Running, jumping, lifting, pushing, stretching — our bodies are naturally built to move and work, strengthen and adapt. Yet the gym is a 20th century phenomenon and 'working out'a concept that has produced one of the world's biggest industries: fitness. An industry whose changing ideals reflect society's cultural shifts on what it means to be fit, and how to have the perfect physique.

Why the Pain? What's the Gain? takes you on a guided tour of humankind's 3000-year wayward search for the perfect workout; from the dirt courtyards of the ancient Greek gymnasium to the birth of the modern gym in 19th-century Paris; to the weekend, beachside Olympic-caliber gymnasts of Santa Monica and Sydney. Daniel Kunitz seeks the answer to a simple yet profound question: Why do we work out?

We will listen in on the innovators and charlatans who lifted the multibillion-dollar business of gym-fitness into being. We will learn how the women's movement sparked an enormous exercise boom and tell the story not only of how we arrived at the big-box gym era but also of the last decade's acceleration into a new fitness frontier.

Daniel Kunitz puts himself through the grit and pain of the cross fit, parkour and tough mudder regimes, searching not just for new-found physique, but on a quest to understand the pain and more precisely the gain of working out.