Question

after prolonged exercise we often develop muscle cramps and fatigue in body. why?

Answer 1

When We do hard exercise Oxygen demand increases due to energy consumption, when there is a lack of oxygen another method for breaking down pyruvate is taken it changes to lactic acid which causes muscle cramps.

Answer 2

During exercise we need a lot of oxygen. When there is a lack of oxygen we breathe anaerobically it produces lactic acid. The accumulation of lactic acid causes muscle cramps or fatigue.
Hope it helps :)