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The hope was recently expressed (Hill 1937, p. 116) th a t with the development of a more accurate and rapid technique for muscle heat measurement, a much more consistent picture might emerge of the energy relations of muscles shortening (or lengthening) and doing positive (or negative) work. This hope has been realized, and some astonishingly simple and accurate relations have been found, relations, moreover, which (among other things) determine the effect of load on speed of shortening, allow the form of the isometric contraction to be predicted, and are the basis of the so-called " visco-elasticity" of skeletal muscle. This paper is divided into three parts. In P art I further developments of the technique are described: everything has depended on the technique, so no apology is needed for a rather full description of it and of the precautions necessary. In P art II the results themselves are described and discussed. In Part II I the " visco-elastic" properties of active muscle are shown to be a con sequence of the properties described in P art II. P a rt I. M e t h o d s o f m e a su r in g t h e e n e r g y LIBERATION OF MUSCLE (a) Galvanometer system. In the earlier stages of the present work a single moving-coil galvanometer of rather short period (0-2-0-3 sec.) was employed, as described in the previous paper (Hill 1937, pp. 115 and 131). Such a galvanometer, however, was far slower than the thermopile, and the interpretation of records obtained by it was difficult, or impossible, without laborious numerical analysis. The galvanometer could not be made quicker without reducing its sensitivity too m uch; amplification therefore was necessary if recording was to be quick enough. Now the rise of temperature which it was particularly desired to measure was th a t associated with shortening, which in a muscle 3 cm. long allowed [ 136 ]

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