I believe at the beginning, the body used up ATP energy in the muscles to support the enduring muscle contraction activities. Once the ATP was used up, the body had to switch to lactic acid fermentation in order to get energy needed support a continued muscle contraction. Muscles need energy to produce contractions, this energy is derived from adenosine triphosphate present in muscles. Muscles tend to contain only limited amount of ATP, therefore when depleted ATP needs to be resynthesized from other sources.