Fitness tracker: How many steps should you take in a day and at what pace?

Fitness trackers have bolstered the case for even light exercise. The persistent mystery, though, is which movements matter the most

Humans, once in constant motion as hunters and gatherers, are moving less than ever. At first, this trend seemed like progress: Transferring our heavy and dangerous work to animals, then machines, enabled more people to live longer. As recently as the 1950s, doctors considered exercise dangerous for people over age 40; for heart disease, which was then killing a record number of Americans, they prescribed bed rest. This was partly based on their concept of what “exercise” was: Early physiologists conducted studies on their (typically young, male) graduate students or on military servicemen — and in order to become more