Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

we never evolved to exercise.

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.

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.6

Full triathlons require extreme obsession and money.

Ernesto reinforced my conviction that what I observed at Ironman was bizarre, and he even caused me to question the sanity of my own efforts to train for a marathon.

By arrangement, the teams had set up two stone cairns about two and a half miles apart, and they agreed that the first team to complete fifteen circuits or to lap the other (in other words, get five miles ahead) would win.

Arnulfo’s team lapped Silvino’s and the race was over after about seventy miles.

Tarahumara run rarájiparis because it is a deeply spiritual ceremony that they consider a powerful form of prayer.6 Many Tarahumara I have interviewed say that the ball-game race makes them feel closer to the Creator.

But nearly every day of their lives, hunter-gatherers and subsistence farmers engage in hours of hard physical work.

Tarahumara men, he discovered they walked on average ten miles a day.

WEIRD (Western, educated, industrialized, rich, and democratic).

But to get to the Hadza is not easy. They live in a ring of inhospitable hills surrounding a seasonal, salty lake in northwestern Tanzania—a hot, arid, sunbaked region that is almost impossible to farm.13

On average, the women walk five miles a day and dig for several hours, whereas the men walk between seven and ten miles a day.15 And when they aren’t being very active, they typically rest or do light work.

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.

nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism.

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.

Your brain and liver each consume about 20 percent of your resting metabolism, and if you are a typically strong human, your muscles expend 16 to 22 percent of your resting metabolism.15 The remaining 40 percent accounts for everything else including your heart, kidneys, guts, skin, and immune system.

FIGURE 3 Energy allocation theory: the different, alternative ways the body can use energy from food.

Stated simply, we evolved to be as inactive as possible. Or to be more precise, our bodies were selected to spend enough but not too much energy on nonreproductive functions including physical activity.

we evolved an unusually expensive way of increasing our reproductive success from an unusually low-energy-budget ancestor. When your expenses are high, every penny saved is valuable.

Whether we take the stairs, jog, or go to the gym, we need to override ancient, powerful instincts to avoid unnecessary physical activity, and it should hardly be surprising that most of us—hunter-gatherers included—naturally avoid exercise.

As a result, standing desks are all the rage, and many people now wear sensors or use their phones to keep track of and limit their sitting time.

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.

To add insult to injury, everything I hate about colds is caused not by the viruses that invaded me but by my body’s inflammatory efforts to combat them.

The body has a finite number of fat cells that expand like balloons. If we store normal amounts of fat, both subcutaneous and organ fat cells stay reasonably sized and harmless.

In fact, none of the mechanisms that inflame us—swollen fat cells, too much fat and sugar in the bloodstream, stress, and inactive muscles—are caused by sitting per se.

Instead, they result from the absence of being sufficiently physically active, which usually means a lot of sitting.

Standing is not exercise, and as yet no well-designed, careful study has shown that standing desks confer substantial health benefits.

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 In other words, we’ve confused cause and effect.

All creatures, even bacteria, have nearly twenty-four-hour internal clocks that generate circadian rhythms—about (circa) a day long—that slow them down or speed them up at different times.

FIGURE 7 Cycles of NREM (non-rapid eye movement) and REM (rapid eye movement) sleep during a typical night’s sleep.

To rid itself of waste, the brain evolved a novel plumbing system that relies on sleep. During NREM sleep, specialized cells throughout the brain expand the spaces between neurons by as much as 60 percent, allowing cerebrospinal fluid that bathes the brain to literally flush away this junk.16

The strongest correlation is that vulnerable prey animals tend to sleep less than the carnivores that want to eat them.20

In warmer months, these foragers slept on average 5.7 to 6.5 hours a day, and during colder months they slept on average 6.6 to 7.1 hours a night. In addition, they rarely napped.

In every culture until recently, infants slept with their mothers. Many cultures consider not sleeping with your child a form of child abuse.44

College students are a special breed of humans, in part because so many of them are enjoying their first taste of being grown up without yet shouldering the responsibilities of being adults.

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.

To quote Jerome Siegel, “In twenty years, people will look back on the sleeping-pill era as we now look back on the acceptance of cigarette smoking.”70

And, critically, ATPs are rechargeable. By breaking down chemical bonds in sugar and fat molecules, cells acquire the energy to restore ADPs to ATPs by adding back the lost phosphate.14

In fact, a fit human can store enough sugar to run nearly fifteen miles.

FIGURE 11 Different processes by which muscles recharge ATPs over time. At first, the energy comes nearly instantly from stored ATP and creatine phosphate (CrP); later, energy comes relatively rapidly from glycolysis; eventually, energy must come from slowly aerobic metabolism. Aerobic metabolism occurs in mitochondria by liberating energy either from pyruvate (an end product of glycolysis) or fatty acids.

Altogether, red fibers are ideal for sustained low-intensity activities like walking or jogging a marathon, pink fibers are best for medium-intensity activities like racing a mile, and white fibers are essential for bursts of extreme power but short duration like sprinting a hundred meters.

Among other payoffs, HIIT increases the heart’s ability to pump blood efficiently by making its chambers larger and more elastic.

Just as paleo dieters believe (illogically) it is healthiest to eat like cavemen, primal fitness enthusiasts believe it is best to work out like our muscled ancestors of yore.6

One potential drawback of bulking up too much is sacrificing power. Strength is how much force I can produce; power is how rapidly I produce it.

Even today, average sedentary human beings benefit more from power than strength. Many activities of daily living such as lifting a bag of groceries and rising from a chair require rapid bursts of force. As we will see later, maintaining these power capabilities is especially vital as we age.16

Finally, and perhaps most definitively, a laboratory analysis of muscle fibers demonstrated that a chimp’s muscles can produce at most 30 percent more force and power than a typical human’s.27

Compared with undoped controls, the men given just testosterone added about six pounds of muscle and got 10 percent stronger;

If you are curling the weight upward by flexing your elbow, your biceps muscle is generating force while shortening, technically known as a concentric muscle action.

If you hold the weight steady without moving it up or down, your biceps will still try to shorten but won’t actually change its length, an isometric muscle action.