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

by Daniel E. Lieberman

You’ll be relieved to know that to measure your DEE, we no longer follow you around all day with a gas mask. Instead, we measure your pee. To be more precise, we’d ask you to drink some very expensive, harmless water that contains a known quantity of rare (heavy) hydrogen and oxygen atoms and then collect samples of your urine over the next few days. This sounds like a creepy magic trick, but by measuring the rate at which these heavy atoms become less abundant in urine, we can calculate the rates at which both the hydrogen and the oxygen atoms leave the body from sweating, urinating, and breathing. Because hydrogen exits the body only in water but oxygen leaves in both water and carbon dioxide, the difference in the concentration of these two atoms in urine allows us to compute exactly how much carbon dioxide someone produced from breathing, hence how much energy he or she used.9 Thousands of people’s metabolisms have been measured this way, providing a remarkable database on human energy expenditure. If you weigh 180 pounds, your DEE is probably about twenty-seven hundred calories a day. Because we already learned your RMR is about seventeen hundred calories a day, that means nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism. Who would have thought that being a couch potato was so expensive? Is

Because fat is a relatively inert tissue that does not contribute much to metabolism,

During the siege of Leningrad, a thousand people starved to death every day.

One of the main ways the starving volunteers spent less energy was to skimp on maintenance. Basically, their metabolisms slowed down and cut back on costly physiological processes that keep the body in a state of balance. Their heart rates decreased by one-third, and their body temperatures dropped from a normal 98.6°F to 95.8°F, causing them to feel constantly cold, even in well-heated rooms. Their bodies also spent less energy replacing cells in their skin and other organs that are normally replenished on a regular basis. Their skin became flaky, their sperm counts fell, and they manufactured fewer blood cells. Keys’s comprehensive measurements even showed they produced less earwax.

Such measurements indicate that nearly two-thirds of a person’s resting metabolism is spent on just three very expensive tissues: brain, liver, and muscle. 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.

The compromises your body makes among these functions depend on your age and energetic circumstances. For example, if you are young and still growing, you probably won’t have enough energy to reproduce, which is why animals usually start having offspring only after they stop growing.

Put simply, regular movement, including getting up every once in a while, helps prevent chronic inflammation by keeping down postprandial levels of fat and sugar.

Sadly, sitting is not always relaxing. Long hours of commuting, a demanding desk job, being sick or disabled, or otherwise being confined to a chair can be stressful situations that elevate the hormone cortisol.

This much-misunderstood hormone doesn’t cause stress but instead is produced when we are stressed, and it evolved to help us cope with threatening situations by making energy available. 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.

In addition to moving our bodies, muscles function as glands, synthesizing and releasing dozens of messenger proteins (termed myokines) with important roles. Among other jobs, myokines influence metabolism, circulation, and bones, and—you guessed it—they also help control inflammation. In fact, when researchers first started to study myokines, 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.

Without going into too many details, we have learned that the body first initiates a proactive inflammatory response to moderate- or high-intensity physical activity to prevent or repair damage caused by the physiological stress of exercise and subsequently activates a second, larger anti-inflammatory response to return us to a non-inflamed state.

Because the anti-inflammatory effects of physical activity are almost always larger and longer than the pro-inflammatory effects, and muscles make up about a third of the body, active muscles have potent anti-inflammatory effects. Even modest levels of physical activ...

The discovery that using your muscles inhibits inflammation provides yet another compelling explanation for why endless hours of sitting are...

But among the men who were fit, those who sat the most had a 65 percent higher risk of inflammatory-related diseases like type 2 diabetes than those who sat the least.

Altogether, these and other alarming studies suggest that even if you are physically active and fit, the more time you spend sitting in a chair, the higher your risk of chronic illnesses linked to inflammation, including some forms of cancer.

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.

for example, just a hundred seconds every half hour—result in lower levels of sugar, fat, and so-called bad cholesterol in their blood.

Finally, muscles, especially in the calves, act as pumps to prevent blood and other fluids from building up in the legs, not just in veins, but also in the lymph system,

Many of your muscles have a roughly fifty-fifty mixture of slow- and fast-twitch fibers, but the muscles you use mostly for generating power like your triceps are about 70 percent fast-twitch fibers, and those you use primarily for walking or other non-forceful activities like the deep muscles of your calf (the soleus) are roughly 85 percent slow-twitch fibers.

In 1976, a pioneering but somewhat painful study that biopsied the outer calf muscles (gastrocnemius) of forty people found that ordinary nonathletes tend to have equal percentages of fast- and slow-twitch fibers, elite sprinters have about 73 percent fast-twitch fibers, and professional distance runners average 70 percent slow-twitch fibers.30 Thousands

Whereas speedsters like greyhounds and cheetahs have highly muscular legs with mostly fast-twitch fibers, animals evolved for endurance like fox terriers and skunks have less powerfully built legs dominated by slow-twitch fibers.

