Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

we never evolved to exercise. What do I mean by that? Well, exercise today is most commonly defined as voluntary physical activity undertaken for the sake of health and fitness. But as such it is a recent phenomenon. Our not-too-distant ancestors who were hunter-gatherers and farmers had to be physically active for hours each day to get enough food, and while they sometimes played or danced for fun or social reasons, no one ever ran or walked several miles just for health. Even the salubrious meaning of the word “exercise” is recent. Adapted from the Latin verb exerceo (to work, train, or practice), the English word “exercise” was first used in the Middle Ages to connote arduous labor like plowing a field.1 While the word has long been used to denote practicing or training to improve skills or health, to be “exercised” also means to be harassed, vexed, or worried about something.

because industrialized societies such as the United States fail to recognize that exercise is a paradoxically modern but healthy behavior, many of our beliefs and attitudes about exercise are myths

Beyond spreading confusion and doubt, the most pernicious consequence of many myths about exercise—especially the one about how it’s normal to exercise—is that we fail to help people to exercise and then unfairly shame and blame them for not doing it.

schadenfreude.

Ernesto told me he was a champion runner in his youth and that he still competed in several races a year. But when I asked him how he trained, he didn’t understand the question. When I described how Americans like me keep fit and prepare for races by running several times a week, he seemed incredulous. As I asked more questions, he made it pretty clear he thought the concept of needless running was preposterous. “Why,” he asked me with evident disbelief, “would anyone run when they didn’t have to?”

Who is more normal from an evolutionary perspective: those of us who push our bodies to do nonessential physical activities, sometimes in extreme, or those of us who prefer to avoid unnecessary exertion?

intensive efforts have failed to identify specific genes that explain much about athletic talent including how and why Kenyan and Ethiopian runners currently dominate distance running.

We must therefore look just as much at the other side of the nature versus nurture debate and consider how environment, especially culture, contributes to everyday people’s athletic abilities and impulses.

In other words, typical hunter-gatherers are about as physically active as Americans or Europeans who include about an hour of exercise in their daily routine.

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.

those of us who no longer engage in physical labor to survive must now weirdly choose to engage in unnecessary physical activity for the sake of health and fitness. In other words, exercise.

while youngsters have always played and sports are a human universal, exercise outside the context of sports was extremely rare until relatively recently.

as recent technological and social developments have diminished industrialized people’s need to be physically active, a growing chorus of experts has never ceased to raise the alarm that we aren’t exercising enough.

Your BMR is the energy you use to maintain the most basic processes of your body necessary to stay alive in a nearly coma-like state.

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.

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.

Keys’s data showed that the bodies of the men he starved saved calories in a manner similar to the way most people economize when they confront a severely reduced income: they prioritized “essential” organs like the brain but abandoned “expendable” costs like reproduction and drastically cut back on “reducible” functions like staying warm, active, and strong. By shrinking their muscles by 40 percent, they saved about 150 calories a day, leaving the starving men feeble and easily fatigued. Their hearts also got smaller by an estimated 17 percent, and their livers and kidneys shrank similarly.16

if longer legs make you faster, and speed helps you escape predators (or be a better predator), then selection will favor long legs. But because speed is obviously always beneficial, why don’t more species have long legs? The explanation is trade-offs. Because variations almost always involve limited alternatives, natural selection has to act on competing costs and benefits. If you are long-legged and big, then you can’t be short-legged and small, which has other advantages depending on your situation. Selection will inevitably favor whichever alternative or compromise most improves your reproductive success in your environment.

you can spend a given calorie in just five ways: growing your body, maintaining your body (resting metabolism), storing energy (as fat), being active, or reproducing.

Like an unsentimental novelist such as Jane Austen, natural selection doesn’t care if we are happy, nice, or wealthy; it just favors heritable traits, including trade-offs, that enable us to have more children.

Does she dash off recklessly at her first opportunity because she finds it intensely pleasurable to gallop at top speed, because she lacks the foresight to conserve energy, or because she needs to release pent-up energy?

The bottom line is that humans evolved to acquire and expend much more energy than chimpanzees.

Despite stereotypes of non-exercisers as lazy couch potatoes, it is deeply and profoundly normal to avoid unnecessarily wasting energy. Rather than blame and shame each other for taking the escalator, we’d do better to recognize that our tendencies to avoid exertion are ancient instincts that make total sense from an evolutionary perspective.

sitting more than three hours a day is responsible for nearly 4 percent of deaths worldwide and that every hour of sitting is as harmful as the benefit from twenty minutes of exercise.

