Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

Walking thus costs calories to raise the body’s center of mass in the first half of stance, then redirect it upward and forward from one step to the next, and to swing the arms and legs.8 While at least one foot is on the ground at all times during a normal walk, the key energetic principle that moves you forward is using your legs like pendulums to exchange potential and kinetic energy.

The other conspicuous and essential adaptation that helps humans walk upright is our uniquely long, curved lower back. Chimpanzees have stiff, short lower backs with usually three lumbar vertebrae, but humans typically have five lumbar vertebrae that create a backward curve. This curve positions the upper body above the hips; without it the torso would always be falling forward, requiring one to use muscles in the hip and back to keep it upright.

Because life is fundamentally about acquiring and using scarce energy to make more life, those better able to conserve energy would have had a reproductive advantage.

Dozens of studies have found that carrying loads less than half one’s body weight typically costs an extra 20 percent of the added weight, and when loads get really heavy, the costs increase exponentially.21 Carrying stuff while walking is generally expensive because we not only spend more calories to elevate more weight during the first half of stance but also have to spend more energy to redirect our body as a whole upward and forward at the end of each step. In addition, when we carry things, our muscles have to work harder to keep us and the load stable.

weights carried higher up in a backpack cost slightly less energy to hoist than those carried closer to the hips as long as you bend forward slightly.

two vertebrae create the curvature in the lower back in males, but by three million years ago australopith females had evolved to spread that curve more gently over three vertebrae and to have larger, more effectively oriented joints.

Further, low- to moderate-intensity activities like walking burn relatively more fat than carbohydrates (hence the “fat-burning zones” on some exercise machines).

In the mid-1960s, a Japanese company, Yamasa Tokei, invented a simple, inexpensive pedometer that measures how many steps you take. The company decided to call the gadget Manpo-kei, which means “ten-thousand-step meter,” because it sounded auspicious and catchy. And it was. The pedometer sold like hotcakes, and ten thousand steps has since been adopted worldwide as a benchmark for minimal daily physical activity.

animals adapted for running (“cursors” in biological parlance) have a special rubber-band-like structure, the nuchal ligament, at the back of their heads that acts like a spring to keep their heads still.

quadrupeds such as horses can run long distances only at a trot. So while horses, dogs, zebras, and antelopes can gallop faster than any human can sprint (the gray bars), they cannot gallop for more than a few miles before having to slow down to a walk or a trot, especially when it is hot.

Every time your legs and feet land on the ground during a run, these tendons stretch as your hips, knees, and ankles bend and your arch flattens. When the tendons recoil, the energy they store is returned to help catapult you back into the air. All animals adapted for running, from kangaroos to deer, have legs with long, springy tendons, but these tendons are short in our close relatives the African apes.

humans independently evolved long tendons such as the Achilles to help us run. 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.

in the endurance speed range humans jump as well as horses. If the horse speeds up, however, we are toast because sprinting humans cannot further extend their stride lengths and can speed up only by increasing stride rate, which is costly and inefficient. Within seconds, the horse will leave the human in the dust. But the horse will have to slow down eventually, often because of heat.

Like all mammals, we cool using the miracle of evaporation: when heat turns water into steam, the energy lost chills the skin underneath. Most animals take advantage of this natural refrigeration by panting—taking short, shallow breaths to evaporate saliva in their throats and on their tongues. As the water evaporates and cools the skin, blood in the veins just beneath is also cooled. This chilled blood then cools the rest of the body. Panting, however, suffers from two constraints.

FIGURE 24 Speed versus stride length (top) and stride rate (bottom) for a good human runner, greyhounds, and horses. E, endurance range; S, sprint range; T, trot; G, gallop. Note that humans closely match horses despite being about seven times smaller. (Modified from Bramble, D. M., and Lieberman, D. E. [2004], Endurance running and the evolution of Homo, Nature 432:345–52)

we alone have five to ten million sweat glands all over our skin, especially on our heads, limbs, and chests.

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.

ordinary humans, like horses and other endurance-adapted animals, evolved voluminous and elastic heart chambers that differ markedly from the smaller, thicker, stiffer hearts of apes and that enable us to efficiently squeeze large volumes of blood with every beat.

We also have an elaborated blood supply to the brain to help cool this vital organ during exercise.

the leg muscles of ordinary humans usually have 50 to 70 percent fatigue-resistant slow-twitch fibers, far more than chimpanzees, which range from 11 to 32 percent.

My favorite of these adaptations is the enlarged gluteus maxi-mus, the largest and plausibly most shapely muscle in the human body.