Whenever biologists want to study the heritability of a trait like running speed, the best data come from twins.

That said, take this percentage with a giant grain of salt because heritability estimates of athletic performance vary widely from study to study. Heritability estimates of speed range from 30 to 90 percent, and those of aerobic capacity range from 40 to 70 percent.

abundant evidence shows that occasional, regular bouts of high-intensity exercise make us not only stronger and faster but also fitter and healthier. This form of training, known as high-intensity interval training (HIIT), involves alternating short bouts of intense anaerobic exercise such as sprinting with less intense periods of recovery. To be clear, HIIT isn’t weight training; it is basically intense cardio.

you keep up a regimen of two sessions a week of HIIT, your muscles will gradually improve their ability to produce high, rapid forces in part by augmenting how many fibers contract simultaneously when stimulated by nerves. In addition, your muscles will change composition.

Although HIIT cannot stimulate your body to produce more fast-twitch muscle fibers, the ones you have will thicken,

HIIT can also modify slower, more fatigue-resistant pink fibers into faster, more fatigable white fibers; lengthen fibers slightly, thus boosting their shortening speed; and increase the percentage of fibers in a muscle that contracts, thereby increasing force.

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

HIIT also augments the number, size, and elasticity of arteries and increases the number of tiny capillaries that infuse muscles. HIIT further improves muscles’ ability to transport glucose from the bloodstream and increases the number of mitochondria within each muscle, thus supplying more energy.

Men reach peak strength in their twenties; women achieve peak strength slightly later but barely lose muscle as they get older.

The ability to lift above your head something twice or more your body weight is a bizarre, dangerous feat that probably had little practical value in the Stone Age.

While obtaining an extra three hundred calories is a trivial task today (accomplished by wolfing down a milk shake), the challenge of foraging daily for those additional calories in the Stone Age would have compromised one’s reproductive success.18

All in all, musclemen and gym rats have physiques you would not have encountered often or perhaps ever among our hunter-gatherer ancestors who lacked the wherewithal to get that ripped, wouldn’t have needed such strength, and would have been taxed by the caloric costs.

Although these studies differ in terms of methods, they collectively reveal that adult chimps are no more than a third stronger than humans. Contrary to the old meme, a chimp couldn’t rip your arm from its socket in an arm-wrestling contest. That said, you’d still probably lose.

In general, the more we load our bones, especially when we are young, the thicker they become.

Imagine you are holding a heavy weight in your hand to do biceps curls. 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. Concentric contractions are the primary means by which muscles move us.

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.

Isometric muscle actions can be challenging, but it is even harder to lower the weight very slowly by extending your elbow. This sort of eccentric muscle action ...

At first you will feel fatigued, but hours later you’ll become sore. This happens because you are making your biceps generate more force than it can easily handle against a resisting force (the weight), literally tearing it apart at the microscopic level. Filaments snap, membranes rip, connective tissues split.

More important, by intentionally shredding the muscle a little, you stimulate growth because the microdamage stimulates affected muscle cells to turn on a cascade of genes. Among other things, these genes augment the total number and thickness of muscle fibers, thus expanding the muscle’s diameter, making the muscle stronger.

you primarily want to gain strength, you’ll get the most bang for your buck by slowly doing a few demanding repetitions of weights that require eccentric or isometric contractions.44 That said, if you are more interested in power and endurance, you’ll derive more benefit from multiple sets of fifteen to twenty rapid concentric repetitions on less demanding weights with only brief rests between sets.45 Lifting weights a few times a week, moreover, is especially helpful to stay healthy and vigorous as we age.

Road rage is one example of how humans sometimes aggress reactively like Travis, but such incidents are rare and shocking because as children we rapidly learn to suppress these reactive instincts.

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.

“Dan, you know that pig can’t hold its head very still.” Frankly, I had never noticed how pigs held their heads, but as I looked with new eyes, I could see he had a point.

Unlike a running dog or horse, which holds its head still like a missile, that pig’s head was flopping about like a fish on a beach.

Our conversation quickly turned to the importance of stabilizing one’s gaze and the hypothesis that animals adapted for running (“cursors” in biological parlance) have a special rubber-band-like structure, the nuchal ligament, at the back o...

Soon thereafter we were looking at casts of fossil hominins. He showed me that nuchal ligaments were present in dogs, horses, antelopes, and other cursors, but absent in pigs and other non-running animals.

Even more excitingly, we could see that gorillas, chimpanzees, and early hominins lack a nuchal ligament, but humans and fossil species from the genus Homo had them.

When quadrupeds gallop, they alternate landing with their forelimbs and hind limbs, using not just their legs but also their spines as springs.5

When they were finally allowed to arise from their beds, the volunteers’ bodies resembled forty-year-olds’ by many metrics: they were fatter, had higher blood pressure, higher cholesterol levels, less muscle mass, and lower fitness.40 The eight ensuing weeks of exercise, however, not only reversed the deterioration but in some cases led to net improvements.