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

by Daniel E. Lieberman

the English word “exercise” was first used in the Middle Ages to connote arduous labor like plowing a field.1 While the word has long been used to denote practicing or training to improve skills or health, to be “exercised” also means to be harassed, vexed, or worried about something.

many of our beliefs and attitudes about exercise are myths

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

typical hunter-gatherers are about as physically active as Americans or Europeans who include about an hour of exercise in their daily routine.

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. Strength and power are not independent, but there is some trade-off between the two: a strong woman may be able to lift a cow above her head, but not rapidly. In contrast, a powerful woman cannot lift as substantial a load, but she can hoist less hefty things more swiftly and repeatedly.

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.

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. As we age, muscle fibers typically dwindle in size and number, and nerves degenerate.50 The result is a loss of strength and power. On average, grip strength in industrialized countries like the United States and the U.K. declines about 25 percent from the age of twenty-five to seventy-five.

As people lose strength, they become less able to perform basic tasks such as rising from a chair, climbing the stairs, and walking normally. Increasing feebleness in turn makes people even less active, leading to a vicious cycle of deterioration.

Humans are the only species capable of throwing overhand fast and on target.

According to one estimate, the Achilles tendon and the spring in the arch of the human foot together return about half the mechanical energy of the body hitting the ground.

the most vital and unique adaptation that enables humans to go the extra mile is our ability to perspire profusely. Running generates copious body heat, warming us pleasantly on cold days but turning dangerous in high heat and humidity.

humans evolved a magnificent cooling system by taking advantage of special water-secreting glands that most animals have only on their paws. Monkeys and apes have small quantities of these so-called eccrine glands elsewhere on their bodies, but we alone have five to ten million sweat glands all over our skin, especially on our heads, limbs, and chests.15 Sweating effectively turns the entire body into a giant, wet tongue. We also lost our fur, which helps air move along the skin’s surface without any barrier, thus enabling us to rapidly dump prodigious quantities of heat.

Humans who train for speed can increase the size of their fast-twitch fibers, but ordinary humans from every population are still slow-twitch dominated, and thus capable of more endurance than apes.

Exercise also causes cells to clean out damaged proteins, lengthen telomeres, repair DNA, and more. All in all, the modest physiological stresses caused by exercise trigger a reparative response yielding a general benefit, a phenomenon sometimes known as hormesis.