CBSE Class 10 Answered

This is due to accumulation of lactic acid in your muscles. 

When a person is doing very heavy exercise and the blood cannot supply enough oxygen, another sort of respiration called anaerobic respiration occurs. This converts glucose into energy without the need for oxygen.

Anaerobic respiration releases less energy than aerobic respiration. But during anaerobic respiration, lactic acid builds up in the muscles. High lactic acid concentrations are painful and felt as cramp.

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