This book addresses how the general principles of biology influence the human capacity for locomotion, and, conversely, how understanding the nature of muscular activity might provide insights into the basic nature of living beings. Through a series of essays, the book relates the evolutionary basis of animal locomotion to recognizing the determinants of exercise capacity. While raising more questions than providing answers, the discussions will assume that without knowing the correct questions to ask, the answers will not be forthcoming. At the root of this book lies the central query: what is it that separates the principles governing the function of living beings from those that dictate the inanimate world? The discussions here address this issue from the expectation that clues to the answer can be obtained through understanding adaptations to the stresses imposed by physical exercise. As such, the book provides thought-provoking analyses of the biological basis of locomotion that will stimulate future efforts to understand these phenomena.