Introduction Lifting a calf, turning a page, and every other voluntary movement happens by means of muscle. No matter how you think of yourself--strong or weak, large or small--you are substantially made of muscle. Most adults are made up of at least 30 percent muscle. Many of us are closer to 40 percent muscle, and a few of us exceed 50 percent. Muscle is also one of the body''s most plastic tissues, changing its size and properties based on people''s habits of diet and care, work and rest. Human skeletal muscle is a primary organ of metabolism, the chemical processes that sustain life. When a child is growing, when an adult is injured or sick, and as everybody executes the constant processes of cellular wear and tear and repair, proteins in muscle do the work of generation, healing, and regeneration. Muscle contraction is the basis of an extensive signaling network in the body, too.

Working muscles produce secretions called myokines that circulate to the brain, liver, heart, intestines, and other organ systems, regulating biological functions that make for thriving life. From early adolescence onward, the kind of muscular work shown in the myth of Milo and prescribed by Thomas DeLorme can yield a wealth of benefits. Progressive resistance exercise can build confidence and reduce anxiety; improve bone density, blood pressure, aerobic fitness, body composition, metabolic health, insulin sensitivity, depression, and sleep; prevent and treat type 2 diabetes and cardiovascular disease; reduce the risk of several types of cancer; increase resistance to injury; and decrease the likelihood of falls and of osteoporotic fracture. In old age, muscle increasingly decides who can live independently and who cannot. Your ability to stand and go where you want to go--your independence, autonomy, and agency--your effectiveness in the world--will depend on muscle, to the last day of your life. In recent decades, we have all witnessed what can happen when people make a practice of lifting weights. The bodies of public figures in many fields have been transformed: Derek Jeter, Serena Williams, and Cris tiano Ronaldo; Madonna, Beyoncé, and Taylor Swift; Marc Jacobs, Oprah Winfrey, and Jeff Bezos; Ruth Bader Ginsburg, Michelle Obama, and Volodymyr Zelensky. At the same time, a related change has happened in the general population.

Signs of the change emerge from almost thirty years of data collected by the Centers for Disease Control between 1988 and 2017, in surveys that asked American adults what forms of physical activity or exercise they spent the most time doing. Among more than fifty types of activities tracked by these surveys over that whole period--from bowling to fishing to running--the one that grew most popular most quickly was lifting weights. The number of people who said they lifted weights more often than they did other types of exercise increased by more than 34 percent in those years. But on the other hand, the absolute number of people who made lifting weights their main form of exercise remained low. That number grew barely more than 1 percent in thirty years--from 3.2 percent to 4.3 percent of the population. And national physical activity surveys in many countries find that vast majorities of people do little or no exercise of any kind that would strengthen their muscles.

The truth is, few of us take much active interest in our own muscles, except for the young and athletic, whose interest tends to be tied to relatively shortterm payoffs: winning games, or dates, or clicks. It can be easy to lose sight of muscle''s importance in every stage and every function of the widest range of lives because muscle is easy to typecast. Just hearing the word muscle can trigger instant thoughts of bodybuilders, or combatsport athletes like boxers and mixed martial artists, or elite soldiers in actual military combat, or influencers and models and movie stars, exuding erotic privilege. For many people, the word muscle can sound inherently sinister, because muscle was glorified in pseudoscientific theories and ideologies that have been used to justify sexism, racism, colonialism, authoritarian rule, and mass atrocities. Books that investigate muscle''s meanings to aesthetics, sex, violence, injustice, and oppression do valuable work. But this book does a different kind of work, focused on the longterm, existential significance of muscles in our lives: the unnerving fact that muscles--and our individual and collaborative abilities to exert muscular strength--modulate our power to act upon the world. In 1937, Charles Scott Sherrington, the Nobel Prize-winning British neurophysiologist whose lifework was to map the nervous system''s amalgamation with the muscular system, reflected that "the importance of muscular contraction to us can be stated by saying that all man can do is to move things, and his muscular contraction is his sole means thereto." If we updated the gendered language, a more accurate one line summary of muscle''s central role in our lives would be hard to devise.

And so, with a slight paraphrase--to open that statement wide, to invite everybody in--Sherrington''s words bear repeating: . all we can do is to move things, and our muscular contraction is our sole means thereto. Try to imagine: How would muscle look different to you, if you had never heard of Arnold Schwarzenegger or The Rock? Or if that''s not possible to imagine, try this instead: Think how the world could look different if every time you heard someone say muscle , the first person you thought of was not some big guy who had taken steroids, but your grandmother. Making that shift is one of the best things you can do for yourself and for the people you love. Status quo views of muscle, by contrast, keep people stuck in destructive zerosum games, pitting aspects of ourselves against each other--the superficial and the serious, brain versus brawn--even though significant evidence shows these conflicts have no legitimate basis in biology. The brain''s posterior cingulate cortex, the seat of empathy, self awareness, and emotional memory--which is also the first part of the brain to atrophy in Alzheimer''s disease, even before people show any signs of memory loss--"actually increases in size when you do weight lifting exercise," according to one of the researchers who discovered this, at the University of Sydney in Australia. For ages, though, most people have been raised on mindbody dualism, the notion that experience can be neatly divided into the physical and mental, or spiritual. Getting ourselves out of the rut requires some reflection.

Often attributed to René Descartes, the seventeenthcentury French philosopher, mindbody dualism has ancient roots. By the fourth century bc, in Athens, Plato taught that a person''s body, or so¯ma , contains and is activated by an incorporeal faculty, a kind of spirit, or psyche¯ , that is superior to the body. Plato said the body is like a tomb or prison for the soul. Mindbody dualism became a tenet of scientific medicine and of the medical dogma that moderate care of each--cultivating a sound mind in a sound body--is the key to health. One of history''s most influential doctors, Galen of Pergamon, who lived in the Roman empire during the second century ad, was fanatic about moderation. Galen denounced athletics as bad for health because athletic competition involved striving for excellence, the opposite of moderation, for the sake of winning a prize. Galen savaged those who disagreed with him, and he saved special venom for big men with lots of meat on their bones. Such athletes "do not even know that they have a soul," he wrote.

"For they are so busy accumulating a mass of flesh and blood that their soul is extinguished as if beneath a heap of filth, and they are incapable of thinking about anything clearly; instead they become mindless like the irrational animals." Ancient medicine''s antipathy to athletics developed in part because of muscle, Galen''s writings imply, and the prejudice endured. In World War II, it was an obstacle for Thomas DeLorme. When the doctor prescribed weight training to rehabilitate injured soldiers, medical colleagues disapproved. A few years later, DeLorme wrote that "the mere mention of large muscles provokes in most people, and especially those of the medical profession, a decided antipathy" because "almost everyone is bewildered and repulsed by the socalled body builder." In the 1950s and 1960s, when mainstream science showed that physical activity is imperative for everyone who wants to live a long and healthy life, positive messages about cardiovascular fitness were commonly joined with negative judgments about muscular fitness. One of the twentieth century''s most popular books about exercise, published in 1968, was Aerobics by Kenneth Cooper, then a thirtysevenyearold United States military physician. Aerobics denigrated muscular fitness and said that lifting weights was "like putting a lovely new coat of paint on an automobile that really needs an engine overhaul.

" Cooper articulated a common prejudice against weight training, based on cardiac concerns, that evidence would later refute. By the time he turned seventy years old, the author of Aerobics was committed to a regimen of lifting weights. Individuals can change more rapidly than institutions and cultures, however. Still today, the medical profession shows relatively little interest in muscle. There is no medical specialty for the treatment of muscle, few doctors routinely measure or assess pa.