Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

numerous studies using better data and sophisticated methods to correct for factors like age, illness, and income have confirmed that people who sleep about seven hours tend to live longer than those who sleep more or less.

sensor-based studies of nonindustrial populations reveal a more complex picture. Whereas most foragers in Tanzania, Botswana, and Bolivia sleep through the night, subsistence farmers in Madagascar often divide their sleep into first and second segments.

The first system is our nearly twenty-four-hour circadian cycle regulated by a specialized group of cells within a region of the brain known as the hypothalamus.52 (The sleep-inducing name for this cluster of cells is the suprachiasmatic nucleus.) These cells wake us up in the morning by signaling to the glands atop our kidneys to produce cortisol, the major hormone that stimulates the body to spend energy. Then as darkness falls, the hypothalamus directs the pineal gland, another structure in the brain, to produce melatonin, the “Dracula hormone,” which helps induce sleep.

If underlying chronic stress from too much time commuting, social conflicts, or endlessly tough homework assignments elevates stress hormones like cortisol above normal levels, we become more alert at night when we’d otherwise become drowsy, or we wake up after one or two NREM and REM cycles.60 Then as we become chronically sleep deprived, we produce more cortisol, especially at night, which can then inhibit sleep, keeping the problem going and promoting insomnia.

Sadly, stresses that elevate cortisol levels and cause sleep deprivation can also slowly erode our health in other ways by depressing the immune system and directing the body to store more organ fat. Lack of sleep also wreaks havoc with the hormones that regulate appetite, increasing levels of a hormone called ghrelin that makes us hungry and simultaneously depressing levels of another hormone called leptin that inhibits the desire to eat.

Because speed is the product of stride length and stride rate (a stride being a full cycle from the time a foot hits the ground to the next time the same foot hits the ground), one can go faster by taking longer strides, by taking more rapid strides, or some combination of the two.

The strategy often works, but if a cheetah ever chases you, zigzagging might be a bad idea. As veterans of Pamplona will attest, the most dangerous parts of the course are the turns where the two-legged humans become even slower and less stable than the four-legged bulls.

And herein lies the basis for why upright humans are comparatively slow: while a dog or chimpanzee has four legs with which to push on the ground to generate power (power is the rate of doing work), we have only two. In fact, when we run, only one leg is on the ground at any given moment to lift and push us forward. Less power means less speed.

Our short, little upright spines do nothing to help us run faster, but instead struggle to keep our inherently tippy upper bodies stable while also dampening the shock wave that travels from the foot up to the head every time we hit the ground.

When it comes to very short sprints, speed is largely a function of strength and skill. Since sprinters’ legs work sort of like hammers that forcefully and rapidly hit the ground, and (as Newton showed) for every action there is an equal and opposite reaction, the harder the legs push downward and backward against the ground, the harder the ground pushes the body upward and forward. For this reason, maximum speeds for hundred- and two-hundred-meter sprints are largely limited by how effectively a runner’s leg muscles can produce force during the fleeting period of time—as little as a tenth of a second in elite sprinters—each foot is on the ground.

As the name implies, each ATP consists of a tiny molecule (an adenosine) attached to three molecules of phosphate (a phosphorus atom surrounded by oxygen atoms). These three phosphates are bound to each other in a chain, one on top of the other, storing energy in the chemical bonds between each phosphate. When the last of these phosphates is broken off using water, the tiny quantity of energy that binds it to the second phosphate is liberated along with one hydrogen ion (H+), leaving behind an ADP (adenosine diphosphate). This liberated energy powers almost everything done by every cell in the body like firing nerves, making proteins, and contracting muscles. 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.

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.

Life demands oxygen, especially if you want to run far. In fact, using oxygen to burn a molecule of sugar yields a whopping eighteen times more ATP than glycolysis. But, once again, there is a trade-off: aerobic metabolism provides substantially more energy but substantially more slowly because it requires a long sequence of steps and an army of enzymes.

Sugars and fats, however, are burned at different rates. Although my body stores enough fat to run about thirteen hundred miles, fat takes many more steps, hence much more time, to break down and burn than sugar. At rest, about 70 percent of a body’s energy comes from slowly burning fat, but the faster we run, the more sugar we must burn. At maximum aerobic capacity we burn exclusively sugar.

For a hundred-meter dash, only 10 percent of your energy comes from aerobic respiration, but that percentage increases to 30 percent over four hundred meters, 60 percent for eight hundred meters, and 80 percent for a mile.23 The farther you go, the more your maximum speed benefits from a high VO2 max (which, as we will see, you can increase by training).

At one extreme are slow-twitch fibers that do not contract rapidly or powerfully but use energy aerobically and don’t fatigue easily. These type I fibers are colloquially known as red muscle because of their darker tinge.28 At the other extreme are fast-twitch (type II) fibers, which come in two types: white and pink. White muscle (type IIX) fibers burn sugar to generate powerful and rapid forces but fatigue rapidly. Pink muscle (type IIA) fibers produce moderately powerful forces aerobically and thus fatigue at an intermediate rate.

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.

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.

In addition to being more fast-twitch dominated, sprinters have larger muscles than distance runners.

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.

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

The basic principle behind resistance exercise is to make your muscles generate force against an opposing force such as your body’s own mass or an external load like a dumbbell, a stack of weights, or a cow. In essence, you use something heavy to resist your muscles’ efforts to contract.

