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 fit than they already were, these subjects needed to work out hard. “The mantra was, You have to go to a gym, you have to do high-intensity physical activity,” says Abby C. King, a professor of health research and policy and medicine at Stanford University: “this sort of ‘no pain, no gain’ phenomenon.”
That notion began to change with the 1968 publication of “Aerobics,” by Kenneth Cooper, an Air Force physician, who argued that anyone could take measures to prevent heart disease with regular “aerobic” exercise, like swimming or jogging, that increases heart rate and oxygen uptake, “improving the overall condition of the body” and thereby “building a bulwark against many forms of illness and disease.” But it was hard to tease apart whether physical activity made people healthier or whether healthier people were more likely to be active. In a landmark study published in 1989, Cooper and colleagues tried to address this problem by considering subjects’ physical fitness, a metric determined by assessing performance on a treadmill test. Theirs is believed to be the first long-term study of men and women to show that the higher a person’s fitness level, the lower their risk of mortality, especially from cardiovascular disease and cancer. But physical fitness, they noted, is not the same as physical activity, the amount of movement a person gets in the course of their daily life. The only way researchers could learn about the latter was by asking people to describe their behavior — a much less precise method than measuring their cardiovascular capacity in a lab.
Not having an objective way of measuring how much activity people were doing made it difficult to observe the full range of that activity’s health benefits. Until 2008, the federal government usually offered physical-activity recommendations as part of its official dietary guidelines, as a way of balancing energy intake with expenditure. But by that year, when the United States Department of Health and Human Services issued its first set of physical-activity guidelines, there was ample evidence that the more “moderate” to “vigorous” activity people reported doing, like brisk walking or raking leaves, the lower their risk of diabetes, certain cancers and cardiovascular disease. Now the department includes Alzheimer’s disease, depression, anxiety and insomnia and recommends that adults get at least 150 minutes of moderate-to-vigorous exercise a week.
Such a broad goal was necessary largely because activity that takes place at a lower intensity for less than 10 minutes had been nearly impossible to study. “We never asked about light-intensity physical activity, because we realized it’s poorly reported,” says I-Min Lee, a professor of medicine at Harvard Medical School. “Putzing around your house, picking up after yourself, doing a little bit of light gardening — how well do people remember that?”