When we do vigorous physical exercise, cellular respiration is deprived of rich quantities of oxygen. So, Cell starts to produce energy without Oxygen ( called anaerobic respiration ) then, Glucose is broken into lactic acid and energy. The lactic acid then accumulates in muscles, causing cramps. 
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During physical exercises, our body muscles need more energy to carry out it need to respire more but it didn't get enough oxygen. so cells in our muscles start respiring anaerobically. in this process, food burns without the presence of oxygen. and it breakdown into lactic acid and energy and this accumulation of lactic acid causes muscle cramps.
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