Concentric contractions are the primary means by which muscles move us. Muscles, however, don’t always shorten. 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 requires your biceps to fire as it lengthens.

This so-called microdamage triggers short-term inflammation, accounting for the swelling and soreness. 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.

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

In contrast, humans confined to bed for far shorter periods lose muscle at an alarming rate.47 After three weeks of bed rest, leg muscles can shrink up to 10 percent.

astronauts in the gravity-free environment of space can lose 20 percent of their muscle mass in just a week or two.49

the process of aging is not as ruinous for muscles as bed rest or spaceflight, but muscular atrophy—the gruesome technical term is “sarcopenia,” Greek for “loss of flesh”—is a major cause of disability and disease among the elderly.

aging does not put an end to muscles’ capacity to respond to resistance exercise; instead, modest levels of resistance exercise slow and sometimes reverse sarcopenia regardless of age thanks to the mechanisms we have already reviewed.

Most obviously, as muscle mass declines, people load their bones less, contributing to osteoporosis. This furtive disease occurs when bones become too frail to sustain the loads they incur, causing them to snap or collapse. Because weakened muscles lead to less physical activity, sarcopenia is also a risk factor for other conditions associated with inactivity, including heart disease and type 2 diabetes.

Finally and importantly, warding off sarcopenia in old age helps prevent depression and other mental health conditions.

Grown-up humans play more than adults of other species, and we fight far less often than other primates like baboons and chimpanzees.

We traded brawn for brains. Instead of relying on speed, power, and strength, humans evolved to cooperate, use tools, and solve problems creatively.

According to Wrangham, humans differ from other animals, especially our ape cousins, in having exceedingly low levels of reactive aggression but much higher levels of proactive aggression. We correspond to Rousseau in terms of reactive aggression and to Hobbes in terms of proactive aggression.

Chimpanzees sometimes engage in proactive aggression, but humans have taken planned, intentional forms of fighting to new heights such as ambushing, kidnapping, premeditated homicide, and, of course, war. Arguably, hunting and combative sports like boxing are also forms of proactive aggression.

Hunter-gatherer males also must cooperate more than males in other species. Men often hunt in small groups and frequently come home empty-handed. By sharing meat from successful hunts, hunters ensure there is enough food to go around every day. Hunter-gatherers also collaborate to take care of children and fend off predators.

decreased size dimorphism, increased cooperation between and among the sexes, and the importance of women’s roles in hunter-gatherer societies have led anthropologists to speculate that humans have been less aggressive since the origins of the genus Homo.

In males today, elevated testosterone contributes to not only higher libidos, more impulsivity, and more reactive aggression but also bigger browridges and larger faces.34 Another molecule that possibly affects facial masculinization is the neurotransmitter serotonin, which reduces aggression; less masculinized faces are associated with higher levels of serotonin.35

Over generations of breeding, farmers have reduced the aggressiveness of these and other animals by selecting for lower levels of testosterone and higher levels of serotonin.36 Correspondingly, many domesticated species have smaller faces.

Many scientists are testing the idea that humans also self-domesticated.40 If so, I’d speculate this process involved two stages. The first reduction occurred early in the genus Homo through selection for increased cooperation with the origins of hunting and gathering. The second reduction might have occurred within our own species, Homo sapiens, as females selected for less reactively aggressive males.

There is an entire field of research, hoplology (from the Greek word hoplos for a plate-armored animal), that studies martial arts, stage combat, and the use of weapons.

Like chimpanzees, early hominins must have employed simple wooden tools, but a seismic shift happened sometime between 3.3 and 2.6 million years ago with the invention of stone tools, about the same time as the oldest evidence for meat eating.

Humans are the only species capable of throwing overhand fast and on target. Actually, only humans (male and female) who practice.

Consider fire and clothing. With these inventions, hominins were able to move into new, colder environments that then permitted selection for features like lighter skin away from the tropics.

Since the bow and arrow was invented 100,000 years ago, it has probably been less advantageous to be big and reactively aggressive.67 Wouldn’t it be ironic if the evolution of projectile weapons helped domesticate humans?

it is primarily among domesticated species that adults play, and among the many reasons humans in every culture play sports, one is to teach cooperation and learn to restrain reactive aggression.

humans are physically weaker than our ancestors not because we evolved to fight less but because we evolved to fight differently: more proactively, with weapons, and often in the context of sports. Along the same lines, we didn’t evolve to do sports to get exercise. As a form of organized, regulated play, sports were developed by each culture to teach skills useful to kill and avoid being killed as well as to teach each other to be cooperative and nonreactive.

If there is one physical activity that most fundamentally illustrates the central point of this book—that we didn’t evolve to exercise but instead to be physically active when necessary—it is walking. Average hunter-gatherer men and women (Hadza included) walk about nine and six miles a day, respectively, not for health or fitness but to survive.

This “swing phase” of a stride is primarily powered by your hip muscles. Your leg’s pendular action flips, however, at the end of the swing phase when your foot collides with the ground. At this instant, your leg becomes an upside-down pendulum whose center of rotation is the ankle. In essence, your leg becomes a stilt during this “stance phase” of the stride.

During the first half of the stance phase, muscles vault your body up and over that leg, elevating your center of mass about two inches (five centimeters). That upward lift expends calories but stores potential energy, just as if you were to raise this book. Then during the second half of stance, your body converts that potential energy to kinetic energy by falling downward and forward, as if you were to drop the book. Eventually, your swing leg collides with the ground, halting your body’s fall and starting a new cycle.