

“What separates you from Usain Bolt is that he hits the ground way harder than you do, with regard to how much he weighs,” Weyand says. He figures a runner like Bolt slams down with roughly 1,000 pounds of force, and he does so for only 0.05 seconds. What’s not known is how Bolt can do that — ask him to lift 1,000 pounds with one leg, and he would surely fail. “We don’t have any idea for sprint-running how these guys are able to hit the ground as hard as they do,” Weyand says. “There’s something about the mechanics of running that amplifies their production of muscular force.”

Fast runners may simply be built for speed. For example, a study published in November in the Journal of Experimental Biology found that sprinters have shorter Achilles tendons and longer toes than nonrunners of similar height. Their muscles may also set them apart. Most people are born with a middle-of-the-road mixture of fast- and slow-twitch muscle fibers. But an athlete may be drawn to a certain sport based on his or her natural ratio of fast- and slow-twitch fibers: Sprinters, who tend to average about 75 to 80 percent fast-twitch muscles, rarely make good marathon runners, and marathoners hardly sprint. Slow-twitch muscles, but not fast-twitch, work largely aerobically, and have a great hunger for oxygen. They sustain mountain climbers and cyclists. Bolt’s sub-10–second, record-breaking bursts are probably an anaerobic feat of fast-twitch muscles.

Sprinters may also use their size to produce the force that pops each foot off the ground in fractions of a second. In 2005, Weyand calculated the body mass index of the world’s 45 top runners in various events between 1990 and 2003, and searched for a relationship between their size and the distances they run. (Distance, not speed alone, affects oxygen need.) Writing in the Journal of Experimental Biology, he reported that an ideal body size exists for each running distance. Being more massive helps generate the forces needed for greater speed, but only up to a certain point. At some threshold, a big runner’s size will work against him.

All of which makes Usain Bolt perplexing to those who study running. At 6’5”, he is larger than sprinters are supposed to be. His size should make it harder for him to accelerate. His longer legs should take more time to reposition. Yet somehow, his large frame supports the fast-twitch physique of a sprinter. He may have been, almost literally, born to run.

The degree to which a person can dramatically change the character of muscles — the balance of fast- to slow-twitch — is controversial. (In other words, an elite sprinter can’t easily switch to endurance running.) However, in 2004, researchers from the Salk Institute for Biological Studies in La Jolla, Calif., created slow-twitch muscles by genetically engineering mice. The scientists manipulated a certain gene that controls musculoskeletal development in embryos, thus creating sedentary mice built like marathon runners. In PLoS Biology, the scientists wrote that the experiment “demonstrates that complex physiologic properties such as fatigue, endurance, and running capacity can be genetically manipulated.” Last year, in Cell, the Salk team described an experimental drug that may have similar ability to spur the development of slow-twitch muscle in adult mice that never hit the exercise wheel.

Steroids bulk up fast-twitch muscles, which is why their use is banned in sports. But is it cheating, Weyand asks rhetorically, “if you’re reprogramming your own DNA?” (SN: 8/2/08, p. 16). The Salk scientists themselves recognized the potential for their experiment to be exploited, reporting their findings to the World Anti-Doping Agency. In the future, however, the line between natural and unnatural might not be so clear.

The gear-head advantage

This may be one of the dangers of learning the secret to speed. Potentially, someone is willing to seize each piece of new information for a competitive advantage. This is nothing new; ancient Greek Olympians swung handheld weights to improve their performance in the long jump. But only in modern athletics has insight into body mechanics dovetailed so spectacularly with innovations in engineering. “A lot of what technology does is to improve the interaction with the environment so you lose less energy,” says Roberts. Klapskates did so famously in the 1990s. While critics at the time chafed, the sport eventually made peace with the new footwear. Today, a competitor is unlikely to win without them.

Controversies don’t always reach such a natural end. The cycling hour record — the distance an athlete can pedal in one hour on a flat track — steadily rose in the 1980s and 1990s as riders began to use new high-tech gear and streamlined riding positions to improve their aerodynamics. In 1999, University of Tennessee researchers used a model that accounted for adjustments in bicycle design, riding position and other modifications. Writing in Medicine and Science in Sports and Exercise, the team reported that about 60 percent of the world records in the previous two decades of cycling were due to better engineering. In 2000, cycling leaders essentially locked the sport in a time machine, declaring that cycling equipment and position had to be similar to designs used to set the hour cycling record in 1972 — an effort, Neptune wrote this summer, “to prevent the hour record from becoming influenced more by technology than by the athletes.” Records set between 1972 and 2000 are still on the books, but in a category called “best hour performance.”

