When there is a lack of oxygen, anaerobic respiration takes place in the muscle cells that produces lactic acid due to which cramps are common. Lactic acid accumulation can cause lactic acidosis responsible for muscle fatigue.

Anaerobic respiration in muscles: Pyruvate is converted to lactic acid.

(Anaerobic respiration is a respiration which takes place in absence of oxygen and instead other electron acceptors are used.)