In the early sixties, a new breakthrough occurred in our understanding of muscle energetics. The author is in the fortunate position of being able to present these findings from the perspective of his own experimental experience, because he himself has worked out a large part of the necessary methodological effort to allow the exact confrontation of chemical changes with mechanical and thermal events, obtained reliably with the help of statistical analysis and planning of experiments. Since then, they have been confirmed, deepened and extended by many other researchers. His early report written in 1964 is updated by many notes and references up to 2018. This book is to be used not only by the experts and the teachers, but also by the younger experimenters as an introduction to this difficult field of muscle energetics.