Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

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.

Treadmill-like devices were first used by the Romans to turn winches and lift heavy objects, and then modified in 1818 by the Victorian inventor William Cubitt to punish prisoners and prevent idleness.

exercise has become commercialized, industrialized, and, above all, medicalized.

Exercise is big business.

But more than anything else, exercise has become a source of anxiety and confusion because while everyone knows that exercise is good for their health, the majority of us struggle to exercise enough, safely, or enjoyably. We are exercised about exercise.

To be clear, I do not contend that exercise isn’t beneficial or that everything you have read about exercise is incorrect.

I will, however, make the case that by ignoring or misinterpreting evolutionary and anthropological perspectives on physical activity, the contemporary, industrial approach to exercise is marred by misconceptions, overstatements, faulty logic, occasional mistruths, and inexcusable finger-pointing.

According to exercists, we were born to exercise because for millions of years our hunter-gatherer ancestors survived through walking, running, climbing, and other physical activities.

Everyone knows they should exercise, but few things are more irritating than being told to exercise, how much, and in what way.

Exhorting us to “Just Do It” is about as helpful as telling a drug addict to “Just Say No.”

The premise of this book is that evolutionary and anthropological perspectives can help us better understand the paradox of exercise—that is, why and how something we never evolved to do is so healthy.

For example, hundreds of studies have looked for correlations between heart disease, exercise habits, and factors like age, sex, and income. These analyses reveal correlations, not causation.

There is nothing inherently wrong with these sorts of studies—as you’ll see, I’ll make use of them throughout the book—but they view exercise too narrowly.

For starters, almost all studies of humans focus on contemporary Westerners or elite athletes.

why exercise affects the body as it does, why so many of us are ambivalent about exercise, and why physical inactivity causes us to age more rapidly and increases our chances of getting sick.

To address these deficiencies, we need to supplement the standard focus on Westerners and athletes with evolutionary and anthropological perspectives.

In short, to really understand exercise, let’s study the natural history of human physical activity and inactivity.

The mantra of this book is that nothing about the biology of exercise makes sense except in the light of evolution, and nothing about exercise as a behavior makes sense except in the light of anthropology.

After an introductory chapter, the first three parts roughly follow the evolutionary story of human physical activity and inactivity, with each chapter spotlighting a different myth.

How can we more effectively manage to exercise, and in what ways? To what extent, how, and why do different types and doses of exercise help prevent or treat the major diseases likely to make us sick and kill us?

you were a subsistence farmer like Ernesto who grows all his own food without the help of machines, why would you ever spend precious time and calories exercising just for the sake of keeping fit or to prove that anything is possible?

Decades of research have indeed confirmed that genes do play key roles in many aspects of sport and exercise, including our motivation to exercise in the first place.3

In addition, studies of professional athletes who push the limits of endurance reveal that the barriers they must overcome include physiological challenges like generating muscle force effectively, fueling themselves efficiently, and controlling their body temperature, but these competitors are even more challenged by psychological hurdles.

To keep going, great athletes must learn to cope with pain, be strategic, and above all believe they can do it.5

The most widespread and intuitively appealing line of thinking about the effects of nurture on physical activity arises from an idea known as the theory of the natural human.

According to this view, championed by the eighteenth-century philosopher Jean-Jacques Rousseau, humans who live in what Rousseau termed a “savage” state of nature reflect our true, inherent selves uncorrupted by civilization.

This theory has been warped into many forms, including the myth of the noble savage, the belief that nonwesternized people whose minds have not been polluted by the social and moral evils ...

To them, chasing that unpredictable ball for mile upon mile is a sacred metaphor for the journey of life, and it induces a spiritual trancelike state.

In other words, the training that enables them to run back-to-back marathons is the physical work that is part and parcel of their everyday life.

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.

What kind of physical activity and how much of it is normal for a “normal” human being?

For example, because most psychologists live and work in the United States and Europe, about 96 percent of the subjects in psychological studies are also from the United States and Europe.8

If we really want to know what ordinary humans do and think about exercise, we need to sample everyday people from a variety of cultures instead of focusing solely on contemporary Americans and Europeans who are, comparatively speaking, WEIRD (Western, educated, industrialized, rich, and democratic).9

So if we genuinely want to know about the exercise habits of evolutionarily “normal” humans, it behooves us to learn about hunter-gatherers, especially those who live in arid, tropical Africa.

All of these tribes trade with neighboring farmers, they smoke tobacco, and their way of life is changing so rapidly that in a few decades they will cease to be hunter-gatherers.10

All in all, assuming that what hunter-gatherers do is evolutionarily “normal,” then comprehensive studies of contemporary foraging populations from Africa, Asia, and the Americas indicate that a typical human workday used to be about seven hours, with much of that time spent on light activities and at most an hour of vigorous activity.19

Your PAL is calculated as the ratio of how much energy you spend in a twenty-four-hour period divided by the amount of energy you would use to sustain your body if you never left your bed.

(1.6). 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.

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.

The problem, of course, is that physical activity helps slow aging and promotes fitness and health. So 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.

My opinion is that while many animals are driven by deep instincts to move, sometimes causing pleasure, exercise as we define it—discretionary, planned physical activity for the sake of physical improvement—is a uniquely human behavior.

And so we promote exercise. Just as we put wheels in cages for mice in labs, over the centuries we have invented a stunning variety of ways and means for our fellow humans to undertake optional physical activity for the sake of health and fitness.

Just as a stove burns gas or wood, your body uses oxygen to burn fat and sugar while giving off carbon dioxide. By quantifying how much oxygen you consume and carbon dioxide you exhale, we can calculate precisely how much energy your body is using at any given moment.5

The unit of measurement we use is a calorie. (Confusingly, the “calorie” used on food labels is actually one kilocalorie, the amount of heat required to raise the temperature of a kilogram of water 1°C, and I will follow the same convention here.)6

If you are an average adult American male who weighs 180 pounds (82 kilograms), your rate of energy expenditure while resting quietly in a chair is approximately seventy calories per hour.

This is your resting metabolic rate (RMR), so named because your resting metabolism comprises all the chemical reactions going on in your body wh...

To correct for these expenses, we could measure your energy expenditure in bed just after you woke up from an eight-hour sleep in a dark 70°F room following a twelve-hour fast.

That measurement, your basal metabolic rate (BMR), would be roughly 10 percent lower than your RMR (in our example, 1,530 calories).

How does the amount of energy you spend at rest compare with your total energy budget?

To compute this ratio, we next need to measure your total daily energy expenditure (DEE), the overall number of calories you spend over the course of twenty-four hours doing everything you do including moving, reading, sneezing, talking, and digesting.