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

by Daniel E. Lieberman

Westerners such as Americans and Europeans constitute only about 12 percent of humanity

Your PAL is calculated as the ratio of how much energy you spend in a twenty-four-hour period divided by the amount of energy you would use to sustain your body if you never left your bed.

most mammals in the wild have PALs of 3.3 or more, nearly twice as high as hunter-gatherers.

average PAL of industrialized adults in the developed world is 1.67,

That modest amount of unspent energy adds up to twenty-six thousand fewer calories spent over the course of a year, enough to run about ten marathons.

a typical gorilla troop travels only a mile per day.

nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism.

20 trillion calories consumed today by human beings on Earth, the majority are devoted to paying for the most basic needs of their bodies at rest.

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

brain and liver each consume about 20 percent of your resting metabolism,

your muscles expend 16 to 22 percent of your res...

resting is not just a state of physical inactivity.

standing costs about 8 to 10 percent more calories than sitting quietly in a simple desk chair.

Organ fat in moderate quantities (about 1 percent of total body weight) is thus normal and beneficial as a short-term energy depot for times when we need rapid access to a lot of calories such as when we walk or jog a long distance.

All bloated fat cells are unhealthy, but swollen organ fat cells are generally more harmful than subcutaneous fat cells because they are more metabolically active and more directly connected to the body’s blood supply. So when organ fat cells swell, they ooze into the bloodstream a great many proteins

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.

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.

By remaining inert for hour upon hour, our bodies never extinguish that faint inflammatory fire that may otherwise be smoldering in the background.

An even more vital function of sleep for the brain is janitorial.

During NREM sleep, specialized cells throughout the brain expand the spaces between neurons by as much as 60 percent, allowing cerebrospinal fluid that bathes the brain to literally flush away this junk.

vulnerable prey animals tend to sleep less than the carnivores that want to eat them.20 Perhaps “the wolf shall dwell with the lamb and the leopard will lie down with the

For this reason, our sleep-wake states are modulated by a second system that is tightly linked to activity levels. This homeostatic system functions like an hourglass that counts how long we’ve

more sleep pressure we accrue from the accumulation of molecules such as adenosine left behind when the brain expends energy.

Although it is impossible to see, everyone including Bolt is now slowing down slightly, and the racers know that victory will go to whoever slows the least.

Because speed is the product of stride length and stride rate

a typical hundred-meter race, Bolt took only forty steps, whereas the rest of the field took about forty-five.

Cheetahs in the wild also run for a maximum of about thirty seconds before slowing down.

two-legged humans can run only half as fast as similar-sized four-legged animals. Greyhounds are about twice as fast as elite sprinters.

Primates’ cumbersome feet are ideal for grasping onto branches and climbing trees, but our stumpy legs diminish speed by shortening each stride.

instead of having one enormous battery that we recharge occasionally, our cells use millions of tiny organic batteries that we have to recharge constantly.

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

a human body stores in toto only about a hundred grams of ATPs at any given moment.

during glycolysis the leftover halves of each sugar, molecules known as pyruvates, accumulate faster than cells can handle. As

those hydrogen ions make muscle cells increasingly acidic, causing fatigue, pain, and decreased function.

using oxygen to burn a molecule of sugar yields a whopping eighteen times more ATP than glycolysis. But,

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.

maximal oxygen uptake, or VO2 max.

Strength is how much force I can produce; power is how rapidly I produce

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

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.

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

upper-body muscle mass is on average 75 percent greater in human males than females.

Average hunter-gatherer men and women (Hadza included) walk about nine and six miles a day,

Another cost-saving method is to carry things with a tumpline, a strap attached to the load that goes around the top of the head. Tumplines demand strong neck muscles and a forwardly bent neck and back.

constrained energy expenditure hypothesis)

exercisers might spend almost the same number of total calories per day as similar-sized but more sedentary individuals despite devoting more energy to being active.

Note that humans can run long distances above the trot speed of dogs, ponies, and sometimes even horses.

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.

speed is a function of how fast you move your legs (stride rate) times how far you travel with every stride (stride length),

good human runners have lower stride rates and similar stride lengths to a full-sized horse.