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

by Daniel E. Lieberman

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.)

This homeostatic system functions like an hourglass that counts how long we’ve been awake, slowly building up pressure for us to sleep. The longer we stay awake, the more sleep pressure we accrue from the accumulation of molecules such as adenosine left behind when the brain expends energy. Then

In a trice,

And finally, adults who are persistently physically inactive are more vulnerable to suffering from insomnia.59

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.

The relatively stubby legs of primates are also relatively thick all the way down: we have fat ankles and big feet. Because dogs, horses, and ostriches have highly tapered hind limbs with small feet, their legs’ center of mass is closer to their hips, making their legs easier to swing. Finally, our primate feet lack claws, which act as natural cleats, or hooves, which act as natural shoes.

energy. Then as the animal’s hind limbs push off, the spine rapidly unbends, releasing elastic energy to help catapult it into the air and increase its stride length.11 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.12

In sum, humans have been slowpokes ever since that fateful transition seven million years ago when our ancestors became bipeds.

life on earth, are called ATPs (adenosine triphosphates). 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.14 The problem

The first process (called the phosphagen system) provides energy fastest but most fleetingly. As Bolt and I start running, our muscle cells contain barely enough ATPs to power a few steps. It seems inadvisable to stockpile so little ATP, but these organic batteries, despite being minuscule, store only one charge each, and they are too bulky and heavy for cells to manufacture and store in large quantities. You use more than thirty pounds of ATP during a one-hour walk and more than your entire body weight of ATP over the course of a typical day—an obviously impossible amount to lug around in reserve.15 Consequently, a human body stores in toto only about a hundred grams of ATPs at any given moment.

the precious short burst of fuel they provide gives muscles time to fire up a second energy recharging process: breaking down sugar. Sugar is synonymous with sweetness, but it’s first and foremost a fuel used to recharge ATPs through a process termed glycolysis (from glyco for “sugar” and lysis for “break down”).

During glycolysis, enzymes swiftly snip sugar molecules in half, liberating the energy from those bonds to charge two ATPs.19 Restoring ATPs from sugar doesn’t require oxygen and is rapid enough to provide almost half the energy used during a thirty-second sprint.20 In fact, 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 s...

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,...

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.

person. This important limit, illustrated in figure 12, is termed maximal oxygen uptake, or VO2 max. Having your VO2 max measured can be a little scary. Typically, you are fitted with a mask connected to a machine that measures oxygen consumption (as described in chapter 2) while you run on a treadmill.

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).24

To address this question, let’s look at a muscle in microscopic view. As you can see in figure 13, muscles are bundles of long, thin cells, called fibers. Each fiber, in turn, is made up of thousands of strands, fibrils, that in turn contain thousands of banded structures called sarcomeres (Greek for “flesh component”). Sarcomeres generate pulling forces because they are made of two key proteins—one thin, the other thick—that try to slide past each other like interlacing the fingers of your two hands. This contractile action occurs whenever a nerve sends an electrical signal to the muscle, causing tiny projections on the thick filaments to pull against the thin filaments much like a tug-of-war

All muscle cells work similarly, but the fibers of skeletal muscles that move our bones come in several varieties. 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. 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.

Like those of any animal, your muscles have a mixture of red, pink, and white muscle fibers whose percentages vary from muscle to muscle. You can see these variations in a cooked chicken. Whereas the bird’s legs and thighs have more red slow-twitch fibers to help them strut around all day, chicken breasts contain mostly white fast-twitch fibers for brief high-power activities like flapping their wings.

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.

So, if like Usain Bolt you have a preponderance of fast-twitch fibers, you have the potential to be fast without much endurance, and if like Frank Shorter you have mostly slow-twitch fibers, then you, too, can be a great marathoner but have no chance to win the hundred-meter dash. And if you are like most of us, you’ll be so-so at both.

osteitis

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.

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.

Recall that muscle is an expensive tissue, accounting for about one-third of a typical person’s body mass and one-fifth of her or his energy budget.

Du Chaillu also claimed to be the first European to observe gorillas in the wild. He describes these apes in his first book, Explorations and Adventures in Equatorial Africa (1861), as terrifying “half man, half beast” and the “King of the African forest.”

One of the many children who read du Chaillu’s book was Merian Cooper, who claims it inspired his 1933 movie, King Kong.

After a few trials, Suzette apparently managed to use her entire body to pull 1,260 pounds, about three to four times more than any burly football player could manage.

Bauman’s amateurish estimate of Suzette’s strength using an untested, uncalibrated, and probably inaccurate instrument is still cited regularly despite repeated failed efforts to replicate her feat. In 1943, a Yale primatologist named Glen Finch carefully replicated Bauman’s experiments on eight adult chimpanzees, none of whom could muster more strength than adult male humans.

