Best Answer: Anaerobic respiration occurs in muscle cells when they do not have access to enough oxygen to complete aerobic respiration to generate all the ATP they need. In human muscle cells, the end product is lactate/lactic acid. Anaerobic respiration occurs when cells do not have enough Oxygen to undergo the process of aerobic respiration. In animal cells (like humans) anaerobic respiration happens mostly on muscle cells through a process called Fermentation that happens outside the Mitochondria.