For most of the past two million years, men and women needed strength for basically one reason: to not die. But as horse, machine and computer power slowly replaced man power, the meaning of strength began to evolve. Now being strong has more cultural and social implications that ever. The connection between mind and body, and body and self-esteem, is complicated and intimate in ways our forebears couldn’t imagine. Doctors and scientists are learning more about the health and longevity benefits of muscle, and even the government now recommends two sessions of strength training per week for everyone. As nerds and Marvel took over our lives and time became ever more scarce, muscle developed into a more desirable commodity. The result: swole Bezos. Today you could survive—thrive, even— without lifting anything heavier than your briefcase. So it raises the question: What is strength really for? There are many answers, and these researchers, powerlifters, soldiers, and everyday guys doing extraordinary things have some of them.