Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

If we really want to know what ordinary humans do and think about exercise, we need to sample everyday people from a variety of cultures instead of focusing solely on contemporary Americans and Europeans who are, comparatively speaking, WEIRD (Western, educated, industrialized, rich, and democratic).

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.

Here’s another, startling way of thinking about these numbers: if you are a typical person who barely exercises, it would take you just an hour or two of walking per day to be as physically active as a hunter-gatherer.

but today less than 20 percent of jobs demand more than light levels of activity, an average reduction of at least a hundred calories per day.24 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.

If you are an average adult American male who weighs 180 pounds (82 kilograms), your rate of energy expenditure while resting quietly in a chair is approximately seventy calories per hour. This is your resting metabolic rate (RMR), so named because your resting metabolism comprises all the chemical reactions going on in your body while you are not being physically active.

If you weigh 180 pounds, your DEE is probably about twenty-seven hundred calories a day. Because we already learned your RMR is about seventeen hundred calories a day, that means nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism.

The key lesson to digest from the starving men’s dramatically lower resting metabolic rates is that human resting metabolisms are flexible. Most critically, resting metabolism is what the body has opted to spend on maintenance, not what it needs to

By some estimates, replacing an hour or two of daily sitting with light activities like walking can lower death rates by 20 to 40 percent.

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.

Long hours of commuting, a demanding desk job, being sick or disabled, or otherwise being confined to a chair can be stressful situations that elevate the hormone cortisol.

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. Short bursts of cortisol are natural and normal, but chronic low levels of cortisol are damaging because they promote obesity and chronic inflammation. Consequently, long hours of stressful sitting while commuting or a high-pressure office job can be a double whammy.

even if you are physically active and fit, the more time you spend sitting in a chair, the higher your risk of chronic illnesses linked to inflammation, including some forms of cancer.

but experiments that ask people to interrupt long periods of sitting even briefly—for example, just a hundred seconds every half hour—result in lower levels of sugar, fat, and so-called bad cholesterol in their blood.52 In turn, less circulating blood sugar and fat prevent inflammation as well as obesity.

To rid itself of waste, the brain evolved a novel plumbing system that relies on sleep. 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.16 These opened spaces also admit enzymes that repair damaged cells and rejuvenate receptors in the brain for neurotransmitters.

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.

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.

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

Fortunately, before our first few steps deplete the leg muscles’ scant supply of ATPs, they quickly tap into another ATP-like molecule known as creatine phosphate that also binds to phosphates and stores energy.

the precious short burst of fuel they provide gives muscles time to fire up a second energy recharging process: breaking down sugar.

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.

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.

We’ll begin with plyometric drills, also known as jumping-training exercises. A typical plyometric exercise might be a sequence of ten or so exaggerated skips in which you jump as high and fast as possible on one leg at a time raising the opposite knee along with both arms. With every landing, your hips, knees, and ankles flex, thus stretching your leg muscles and making it extremely challenging for them to contract explosively.50 These jumps rapidly fatigue your fast-twitch fibers. Next, do an equal number of butt kicks. Then try to repeatedly sprint a hundred or two hundred meters as fast as you can, thus demanding from your muscles rapid, forceful contractions and depleting their ATP and phosphate stores. Such HIIT workouts are hard and can make you sore for days.

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 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.

supplement my weekly quota of aerobic exercise with twice-weekly bouts of strength training that involve eight to ten different resistance exercises with ten to twelve repetitions each.59 Once I hit sixty-five years old, they recommend I increase my weight training to ten to fifteen repetitions.

In this regard, I am drawn to the philosophy of Jean-Jacques Rousseau and his followers who believe that our natural tendency is to behave morally and that many acts of human violence can be traced to corrupting cultural attitudes and conditions.6

Blank slate - see pinker

I thus also give credit to Thomas Hobbes and his followers who see human tendencies toward aggression as ancient, intrinsic, and sometimes adaptive.8 As detailed comprehensively by Steven Pinker, our species has become exponentially less violent only very recently thanks to social and cultural constraints, many fostered by the Enlightenment.9

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

When running in the heat, humans can sweat one liter per hour (sometimes even more), enough to keep cool while racing a marathon in 90°F—something no other animal can

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.19 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.20

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.52

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.

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.

One rarely considered parallel between running and dancing is how both can induce altered states. 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.

Libertarian paternalists favor nudges over coercion. Instead of forcing people to exercise, libertarian paternalists provide incentives.

Singapore does this well

A consensus suggestion is two sessions per week of muscle-strengthening exercises involving all major muscle groups (legs, hips, back, core, shoulders, and arms). Space these sessions several days apart to permit recovery, and they needn’t involve large weights but should include eight to twelve repetitions of each exercise tiring enough to make you want to stop; two or three sets of exercises are more effective than just one.

Stress elevates cortisol, which releases blood sugar, causes organ fat to accumulate, and facilitates inflammation.

Depression takes many forms including major depressive disorder, which is defined as more than two weeks of extreme sadness, loss of pleasure in formerly engaging activities, diminished energy, altered appetite and sleep, poor concentration, low self-esteem, and general purposelessness.

Between 2009 and 2017, rates of depression rose 47 percent among those aged twelve to thirteen and by more than 60 percent among those aged fourteen to seventeen.127

First, physical activity has many direct effects on the brain. One is to flood the brain with mood-altering chemicals. As a reminder, exercise heightens the activity of transmitting molecules in the brain, notably dopamine, serotonin, and norepinephrine.

Exercise also increases levels of other neurotransmitters, glutamate and GABA, that are often depleted in people with depression and anxiety.132 Additional mood-enhancing molecules turned on by exercise include endogenous opioids such as endorphins and endocannabinoids that inhibit pain and produce positive moods.133 Finally, as if this were not enough, remember that physical activity increases levels of BDNF and other growth factors that help maintain brain function.

The anthropologist Thom McDade has shown that people who grow up in these sorts of evolutionarily “normal,” unsterile environments have different inflammatory immune responses from those of us who grew up with high levels of sanitation. When humans who grew up in pathogen-rich conditions get an infection, their inflammatory response is sudden, strong, but short-lived. In contrast, the immune systems of people who grew up in highly hygienic environments with dishwashers, indoor plumbing, bleach, and lots of soap are different. When they get an infection, their inflammatory response tends to be much slower and less intense and lasts much longer. In other words, too much hygiene, especially when we are young, may predispose us to more chronic inflammation as we age.

Cf ellen and robert

The body’s main energy store are adenosine molecules with three phosphates attached: ATP (adenosine triphosphate). When ATPs break down, releasing energy, adenosine molecules slowly accumulate in the brain, helping make you drowsy. Caffeine keeps you awake by binding to the receptors in the brain that normally bind to adenosine, blocking their effect.