Swimming may now be similarly a victim of its own rocket science. After the 2004 Olympics, Speedo teamed with NASA engineers to design a space-age swimsuit. The result was the LZR Racer, which Michael Phelps unveiled in 2008, just before donning one of the new suits into Olympic stardom. In fact, the Beijing Games, which marked the international debut of the suits, saw nearly every world record taken by swimmers in LZR Racers. The ultra-lightweight material — the suit is about half polyurethane — not only reduces drag, but also compresses the body to keep a swimmer in an optimal position during the race.

The new suit set off an arms race in swimsuit technology, with even faster designs made possible through better body compression and increased buoyancy (from trapped air). Biedermann beat Phelps wearing a suit that was entirely polyurethane. Swimming’s governing body, FINA, has said it plans to ban the suits in 2010, but has not announced what, if anything, will happen to records set during the polyurethane spree.

Is running due for a similar crisis? Mark Denny, the Stanford University statistician who made the 9.48-second 100-meter prediction, doesn’t think so. “Running is about as pure a sport as you can get,” he says, pointing out that women’s records in the 100 meters have largely hovered around 10.6 seconds since the 1980s. (A faster time is on the books, he says, but evidence suggests it was wind-aided.) Wind aside, running is about the body, the shoes and the ground, and so it has largely remained for almost three decades. Tracks changed in the 1970s, when Harvard researchers designed a surface that reduced foot contact time and increased its spring. By 1980, world records had begun to fall on the redesigned tracks. Since then, the sport has not seen equipment changes capable of dramatically lowering finish times.

Roberts also agrees that running is less receptive to technological intervention because humans evolved doing it. “The closer you get to something we’ve done naturally, the less able you are to change it with technology,” he says.

Prosthetics’ edge

That’s true, Weyand says, but he is hes­itant to say that running isn’t open to enhancements in engineering. He recent­ly helped evaluate one of the most profound developments: artificial limbs used by South African runner Oscar Pistorius, who had both legs amputated as a child. The International Association of Athletics Federations had barred Pistorius from competing against able-bodied runners, saying his J-shaped carbon-fiber prosthetics give him an advantage. (He was ultimately allowed to compete last year, following a legal appeal that reversed the ruling.) In September, in the Journal of Applied Physiology, Weyand and colleagues published an analysis of Pistorius’ running ability, reporting that his mechanics differed from human legs. Weyand’s team reported that Pistorius hits with less force and spends longer with each “foot” on the ground than runners with intact legs. The paper did not directly assess performance advantages.

But in an article in press in the same journal, Weyand and Matthew Bundle of the University of Wyoming release their conclusion: Pistorius has an edge over other runners. He can reposition his lightweight legs more rapidly than any sprinter ever measured, including Usain Bolt. In addition, Pistorius doesn’t have to push as hard to produce the same force, much like a bicycle rider can switch to a lower gear and pedal less without losing speed. Other members of the investigation team, however, maintain that Pistorius does not gain an advantage from his artificial limbs. This means, through Pistorius, that scientists are now for the first time faced with the question of defining that which makes running human. In other words, the limit to speed may not lie in the body itself, but in how far we allow technology to take us.

Laura Beil is a freelance science writer in Cedar Hill, Texas.

Strife in the fast lane: 50 years of world records
Technology can help athletes push limits of speed, but rarely without controversy

Speed skating Finishing times in the 1,500-meter race were falling even before 1997, when klapskates (pictured, 1998) entered the sport. The skates allow the blade to remain longer on the ice, giving a skater more power with each push. Over the objection of some sports purists, skating authorities allowed the skates to remain in use.

Swimming The use of body-compressing, drag-minimizing LZR swimsuits (modeled in early 2008 by U.S. swimmers Michael Phelps and Natalie Coughlin) set off a world record feeding frenzy. Even faster, all-polyurethane suits dominated the 2009 world championships, where 43 world records fell. Polyurethane suits will be banned from competition in 2010.

Cycling The top speed of cyclists, measured by the hour record, shifted into high gear after changes in bicycle design and position. The banned Obree position, named for Graeme Obree (shown in 1996 in the superman pose, also now banned) involves a deep, aero‑dynamic tuck. One study largely credits better design for records set from 1967 to 1996.

Source: All Graphs from Neptune et al./Ann. Rev. biomed. eng. 2009

About Laura Beil

Laura Beil is a contributing correspondent. Based outside Dallas, Beil specializes in reporting on medicine, health policy and science.