* Illustration: Martin Woodtli * Shouldn't evolution have taken care of our blown-out knees and aching backs? Instead, evolution may be to blame; chimps don't tear menisci or herniate discs like we do. It could be that our knees and backs are such a pain because we insist on standing upright. When our ancestors stood up on two feet, they stumbled upon a much more efficient way of walking that uses 75 percent less energy than knuckle-dragging. Over the next several million years, natural selection refined the structural modifications that keep our torso centered over our lower half as we put one foot in front of the other: longer legs, knobby knees, an elongated spine. But those adaptations can be a bit of a kludge.
By stacking the thigh bone directly on top of the shin bone, we've saddled our knees with more weight than they can handle. Even when you're just strolling down the street, the pressure exerted on your knees is three to five times your body weight. During more strenuous activity like running or climbing stairs, it can be double that. By the time you hit 30, the cartilage that distributes your weight across the joint has already begun to wear away. The tissue loss may eventually lead to osteoarthritis, and the shin and thigh bones will begin to rub against each other. This so-called wear-and-tear arthritis sends about 5.5 million people to doctors every year.
The same thing happens to our vertebrae, which weren't designed to sit on top of each other in one weight-bearing column. With time, the cushions between the bones lose some of their spring, and then one day, as you lift a grocery bag or slump in your chair, a disc can bulge out of place. Pain, numbness, and spasms result because the disc is now pressing on a nerve that connects your spinal cord to the rest of your body. More than 80 percent of working Americans will suffer from back pain that's severe enough to limit their activity at some point during their careers.
Surgery is just a temporary fix for our anatomical woes. After a decade, replacement cartilage can wear away, too. But we can't blame all of our aches on design flaws. The fact that we spend so much time hunched over a keyboard — level 70 World of Warcraft players, are you listening? — can't be good. It might help if we all got up off our highly evolved duffs more often.
