When muscle cells break down glucose under oxygen deficient conditions (anaerobic respiration), they form lactic acid.

Here's why:

* Glycolysis: The first stage of glucose breakdown happens in both aerobic and anaerobic respiration. It produces pyruvate.

* Anaerobic Conditions: In the absence of oxygen, pyruvate cannot enter the mitochondria for further breakdown in the Krebs cycle and electron transport chain.

* Lactic Acid Formation: Instead, pyruvate is converted to lactic acid through a process called lactate fermentation. This is a way to regenerate NAD+ which is needed for glycolysis to continue.

Note: Lactic acid accumulation in muscle cells contributes to muscle fatigue and soreness.