Originally published in 1971, this is an account of the centuries of experiment and speculation that have led to our understanding of how muscles work. The book traces all the developments in the field since 1600 and devotes special attention to the breakthroughs made in the last century. It considers the nature of the muscle machine and its fuel, as well as the intricate regulation of energy supply under different conditions. It also examines the very varied kinds of muscle and the effects of some diseases on their structure and function. Finally it shows how the energy metabolism first elucidated for the mechanical work of muscle has been found to explain most of the other kinds of work done by cells, as in light production, secretion, ionic transport and electrical discharge. The biochemistry of muscle, carbohydrate metabolism and phosphorylations was the author's chosen field of research for over forty years.