Adaptation for stability while running

Other adaptations to keep us stable during running include the ability to rotate our trunks as we pump our arms opposite our legs,22 and the previously mentioned nuchal ligament, which helps keep our heads from jiggling too much.

FIGURE 25 Good running form (on right) compared with common poor form (left).

I think most experienced runners and coaches agree on four key, related elements illustrated in figure 25: (1) not overstriding, which means landing with your feet too far in front of your body; (2) taking about 170–180 steps a minute; (3) not leaning too much, especially at the waist; (4) landing with a nearly horizontal foot, thus avoiding a large, rapid impact force with the ground.

Avoid overstriding. Get your knees up when you swing your legs forward so you land with a vertical shank and your foot below the knee, not too far in front of the hips. This prevents the legs from landing too stiffly and causing overly high breaking forces that slow you down. Step rate usually increases with speed, but experienced endurance runners generally take 170–180 steps a minute regardless of speed. They thus speed up economically by jumping farther (running is jumping from one leg to another), and a high step rate prevents overstriding. Lean forward slightly, but not too much at the waist. Too much upper-body lean requires you to spend more energy preventing your torso from toppling forward, and it encourages overstriding. Land gently with your feet nearly horizontal. If you are barefoot and don’t overstride, it is almost impossible not to land on the ball of your foot before letting down your heel in what is called a forefoot or mid-foot strike. Forefoot and mid-foot strikes usually don’t generate an impact peak on the ground—a rapid, large collisional force that is painful without shoes. Forefoot and mid-foot strikes also generate rotational forces (torques) that are lower in the knee but higher in the ankle, requiring strong calf muscles and Achilles tendons, which can lead to problems for people trying to transition to this way of running. If you change how your foot lands, do so gradually and build up strength.

Long periods of vigorous exercise stimulate mood-enhancing chemicals in the brain including opioids, endorphins, and, best of all, endocannabinoids (like the active compound in marijuana). The result is a runner’s or dancer’s high.

Fountain of Youth runs with sweat. That sweat, moreover, needs to keep flowing as we age.

“Men do not quit playing because they grow old; they grow old because they quit playing.”

This unique behavior is strongly linked to our species’ exceptional longevity in which we typically live beyond the age at which we cease to reproduce. Similarly long post-reproductive life spans are rare in the animal world.

“shadow of natural selection.”11 Theoretically, once an individual falls into this dreaded shadow, it becomes biologically and evolutionarily obsolete because natural selection should no longer act to combat natural processes of aging.

Human hunter-gatherers, in contrast, typically wean their offspring after three years and become pregnant again long before their little ones are able to feed or fend for themselves, let alone stay out of danger. A typical hunter-gatherer mother, for example, might have a six-month-old infant, a four-year-old child, and an eight-year-old juvenile. Because she is usually capable of gathering only about two thousand calories a day, she cannot get enough food to provide for her own substantial caloric needs, which exceed two thousand calories, as well as the needs of her several offspring, none of whom are old enough to forage on their own.12 She needs help.

Contrary to the widespread assumption that hunter-gatherers die young, foragers who survive the precarious first few years of infancy are most likely to live to be sixty-eight to seventy-eight years old.

Instead of becoming obsolete, middle-aged and elderly hunter-gatherers bolster their reproductive success by provisioning children and grandchildren, doing child care, processing food, passing on expertise, and otherwise helping younger generations. Once this novel cooperative strategy—the essence of the hunting and gathering way of life—started to emerge during the Stone Age, natural selection had the chance to select for longevity.

I propose a corollary to the grandmother hypothesis, which I call the active grandparent hypothesis. According to this idea, human longevity was not only selected for but also made possible by having to work moderately during old age to help as many children, grandchildren, and other younger relatives as possible to survive and thrive. That is, while there might have been selection for genes (as yet unidentified) that help humans live past the age of fifty, there was also selection for genes that repair and maintain our bodies when we are physically active. As a result, many of the mechanisms that slow aging and extend life are turned on by physical activity, especially as we get older. Human health and longevity are thus extended both by and for physical activity.

natural selection favored older individuals whose bodies stimulated repair and maintenance mechanisms in response to the stresses caused by these activities. And because middle-aged and elderly humans never had the opportunity to retire and kick up their heels, there was never strong selection to turn on these mechanisms to the same degree without the stresses caused by physical activity.

All Hadza women dig, but grandmothers dig more than mothers in part because they don’t have to nurse or spend as much time taking care of little ones.

