Metabolism and the production of heat in muscles
Hill and Meyerhof investigated how muscle converted chemical to tension energy using preparations of frog muscle, which remains alive for hours, sometimes days, contracting when stimulated and then returning to rest.
Hill carried out thermo-electric experiments and found that muscles had a working phase, which did not require oxygen, and a phase of recovery which did. He showed that the muscle had two distinct states: resting (before work) and recovery (after work). The formation of lactic acid was the earliest known chemical process in muscle, but its function was unclear. Hill believed, controversially, that lactic acid may be vital to muscle function, and that lactic acid was not used up, but was converted to glycogen during the recovery phase.
Meyerhof used chemical methods to investigate muscle preparations. He found that lactic acid was formed from carbohydrates such as glycogen, and that the combustion of four lactic acid molecules leads to one being oxidized and three being turned back into carbohydrates. Hill and Meyerhof’s work created a new understanding of how muscle worked: lactic acid is required for the muscle fibers to contract, and gradually spreads throughout the muscle, until it is unable to relax between stimuli. The muscle is then exhausted and must convert the lactic acid back to glycogen.