Howard G. Knuttgen of Biology, Boston University, 2 Cummington Department Street, Boston, 02215 Massachusetts, USA The relationship of the formation of lactate acid to skeletal muscle energy release in exercising humans was first explored by A. V. Hill and co-workers (2l, 22). The term "oxygen debt" was suggested by them to describe the excess oxygen consumption of recovery which they felt was closely related. A combination of their work and the earlier work of Krogh and Lindhard (35) re- sulted at that time in the belief that a certain amount of energy release during the transition from rest...