Among the various conditions that influence the energy exchange of the body, muscle work stands out as the most conspicuous. It needs no argument to convince one that all forms of muscular activity involve liberation of the energy stored up in the tissues of the body; and consequently that all work accomplished means chemical decomposition, in which complex molecules are broken down into simple ones with liberation of the contained energy, the energy exchange being proportional to the amount of work done. As we have seen, the basal energy exchange of the normal individual is ascertained by studying his heat production while at rest--best during sleep--without food, when involuntary muscle activity and heat production are at their lowest. The maximum energy exchange is seen in the individual at hard muscular work. Heat production is then at its highest, as can be ascertained by direct calorimetric observation; or, by studying the output of excretory products, which measure the extent of the oxidative processes from which comes the energy for the accomplishment of the work. As an illustration of the general effect of muscular work on the energy exchange of the body, we may cite a summary of some results reported by Atwater and Benedict,[37] the figures given being average results, from several individuals, and covering different periods of time. Though not strictly comparable in all details, they are sufficiently so to illustrate the main principle. [37] Atwater and Benedict: Experiments on the Metabolism of Matter and Energy in the Human Body 1900–1902. Bulletin No. 136, Office of Experiment Stations, U. S. Department of Agriculture, 1903, p. 141. HEAT GIVEN OFF BY BODY, INCLUDING FOR WORK EXPERIMENTS THE HEAT EQUIVALENT OF THE EXTERNAL MUSCULAR WORK.