We have already discussed Lundsgaard's realisation in 1934 that phosphocreatine breakdown must be considered a recovery process, leading to the restitution of some unknown energy-rich substance. The evidence from his experiments on whole muscle seemed to rule out ATP as this substance, since ATP breakdown was observed only after stimulation to exhaustion, just before rigor supervened. Lohmann's experiments a little later on muscle extracts, however, bore the clear implication that ATP breakdown must precede phosphocreatine breakdown. The idea of ATP hydrolysis as the energy-yielding reaction closest to the muscle machine was greatly strengthened by Engelhardt & Lyubimova's discovery of the ATPase activity of myosin and the subsequent work on interactions in vitro of ATP and actomyosin gels. Since no decrease in ATP content of normally contracting, unexhausted muscle was observed, the confident assumption was generally made that resynthesis of the ATP used was too rapid to permit of measurement of its hydrolysis. This attitude was too complacent for although the assumption turned out to be right in the end, its rigorous proof was attended by extreme difficulties.

We shall consider first in this chapter the various ways and means which experimenters tried hoping to get light on the chemical reactions accompanying contraction, and the evidence which came out of such work for ATP dephosphorylation during a short series of twitches or a short tetanus.