Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

The benefits of regular HIIT go well beyond its effect on muscles. Among other payoffs, HIIT increases the heart’s ability to pump blood efficiently by making its chambers larger and more elastic. HIIT also augments the number, size, and elasticity of arteries and increases the number of tiny capillaries that infuse muscles. HIIT further improves muscles’ ability to transport glucose from the bloodstream and increases the number of mitochondria within each muscle, thus supplying more energy.54 These and other adaptations lower blood pressure and help prevent heart disease, diabetes, and more.

Wondering how lions could develop such strength without using weights, Angelo figured they must get strong by “pitting one muscle against another.” He started to experiment with what he called “dynamic tension,” which today we call isometric training.

CrossFitters encourage each other to keep up a relentless, total-effort exercise session. At the end, everyone is physically spent but ecstatic. Apart from the impressive fitness effects of such routines, most CrossFitters believe they are participating in an ancient tradition of total-body athleticism based on the considerable strength that was supposedly necessary for human survival.

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.

Even today, 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.16

Recall that muscle is an expensive tissue, accounting for about one-third of a typical person’s body mass and one-fifth of her or his energy budget.

One of the many children who read du Chaillu’s book was Merian Cooper, who claims it inspired his 1933 movie, King Kong.

The same methods suggest that Neanderthal males averaged 5 feet 5 inches (166 centimeters) and 172 pounds (78 kilograms) while females were 5 feet 2 inches (157 centimeters) and 145 pounds (66 kilograms). Neanderthals were thus shorter but heavier than most humans today.

In general, the more we load our bones, especially when we are young, the thicker they become.

This hypothesis is relevant to our discussion of strength because testosterone also helps build muscle, which is why some athletes use it illegally.

Charlotte’s heroic deed is an example of hysterical strength, the ability of everyday people to muster superhuman feats of muscle in life-or-death situations. In such emergencies, the body releases massive amounts of adrenaline and cortisol, allowing the heroine to maximally contract every muscle fiber in her body.

Regardless, most of us are stronger than we think and never achieve our full potential because the nervous system sensibly inhibits us from going all out, thus tearing muscles, breaking bones, and possibly killing ourselves.

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.

If you are curling the weight upward by flexing your elbow, your biceps muscle is generating force while shortening, technically known as a concentric muscle action. Concentric contractions are the primary means by which muscles move us.

Muscles, however, don’t always shorten. If you hold the weight steady without moving it up or down, your biceps will still try to shorten but won’t actually change its length, an isometric muscle action.

it is even harder to lower the weight very slowly by extending your elbow. This sort of eccentric muscle action requires y...

Will your biceps get stronger if you focus on concentric, isometric, or ec...

Concentric contractions are critical for movement, but as Charles Atlas supposedly intuited in the Brooklyn Zoo, they are generally less potent for building muscle...

This happens because you are making your biceps generate more force than it can easily handle against a resisting force (the weight), literally tearing it apart at the microscopic level.

This so-called microdamage triggers short-term inflammation, accounting for the swelling and soreness.

You do have to repeatedly stress your muscles beyond their customary capacity, but shredding them isn’t always necessary to turn on the genes that promote growth.

If you primarily want to gain strength, you’ll get the most bang for your buck by slowly doing a few demanding repetitions of weights that require eccentric or isometric contractions.

That said, if you are more interested in power and endurance, you’ll derive more benefit from multiple sets of fifteen to twenty rapid concentric repetitions on less demanding weights with only brief rests between sets.

In contrast, humans confined to bed for far shorter periods lose muscle at an alarming rate.47 After three weeks of bed rest, leg muscles can shrink up to 10 percent.

Even worse, astronauts in the gravity-free environment of space can lose 20 percent of their muscle mass in just a week or two.

As we age, muscle fibers typically dwindle in size and number, and nerves degenerate.50 The result is a loss of strength and power.

Sarcopenia is a silent epidemic of aging that needs more attention, especially because debilitating declines in muscle function and capacity are largely preventable.

modest levels of resistance exercise slow and sometimes reverse sarcopenia regardless of age thanks to the mechanisms we have already reviewed.

Most obviously, 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.

Happily, numerous studies confirm that non-extreme levels of resistance exercise confer significant metabolic and cardiovascular benefits, including improving muscles’ ability to use sugar and lowering levels of harmful cholesterol.

