Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

exercise today is most commonly defined as voluntary physical activity undertaken for the sake of health and fitness.

What kind of toll does all the training necessary to do a full triathlon have on the athletes’ friends, families, and marriages?

In the women’s race, known as ariwete, teams of teenage girls and young women run about twenty-five miles while chasing a cloth hoop. In the men’s race, the rarájipari, teams of men run up to eighty miles while kicking an orange-sized wooden ball.

this myth implies that people like you and me who neither can nor will accomplish such feats are, from an evolutionary perspective, abnormal because civilization has turned us into etiolated wimps.

Not only is the myth of the athletic savage an example of truthiness—something that feels true because we want it to be true—it trivializes the physical and psychological challenges faced by all athletes everywhere, the Tarahumara included.

Here’s another, startling way of thinking about these numbers: if you are a typical person who barely exercises, it would take you just an hour or two of walking per day to be as physically active as a hunter-gatherer.

their bodies transformed to use less energy even while resting. After twenty-four weeks of starving, the volunteers’ resting and basal metabolic rates plummeted by 40 percent, far more than expected from the weight they lost.

The key lesson to digest from the starving men’s dramatically lower resting metabolic rates is that human resting metabolisms are flexible. Most critically, resting metabolism is what the body has opted to spend on maintenance, not what it needs to spend. One

If you are sitting while reading this, for every five breaths you take, one pays for your brain, another for your liver, a third for your muscles, and the last two pay for the rest of your body.

Stated differently, apes and sedentary industrialized people are unusually inactive compared with most mammals, and hunter-gatherers are in-between.

In a delightfully comprehensive study, the anthropologist Gordon Hewes documented more than a hundred postures that humans from 480 different cultures adopt when they sit without a chair.

rural teenagers in Kenya who rarely sit in chairs with backrests have 21 to 41 percent stronger backs than teenagers from the city who regularly sit in the sorts of chairs you and I usually use.

Every hour spent resting comfortably in a chair is an hour not spent exercising or actively doing things. The second concern is that long periods of uninterrupted inactivity harmfully elevate levels of sugar and fat in the bloodstream. Third and most alarmingly, hours of sitting may trigger our immune systems to attack our bodies through a process known as inflammation. Don’t panic, but as you sit comfortably reading this, your body may be on fire. On

The good news is that the biggest causes of chronic inflammation are largely avoidable, preventable, or addressable: smoking, obesity, overconsumption of certain pro-inflammatory foods (a chief one being red meat), and—surprise, surprise—physical inactivity.

The majority of that fat (about 90 to 95 percent) is subcutaneous, so named because it is stored in billions of cells distributed in buttocks, breasts, cheeks, feet, and other nameless places just below the skin.

The other major type of fat is cached in cells in and around our bellies and other organs including the heart, liver, and muscles.

A second way lengthy periods of sitting may incite widespread, low-grade inflammation is by slowing the rate we take up fats and sugars from the bloodstream.

Another worrisome way sitting can provoke inflammation is through psychosocial stress.

Last, and perhaps most important, prolonged sitting can kindle chronic inflammation by allowing muscles to remain persistently inactive.

they were astonished to discover that muscles regulate inflammation during bouts of moderate to intense physical activity similarly to the way the immune system mounts an inflammatory response to an infection or a wound.

A multiyear analysis of almost five thousand Americans found that people who broke up their sitting time with frequent short breaks had up to 25 percent less inflammation than those who rarely rose from their chairs despite sitting the same number of hours.48

In fact, people who rarely sat for more than twelve minutes at a time had lower death rates, and those who tended to sit for half an hour or longer at a stretch without getting up had especially high death rates.

Instead, leisure-time sitting best predicts mortality, suggesting that socioeconomic status and exercise habits in mornings, evenings, and weekends have important health effects beyond how much one sits during weekdays at the office.

Instead, the best predictor of avoiding back pain is having a strong lower back with muscles that are more resistant to fatigue; in turn, people with strong, fatigue-resistant backs are more likely to have better posture.71

One conspicuous benefit is cognitive: sleep helps us remember important things and helps synthesize and integrate them. It sounds like magic, but while we sleep, our brains file and then analyze information.

An even more vital function of sleep for the brain is janitorial. The zillions of chemical reactions that make life possible inevitably create waste products known as metabolites, some highly reactive and damaging.12 Because the power-hungry brain uses one-fifth of the body’s calories, it generates abundant and highly concentrated metabolites. Some of these garbagy molecules such as beta-amyloid clog up neurons.13

Hewes, G. (1953), Worldwide distribution of certain postural habits, American Anthropologist 57:231–44.