One of the most reliable measures of age-related fitness is walking speed—a measure that correlates strongly with life expectancy.

Aging is inexorable, but senescence, the deterioration of function associated with advancing years, correlates much less strongly with age. Instead, senescence is also influenced strongly by environmental factors like diet, physical activity, or radiation, and thus can be slowed, sometimes prevented, and even partly reversed.

Menopause, for example, is a normal consequence of aging that happens when a woman’s ovaries run out of eggs. In contrast, type 2 diabetes occurs among some older people for reasons not intrinsic to the aging process itself but instead from factors like obesity and physical inactivity whose damaging effects accumulate with age.

The oxygen we breathe generates energy in cells but leaves behind unstable oxygen molecules with free, unpaired electrons. These reactive oxygen species (charmingly also called free radicals) steal electrons indiscriminately from other molecules, thereby “oxidizing” them. That theft sets off a slow chain reaction by creating other unstable, electron-hungry molecules obliged to steal electrons from yet more molecules. Oxidation burns things gradually and steadily. Just as oxidation causes metal to rust and apple flesh to brown, it damages cells throughout the body by zapping DNA, scarring the walls of arteries, inactivating enzymes, and mangling proteins. Paradoxically, the more oxygen we use, the more we generate reactive oxygen species, so theoretically vigorous physical activities that consume lots of oxygen should accelerate senescence.

Mitochondria, however, burn oxygen, creating reactive oxygen species that, unchecked, cause self-inflicted damage. When mitochondria cease to function properly or dwindle in number, they cause senescence and illness.

Another self-sabotaging reaction that results from being alive and using energy is browning, technically glycation. Browning occurs when sugar and protein react with the help of heat. Glycation gives cooked foods like baked bread and roasted meat their dark, aromatic, tasty exteriors, but what’s good for cookies is bad for kidneys. These reactions can damage tissues and produce compounds (advanced glycation end products) that stiffen blood vessels, wrinkle skin, harden the lenses in our eyes, clog up kidneys, and more. These and other kinds of damage then trigger inflammation.

Over time, tiny molecules glue themselves to the DNA in cells. These so-called epigenetic (on top of the genome) modifications can affect which genes are expressed in particular cells.31 Because environmental factors like diet, stress, and exercise partly influence epigenetic modifications, the older we are, the more of them we accumulate.32 Most epigenetic modifications are harmless, but the more you have for a given age, the higher your risk of dying.

The most common explanation for why exercise slows and sometimes turns back the gradual slide toward poor health is that physical activity prevents or ameliorates bad things that accelerate senescence. Top of the list is fat. Exercise staves off and sometimes reverses the accumulation of excess fat, especially belly fat, a chief cause of inflammation and other problems.

Exercise also lowers bloodstream levels of sugar, fat, and unhealthy cholesterol that slowly contribute to hardening of the arteries, damage proteins, and otherwise gum up the works.

exercise also improves cardiovascular function, lowers levels of stress hormones, revs up metabolisms...

For example, demanding physical activities can increase the strength of bones and muscles, increase cells’ abilities to take up glucose from the blood, and both augment and replace mitochondria in muscles. In addition, repair mechanisms sometimes overshoot the damage induced by exercise, leading to a net benefit.

while physical activity initially stimulates inflammation, especially via muscles, it subsequently causes muscles to produce an even stronger, more lasting, and more widespread anti-inflammatory response whose long-term effect is less inflammation not just in the affected muscle but elsewhere.45 As a result, physically active people tend to have lower baseline levels of inflammation.

FIGURE 29 The Stanford Runners Study. This study measured the probability of surviving in a given year (top) and disability (bottom) over two decades in a group of amateur runners over fifty in 1984 compared with a group of healthy but sedentary controls. After more than twenty years the runners had 20 percent higher survival rates and 50 percent less disability. (Modified with permission from Chakravarty, E. F., et al. [2008], Reduced disability and mortality among aging runners: a 21-year longitudinal study. Archives of Internal Medicine 168:1638–46)

For generation after generation, our ancestors young and old woke up each morning thankful to be alive and with no choice but to spend several hours walking, digging, and doing other physical activities to survive to the next day. Sometimes they also played or danced for enjoyment and social reasons. Otherwise, they generally steered clear of nonessential physical activities that divert energy from the only thing evolution really cares about: reproduction. The resulting paradox is that our bodies never evolved to function optimally without lifelong physical activity but our minds never evolved to get us moving unless it is necessary, pleasurable, or otherwise rewarding.