Describe a situation that could cause lactic acid to build up in human muscle cells?

Anaerobic exercise is one situation that would cause a buildup of lactic acid in human muscle cells. As the cells switched over from aerobic respiration to fermentation in order to produce ATP, lactic acid would be produced in relatively large quantities as a byproduct. This allows NADH and FADH2 electron carriers to be oxidized without the electron transport chain and would allow Glycolysis to continue.