Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

Slowly and gradually, exercise switches from being a negative feedback loop in which discomfort and lack of reward inhibit us from exercising again to being a positive feedback loop in which exercise becomes satisfying.

She sent the website StickK.com a thousand dollars, pledged to walk four miles a day, and designated her husband as her official referee.

And yet there are several noncontroversial exceptions to the principle of not forcing people to exercise.

From a purely utilitarian perspective, how is requiring exercise different from mandating seat belt use? According to the National Transportation Safety Board, seat belts prevent approximately 10,000 deaths a year in the United States.29 According to the Centers for Disease Control, inadequate physical activity causes about 300,000 deaths a year in the United States—thirty times more.30 Worldwide, physical inactivity causes about 5.3 million deaths per year—about as many as caused by smoking.

The dilemma is that most people who struggle unsuccessfully to exercise also want to exercise.

It’s not anyone’s fault that we inherited tendencies (some of us more than others33) to avoid unnecessary physical activity but were born to a world in which physical activity is no longer required and increasingly hard to do thanks to commuting, desk jobs, elevators, shopping carts, streets without sidewalks, buildings without easily accessible stairs, and so on.

Nudges influence our behaviors without force, without limiting our choices, and without shifting our economic incentives.

Put out your exercise clothes the night before you exercise so you wear them first thing in the morning and are ready to go (alternatively, sleep in your exercise clothes).

Shoves are more drastic forms of self-coercion, along the lines of my friend’s walking to avoid sending money to the NRA.

Note that all of these methods share one essential quality: they involve social commitment.

In fact, skipping exercise is sometimes counterproductive for those who are pressed for time.

The challenge is to support and assist everyone at every level nonjudgmentally and in ways and degrees they find acceptable and rewarding.

Although there can be no optimal exercise prescription, physical activity nonetheless stimulates growth, maintenance, and repair mechanisms that build capacities and slow aging.

This study when published was the first unequivocal evidence for a powerful dose-response relationship between exercise and mortality.

Exercise is no panacea, but the more you exercise, the longer you are likely to live, and the effects of physical activity on longevity become vastly greater as we age.

By the 1990s, so many studies had accumulated that three major health organizations decided to convene expert panels to review the evidence and make recommendations. In 1995 and 1996, all three panels published essentially the same advice: to reduce the overall risk of chronic disease, adults should engage in at least 30 minutes of moderate-intensity exercise at least five times a week.

As you can see, the biggest reduction in mortality, about a 30 percent drop, is between sedentary individuals and those who exercise sixty minutes a week.

Because the analysis also examined exercise dose in terms of intensity, the study further concluded that half an hour of vigorous exercise and an hour of moderate exercise confer the same benefit.

In the end, the 2018 HHS panel concluded that some physical activity is better than none, that more physical activity provides additional health benefits, and that for “substantial health benefits” adults should do at least 150 minutes per week of moderate-intensity or 75 minutes per week of vigorous-intensity aerobic physical activity, or an equivalent combination of the two.

(Moderate-intensity aerobic activity is defined as between 50 and 70 percent of your maximum heart rate; vigorous-intensity aerobic activity is 70 to 85 percent of your maximum heart rate.)

The benefits of even a little exercise are substantial, but the benefits eventually level off.

I’d like to draw your attention to several noteworthy insights evident from this figure.

The first is the variation among the studies.

Second, despite this variation, the dose-response relationship between physical activity and mortality follows a common pattern.

When the researchers tabulated deaths among these groups over the succeeding twelve years, they found that joggers who ran slowly over moderate distances had 30 percent lower death rates than sedentary individuals, but the serious runners who ran the most and the fastest died at the same rate as the non-exercisers.

A number of studies have found that elite athletes, especially those who do endurance sports, live longer and require less medical care than nonathletes.

“these findings reinforce the notion that light to moderate doses of exercise have a substantial positive impact on health but that continued dose escalation appears neither incrementally better nor worse.”

Additional studies found lower levels of disease-fighting white blood cells in the bloodstream and saliva immediately following intense bouts of vigorous exercise.

These and other data led to the hypothesis that the energetic demands of extreme exercise create a temporary “open window” for infection.

