For a species that prides itself on its athletic prowess, human beings are a pretty poky group. Lions can sprint at up to 50 m.p.h. when they're chasing down prey. Cheetahs move even faster, flooring it to a sizzling 70 m.p.h. But most humans--with our willowy spines and awkward, upright gait--would have trouble cracking 25 m.p.h. with a tail wind, a flat track and a good pair of shoes.
Nonetheless, the human race is undeniably becoming a faster race. Since the beginning of the past century, track-and-field records have fallen in everything from sprints to miles to marathons. The performance arc...