Explain why short bursts of anaerobic exercise is possible (in mammals), but impossible at longer durations? (4)

In the absence of oxygen, the anaerobic glycolysis pathway can occur, which produces roughly 1/15th of the normal yield of ATP from aerobic respiration, as well as two molecules of NADH (reduced NAD). Similarly, in short bursts of exercise, phosphocreatine is broken down in muscles to provide a temporary supply of ATP. Both of these short-term supplies of ATP allow for short durations of exercise, but over long term, the build up of lactic acid from this pathway causes muscle fatigue, as it is toxic, resulting in the inability to move these muscles due to spasm and cramp, making longer term exercise impossible anaerobically.