This is becoz after the heavy exercise there is less amount of oxygen in the body. This leads to break down of pyruvates into lactic acid and energy. So due to the action of lactic acid the sportsmen feel cramp.
                                  in less
                                 amount of O2    
Glucose------>Pyruvate-------------------------> Lactic acid + Energy
1 5 1
This is because when they are excercising in lack of oxygen in the body bacuse all the oxygen is used up than lacti acid id formed this causes cramps in the body