Question

why do we get muscle cramps after heavy exersise

Answer 1

We get muscle cramps because during heavy exercise we undergo anaerobic respiration due that formation of lactic acid occurs. I.e.
Food + oxygen = energy(2 AtP) + CO2+ lactic acid

Answer 2

During heavy exercise , our body requires instant energy and it is fulfilled through anaerobic respiration , since anerobic respiration is a two step process , it gives faster energy.Thus, muscle cells break down glucose to produce lactic acid and energy. Alot of latic acid gets accumulated in our muscles and this causes muscle cramps.