托福阅读练习：Why Walking Weakens Wee Ones

会计考友

　　If you’ve ever had to pick up and carry a tired child through the mall, you know that walking sometimes tuckers out the toddlers. Now scientists think they know why. The results appear in the Journal of Experimental Biology. [P. Weyand et al., "The mass-specific energy cost of human walking is set by stature," J. Exp. Biol., link to come]
　　Larger animals tend to use less energy per gram of body mass than smaller ones do, even at rest. But what happens when they’re in motion? Researchers recruited walkers of different ages and sizes, from a three-foot-tall kindergartner to a six-foot adult. While the subjects logged miles on the treadmill, the scientists measured their stride and metabolism.
　　And they found that people pretty much walk the same way, regardless of their stature. If you scaled a five-year-old up to be six-and-a-half feet tall, the giant child would lope along just like a similarly sized adult.
　　What’s more, walkers of all heights use the same amount of energy per step. That means that big people don’t conserve energy by strolling in a more economical style. They expend less energy because—just as you may have always suspected—they simply need fewer strides to cover the same ground. Which means that for some people a walk in the park ain’t necessarily a walk in the park.