Its most recent expert panel’s review of the evidence suggests I 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.

Shopping carts, baby carriages, wheels on suitcases, forklifts, and other devices emancipate us from having to lift or carry anything anymore. Thus to get resistance exercise, we do bizarre things like repeatedly lift weights in the gym.

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.

According to Wrangham, humans differ from other animals, especially our ape cousins, in having exceedingly low levels of reactive aggression but much higher levels of proactive aggression.

Yet nonreactive adult humans can excel at purposeful, planned forms of hostility. This kind of proactive aggression is characterized by predetermined goals, premeditated plans of action, attention to the target, and lack of emotional arousal.

Arguably, hunting and combative sports like boxing are also forms of proactive aggression. And, importantly, hunting and other forms of planned aggression are utterly different psychologically from reactive aggression.

Violent criminals, ruthless dictators, torturers, and other proactive aggressors can simultaneously be loving spouses and parents, reliable friends, and patriotic fellow citizens who remain utterly calm and pleasant in situations that would send a chimpanzee or a toddler into a rage.

According to Lovejoy, early hominin females favored males who were better at walking upright and thus better able to carry food with which to provision them. To entice these tottering males to keep coming back with food, females encouraged exclusive long-term monogamous relationships by concealing their menstrual cycles and having permanently large breasts (female chimps advertise when they ovulate with eye-catching swellings, and their breasts shrink when they are not nursing). Put crudely, females selected for cooperative males by exchanging sex for food.

If I had to fight other guys without weapons to get a girlfriend or wife, I would have a strong advantage if I were as big as possible, and I’d have little hope of passing on my genes if I were tiny. Unsurprisingly, whenever species have high levels of male-male competition, selection drives up body size in just males.

It is widely acknowledged, however, that sports differ from play in one key respect: whereas play is unorganized and unstructured with no particular rules or outcomes, sports are competitive physical activities between opponents according to established rules and criteria for winning.70 By this definition, some pastimes that require little strength or fitness are classified as sports including darts and bowling.

As with war, suppressing reactive aggression and following rules are fundamental to most sports. Indeed, sports might have evolved as a way to teach impulse control along with skills useful for hunting and controlled proactive fighting.

among the many reasons humans in every culture play sports, one is to teach cooperation and learn to restrain reactive aggression.

From an evolutionary perspective, individuals may also be drawn to sports because they can improve their reproductive success.

As a form of organized, regulated play, sports were developed by each culture to teach skills useful to kill and avoid being killed as well as to teach each other to be cooperative and nonreactive. Sports took on the role of providing exercise only when aristocrats and then white-collar workers stopped being physically active on the job.

During this period of rapid climate change, the rain forest that covered much of Africa shrank and split into thousands of fragments interspersed with drier, open woodlands. For apes living in the depth of the rain forests, life went on as usual, but those at the margins of the forest must have faced a crisis. As woodlands replaced the forest, the fruits that dominated their diet became less abundant and more dispersed. They had to travel farther to get the same amount of food. 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. However, because these apes still benefited from using their long arms, fingers, and toes to climb trees, natural selection evidently favored those who walked efficiently without compromising their ability to still climb effectively. The solution was bipedalism.

Individuals who could still scamper nimbly up and down trees but also had hips, spines, and feet that helped them save hundreds of calories a day by walking upright probably had higher reproductive success. Despite being slower and less stable on two legs, over many generations these apes became gradually better at walking upright until eventually they were a new species. We are their descendants.

The forces that drove our ancestors to walk upright eons ago may seem irrelevant today, but they aren’t. For millions of years until the postindustrial era, our ancestors had to walk something like five to nine miles every day to survive. We evolved to be endurance walkers. Yet, like our ancestors, most of us retain a deep-seated drive to spend as little energy as possible by walking only when necessary. That instinct to conserve calories points to another key difference between walking today and in the past: how much we carry things like babies, food, fuel, and water.

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

Evidently the problem was serious enough to lead to selection on the female spine. As Katherine Whitcome, Liza Shapiro, and I showed, 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.

Reason why women do not have back pain.

Biological systems such as bodies are messy, and anyone who has struggled to lose weight knows that simple theories rarely apply to the convoluted realities of weight loss. What works for one person fails for another, and while many people successfully shed pounds when they start a new weight-loss plan, satisfaction often turns to frustration as the initial rate of weight loss diminishes and then reverses.