Question

why do we get muscular cramps after heavy exercise

Answer 1

Electrolyte depletion and dehydration theory. It is widely believed that excessive sweating due to strenuous exercise can lead to muscle cramps. Deficiency of sodium and other electrolytes may lead to contracted interstitial fluid compartments, which may exacerbate themuscle cramping.
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Answer 2

When we are exercising, especially heavy excercise, our muscle cells constantly require energy. But for energy to be synthesised, constant and a fast supply of oxygen is needed. Therefore the muscle opt for the anaerobic mode of respiration which results in the formation of lactic acid. This lactic acid is what that causes cramps.