William King formally defined the species Homo neanderthalensis in 1864, just five years after Darwin published The Origin of Species and three years after du Chaillu’s accounts, he left little doubt about his distaste for these savage antecedents “whose thoughts and desires…never soared above those of a brute.”

Charlotte’s heroic deed is an example of hysterical strength, the ability of everyday people to muster superhuman feats of muscle in life-or-death situations. In such emergencies, the body releases massive amounts of adrenaline and cortisol, allowing the heroine to maximally contract every muscle fiber in her body.

Since any interruption in blood supply might cause us to faint, perhaps fatally, high-resistance exercise requires the heart to generate high pressures that have to be withstood, especially by the heart itself and by the aorta. For this reason, as blood pressure shoots up, we instinctively inflate our chests and briefly hold our breath. 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

This sort of eccentric muscle action requires your biceps to fire as it lengthens. Will your biceps get stronger if you focus on concentric, isometric, or eccentric muscle actions?

Concentric contractions are critical for movement, but as Charles Atlas supposedly intuited in the Brooklyn Zoo, they are generally less potent for building muscle than eccentric and isometric muscle actions.40 Athletes, trainers, bodybuilders, and others interested in getting stronger therefore tend to incorporate plenty of eccentric and isometric kinds

Although “no pain, no gain” is a bedrock principle and mantra for serious weight lifters, you may be relieved to know you can strengthen your muscles without getting horridly sore and walking around like a mummy. You do have to repeatedly stress your muscles beyond their customary capacity, but shredding them isn’t always necessary to turn on the genes that promote growth.43 If 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

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.

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.

While modern weight lifting might amuse our hunter-gatherer ancestors, they’d probably be relieved to know that, like them, we don’t need to be more than moderately muscular to ward off sarcopenia and other associated diseases like osteoporosis.

—George Orwell, The Lion and the Unicorn (1941)

Even the most belligerent human groups ever studied engage in violence about 250 to 600 times less frequently than chimpanzees.

chimpanzee, however, you’d probably respond to my theft with instantaneous, uninhibited violence. Unless I were the dominant male in the troop, without pausing to think, you’d give me a thumping and retrieve your book. One widely reported case of this sort of reactive aggression that is only too common among chimpanzees involved an adult chimp named Travis who had spent his entire life peacefully as part of Sandra and Jerome Herold’s family. Then, in February 2009, at the age of fifteen, he flew off the handle after one of Sandra’s friends, Charla, picked up one of his favorite toys. Travis’s immediate and savage attack left Charla with no hands and without much of her face including her nose, eyes, and lips.11

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,

The slight corporeal strength of man, his little speed, his want of natural weapons, &c., are more than counterbalanced, firstly by his intellectual powers, through which he has, whilst still remaining in a barbarous state, formed for himself weapons, tools, &c., and secondly by his social qualities which lead him to give aid to his fellow-men and to receive it in return.17

Put crudely, females selected for cooperative males by exchanging sex for food. If so, then selection against reactive aggression and frequent fighting is as old as the hominin lineage.22

The biggest problem is that early hominin males appear to be at least 50 percent bigger than females.23 Lucy, the famous Australopithecus afarensis female from 3.2 million years ago, weighed slightly less than thirty kilograms, but males of her species weighed about fifty kilograms. This difference in body size, termed sexual dimorphism, is a reliable indicator of competition between males within species.

Hunting and gathering matter. Despite evidence that hunter-gatherers are no angels (by some estimates, almost one-third of male deaths in such societies arise from violence27), you simply cannot survive as a hunter-gatherer without being highly cooperative—far more than chimpanzees.

Whole-body scans show that males average 61 percent more muscle mass then females, with most of that difference in the upper body.30 Men’s extra brawn, moreover, is added during puberty, when testosterone levels shoot up, accelerating muscle growth in the arms, shoulders, and neck.31 In this regard, human men resemble male kangaroos, whose upper bodies also enlarge during adolescence to help them fight.32 Enhanced upper-body muscularity in male humans might also have been selected for hunting, but we cannot rule out aggression. The second fact is literally staring us in the face. Consider

The earliest H. sapiens males have smaller, less robust faces than Neanderthals and other non-modern humans, but truly lightly built, “feminized” faces don’t appear until less than 100,000 years ago.33 It is intriguing to hypothesize that these big faces reflect higher levels of testosterone during adolescence. 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 FIGURE 16 Side and front views of skulls from a male Neanderthal and male H. sapiens from different time periods.

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.