For some time, doctors have noted that many competitive runners have CAC scores greater than 100 and assumed these patients were at elevated risk for heart disease.25 But these risk estimates are based on nonathletes and do not take into consideration the size and density of the plaques, the size of the coronary arteries around them, or the likelihood that the plaques will grow, detach, or do anything else that could cause a heart attack.

One massive analysis of almost twenty-two thousand middle-aged and elderly men found that the most physically active individuals had the highest CAC scores but the lowest risk of heart disease.

You also increase your risk of musculoskeletal injuries if you haven’t adapted your bones, muscles, and other tissues to handle the stresses of repeated high forces of Olympic-level weight lifting, playing five sets of tennis a day, running marathons, or overdoing some other sport that obsesses you.

Although people, mostly the privileged, have been exercising for the sake of health since at least the time of Socrates, few exercisers until recently planned what mixture of cardio and weights to do every week.

But after World War II, exercise gradually began to be medicalized. As evidence accumulated, doctors and medical scientists increasingly viewed being sedentary as a pathological condition, and exercise became a form of treatment.

As director of the U.S. Air Force’s Aerospace Medical Laboratory in San Antonio, Cooper was charged with training astronauts to overcome the muscle-depleting, bone-wasting effects of being in gravity-free space.

As he forced astronauts to walk, run, cycle, and swim, he developed a point system that ultimately turned into a twelve-minute test that measured cardiorespiratory fitness. Cooper published his test and the science behind it in a 1968 book titled Aerobics (a term he coined) that became an international best seller and was a major impetus behind the 1970s fitness boom.33 To this day, when people think of exercise, they usually have in mind sustained moderate-intensity aerobic exercise. Because that’s a mouthful of jargon, let’s use the term “aerobic exercise.”

Aerobic exercise is sustained physical activity fueled by burning oxygen. The key metrics are heart rate and oxygen use. By convention, aerobic exercise elevates your pulse to between 50 and 70 percent of maximum (most people’s maximum heart rate is between 150 and 200 beats per minute depending on fitness and age).34 Another way to ...

Thousands of studies since 1968 have firmly established the many diverse benefits of aerobic exercise.

Because the fundamental challenge of aerobic activity is to deliver more oxygen at a faster rate to muscles and other organs, this demand stimulates the chambers of the heart to grow stronger, more capacious, and more elastic.

And aerobic exercise raises so-called good cholesterol (HDL) and lowers so-called bad cholesterol (LDL) and circulating fats (triglycerides).

Within muscles, it increases the number of mitochondria, promotes the growth of muscle fibers, and increases their ability to store carbohydrates and burn fat. In terms of metabolism, it burns harmful organ fat, improves the body’s ability to use sugar, lowers levels of inflammation, and beneficially adjusts the levels of many hormones including estrogen, testosterone, cortisol, and growth hormone.

Short bursts of intense cardio elevate heart rate and oxygen consumption close to their upper limit, usually above 85 or 90 percent of maximum rate.

Athletes have long known that repeated surges of this intensity, termed high-intensity interval training, are an effective way to improve performance.

In recent years, HIIT has also gone mainstream as exercise scientists started to study, appreciate, and laud its many potential health benefits for ordinary people.

HIIT stresses the cardiovascular system more acutely than moderate-intensity aerobic exercise, it can yield rapid, dramatic benefits.

It bears repeating that when working against substantial loads, muscles can shorten (concentric contractions), but they are more stressed and grow larger and stronger in response to forceful contractions in which they stay the same length (isometric contractions) or stretch (eccentric contractions).

In the eighteenth century it was fashionable to lift church bells that were silenced (made “dumb”) by having their clappers removed, hence the term “dumbbells.”

However you do them, resistance activities are critical for maintaining muscle mass, especially fast-twitch fibers that generate strength and power.

Resistance exercise can also help prevent bone loss, augment muscles’ ability to use sugar, enhance some metabolic functions, and improve cholesterol levels.

Yet everyone benefits from mixing it up because weights, moderate-intensity aerobic exercise, and HIIT have different, complementary effects on the body.