FIGURE 5 The spine and pelvis during standing and sitting. Compared with the chimpanzee (left), the human lower spine (the lumbar region) has a curvature (a lordosis) that positions our center of mass (circle) above our hips when we stand. When we squat on the ground (the way people often sat for millions of years) or slouch when sitting in a chair with a backrest, we tend to rotate the pelvis backward and flatten the lower spine, reducing this lordosis. (Note that I have shown just a few of the many postures people adopt when sitting.)

Cortisol shunts sugar and fats into the bloodstream, it makes us crave sugar-rich and fat-rich foods, and it directs us to store organ fat rather than subcutaneous fat. Short bursts of cortisol are natural and normal, but chronic low levels of cortisol are damaging because they promote obesity and chronic inflammation. Consequently, long hours of stressful sitting while commuting or a high-pressure office job can be a double whammy.

does a daily bout of hard exercise negate the effects of sitting for the rest of the day?

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.

when you look closely, there is little empirical evidence that average sleep duration in the industrial world has decreased in the last fifty years.28 The more we look, the less we can profess eight hours to be normal.

If you require quiet and dark to fall asleep, you are evolutionarily unusual.

Why are so many people today stressed out by a behavior as intrinsically restful and effortless as falling sleep? And how does physical activity help?

we have become prey to what has been termed the sleep-industrial complex. People stressed about sleep are enticed to spend a fortune on hi-tech mattresses, sound machines, noise-canceling headphones, light-blocking curtains, gizmos to halt their bedfellow’s snoring, eye masks, and something called high-performance bedding. These mostly harmless gadgets would no doubt amuse our ancestors who slept on the ground by a fire, but we should be downright alarmed by the abuse of sleeping pills. Sleeping pills, which are highly habit forming, are a multibillion-dollar industry.

“In twenty years, people will look back on the sleeping-pill era as we now look back on the acceptance of cigarette smoking.”

It bears repeating that sleep and physical activity are inextricably linked: the more physically active we are, the better we sleep because physical activity builds up sleep pressure and reduces chronic stress, hence insomnia.

If, as some religions teach, we are made in God’s image, then God must be a slow runner.

to avoid being a tiger’s dinner, you need only to run slightly faster than the next guy. That would be no problem for Usain Bolt, who can sprint more than twice as fast as I can. But, like most distance runners, I can probably run much farther than Bolt. To what extent does my greater endurance come at the expense of speed?

a fit human can store enough sugar to run nearly fifteen miles. But there is a consequential catch: during glycolysis the leftover halves of each sugar, molecules known as pyruvates, accumulate faster than cells can handle. As pyruvates pile up to intolerable levels, enzymes convert each pyruvate into a molecule called lactate along with a hydrogen ion (H+). Although lactate is harmless and eventually used to recharge ATPs, those hydrogen ions make muscle cells increasingly acidic, causing fatigue, pain, and decreased function.21 Within about thirty seconds, a sprinter’s legs feel as if they are burning. It then takes a lengthy period of time to slowly neutralize the acid and shuttle the surplus lactate into the third, final, but long-term aerobic energy process.

The more we study the effects of HIIT, the more it appears that HIIT should be part of any fitness regimen, regardless of whether you are an Olympian or an average person struggling to get fit.

Nobody picks on a strong man. —Charles Atlas

This vital reflex, known as the Valsalva maneuver, lessens stress on the heart, and it also helps rigidify the trunk and stabilize the spine.38

Lifting weights a few times a week, moreover, is especially helpful to stay healthy and vigorous as we age.

Because dead people can’t have babies or help existing children survive, any heritable advantages or disadvantages that affect fighting and hunting ability should have strong effects on selection.

Humans are the only species capable of throwing overhand fast and on target.

upper-body muscle mass is on average 75 percent greater in human males than females.66 As we have seen, strength in the shoulders, arms, and torso matters for throwing, not to mention wrestling and other competitions.

But once the crash diet was over and the policemen went back to their normal diets, only the officers who continued to exercise avoided weight regain; all the rest regained most or all of the pounds they initially lost.50 Many other studies confirm that physical activity, including walking, helps keep those lost pounds off.51

anything less than five thousand steps a day falls under the threshold of being “sedentary,”

In essence, running is jumping from one leg to another.

scavenging, yet by 2 million years ago there is clear archaeological evidence that they also hunted large animals like wildebeests and kudu.