Born to Run
Sheep on treadmills and bones of extinct animals reveal secrets of motion.
It isn’t something you see every day in the local gym: sheep running on treadmills. But if you had stumbled into the research labs on the South Farms of the University of Illinois not too long ago, you might have done a double take at this startling sight.
It was all part of an anthropological analysis of how movement patterns affect our bones. The data collected has helped researchers in probing the bones of extinct animals and determining how early primates might have moved around in their environments, says John Polk, a U of I anthropologist who has spent much of his career studying how the human skeleton evolved so we can walk upright and run with endurance.
According to Polk, their research team had sheep walk and run on a treadmill at different angles to find out how walking with flexed knees or extended knees affects patterns of bone density on joint surfaces. The sheep were contained by a box placed over the treadmill, but the front and one side of the box were made of plexiglass so the running sheep could see directly ahead of them.
“If you put another sheep in front of them, they’ll run towards it,” Polk says. “Sheep are wonderful treadmill runners.”
Researchers discovered that when the sheep walked with legs flexed, the bones were denser on the posterior, or back, side of the knee joint; and when they walked with extended legs, bones were denser on the front of the knee joint. This knowledge has made it possible for anthropologists and paleontologists to determine, by measuring density patterns in the bones of extinct animals, whether a particular primate walked in a crouched or more upright position.
Polk is interested in two major transitions among early humans—the shift to walking upright about 5 to 7 million years ago and the origin of endurance running about 2 million years ago. For instance, the human ancestor, Australopithecus, had shorter limbs and may have used more flexed lower-limb postures when they walked.
“They didn’t have the anatomy to run long distances, but they were not ecologically required to do so,” Polk says. “They probably didn’t walk exactly like us either, and it is quite possible that there is more than one way to be an upright biped. The australopithecines lived in forests, and meat wasn’t a big portion of their diet, so endurance running was not required in tracking down prey.”
But when long-distance hunting arose, that all changed.
Polk has been intrigued by research at Harvard, where he did his post-doctoral work, which argues that humans developed their endurance running ability because hunting required it. Human hunters would “run down” animals in hot environments until their prey collapsed due to hyperthermia and exhaustion.
“Baby, we were born to run,” sang megastar Bruce Springsteen back in the 1970s. This lyric rings true, but Polk says that it goes deeper than that. Humans weren’t just born to run. We also evolved to run.
“The modern human body shape resulted from the fact that we are great endurance runners,” he points out. “A lot of our anatomy is well-adapted to endurance running. We have relatively long limbs and narrower hips compared to other primates or animals. Our Achilles tendons are long and can store and recover elastic energy more effectively than other primates, and we sweat more and have less hair than other animals, making us very good at dumping heat in hot environments.”
Like other specialized running animals, humans also have a tendinous band connecting the back of the head to our upper backs, stabilizing our heads during running. In addition, we can regulate the number of breaths per stride, whereas quadrupeds can take only one breath per stride.
When quadrupeds stretch out during a run, they have to breathe in; then they breathe out as their stride compresses. This is one of the few ways quadrupeds can try to cool down, he says, and if they’re running for long periods of time, the heat dumping is limited and body temperature can rise dramatically. Because humans can regulate their breathing, they can keep cool and have greater endurance.
Polk says that aside from a couple of exceptions, such as pronghorn antelopes or perhaps some wild dogs, very few wild animals would be able to run a marathon, as humans do. Pronghorns can maintain an incredible 40 mile-per-hour pace for a half hour or more. Other animals, such as sled dogs, can be bred and trained for endurance, but the ability does not come naturally. Horses are sprinters, and although some breeds can also be trained for distance running, there is always the risk of being run to exhaustion.
Over the years, Polk’s laboratory has been putting bones to the test, using CT scans and other devices to find out how people and animals, both living and extinct, came to walk and run the way they do. Polk has also created three-dimensional models of top college sprinters to figure out the secret of their success, and has been working with colleagues across the University to develop new ways to diagnose the motion problems of people with gait impairments (see accompanying sidebar).
“If you want to look at how extinct animals moved, we need to understand how living animals move,” he says. “So we’re interested in how the forces we encounter during motion act on our skeletons and can alter bone properties. This helps us figure out what kinds of skeletal and physiological changes were involved in both the transition to walking upright, and in the transition from habitual walking to more active running.”
Polk has spent countless hours studying how animals run, but he didn’t actually run in a long-distance race himself until he tackled a half marathon in the spring of 2010. He says his performance was passable, and he was pleased that at least he was not lapped by any of the full marathon runners.
The other good news: No sheep outdistanced him either.
By Doug Peterson