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Consider the simple human act of walking or running. At his laboratory in the anthropology department atHarvardUniversity, Dan Lieberman does just that, using biomechanical studies to see how we use our body parts in various aspects of movement. As a volunteer subject in one of his experiments last fall, I was wired up and put through paces on a treadmill. On my feet were pressure sensors to show my heel and toe strikes. Electromyographic sensors revealed the firing of my muscles, and accelerometers and rate gyros on my head detected its pitching, rolling, and yawing movements. Small silver foam balls attached to my joints—ankle, knee, hip, elbow, shoulder—acted as reflectors for three infrared cameras mapping in three-dimensional space the location of my limb segments.
These biomechanical windows on walking and running illuminate just how astonishing a feat of balance, coordination, and efficiency is upright locomotion. The legs on a walking human body act not unlike inverted pendulums. Using a stiff leg as a point of support, the body swings up and over it in an arc, so that the potential energy gained in the rise roughly equals the kinetic energy generated in the descent. By this trick the body stores and recovers so much of the energy used with each stride that it reduces its own workload by as much as 65 percent.
The key lies in our human features: the ability to fully extend our knees; the way our lower back curves forward and our thighbone slopes inward from hip to knee so that our feet straddle our center of gravity; and the action of the gluteal abductors, the muscles attached to the pelvis that contract to prevent us from toppling over sideways mid-stride when our weight is on a single foot.
In running, we shift from this swinging pendulum mode to a bouncy pogo-stick mode, using the tendons in our legs as elastic springs. Lieberman's recent studies with Dennis Bramble of the University of Utah suggest that running—which our ancestors mastered some two million years ago—was instrumental in the evolution of several features, including our extra leg tendons, our relatively hairless skin and copious sweat glands (which facilitate cooling), and our enlarged gluteus maximus, the biggest muscle in the body, which wraps the rear end and acts to stabilize the trunk, preventing us from pitching forward. Now Lieberman is studying the role in upright locomotion of a tiny slip of muscle in the neck called the cleidocranial trapezius—all that remains of a massive shoulder muscle in chimps and other apes—which steadies our head during running, preventing it from bobbling. Watching the graphs from the experiment on a computer screen, one can't help but marvel at the effectiveness of the system, the little cleidocranial portion of the trapezius steadying the head; the regular pumping action of arms and shoulders stabilizing the body; the consistent springlike rhythms of our long-legged stride.
"Compare this with the chimp," Lieberman says. "Chimps pay a hefty price in energy for being built the way they are. They can't extend their knees and lock their legs straight, as humans can. Instead, they have to use muscle power to support their body weight when they're walking upright, and they waste energy rocking back and forth."
Chimps are our closest living evolutionary relatives and, as such, are especially well suited to teach us about ourselves. Almost every bone in a chimp’s body correlates with a bone in a human body. Whatever skeletal distinctions exist are primarily related to the human pattern of walking upright—hence the keen interest in parsing these distinctions among those who study the origins of human bipedalism.
Two-legged walking in a chimp is an occasional, transitory behavior. In humans, it is a way of life, one that carries with it myriad benefits, perhaps chief among them, freed hands. But upright posture and locomotion come with a host of uniquely human maladies.
