The origin, or precursor, of lactic acid is glucose (a monosaccaride sugar). During contraction, there are chemical changes in striated muscles in which glucose breaks down into two components: phosphoric acid on one side, and lactic acid on the other. This occurs under the influence of clastase, which is a muscular enzymes complex acting as a catalyst. Thus, it can be said that acid lactic is the byproduct of muscular activity. The breaking down of glucose into lactic acid and phosphoric acid is called glycolysis.
The less oxygen there is in the muscle, the more lactic acid will be produced. As this chemical change in muscle occur in the total absence of oxygen during contraction, this phase is called anaerobic phase. When the muscular work is constant and continued in time, there is a muscular fatigue, that is to say that there is lactic acid accumulation in the muscle. However, under the presence of oxygen during relaxation, the amount of lactic acid is reduced; this phase is called aerobic phase.
Otto Fritz Meyerhof (1884-1951), a German biochemist, was the first scientist to study the chemical changes in muscle contraction.