The most common reason for muscle fatigue is the lack of enough to meet the metabolic demands placed on the muscle.

Oxygen

If the oxygen remains low lactic acid accumulates resulting in muscle fatigue. The demand for energy increase, glucose is then broken down and the hydrogen amount accumulates. Oxygen is the final acceptor for Hydrogen. If there is not enough oxygen hydrogen will bind to the pyruvic acid and will start the process of fermentation that will lead to accumulation of lactic acid. This will also develop oxygen debt.