When working muscle cells are deprived of oxygen they produce?

Lactic acid

During intense exertion or activity involving the muscles, there arises a need for a sudden short term burst of energy. This need for energy is so demanding that it cannot be satisfied through the normal mechanism  whereby energy is supplied through aerobic respiration using oxygen.Therefore the muscle cells must rely on other mechanisms that do not rely on oxygen to fuel muscle contraction and this is where they resort to anaerobic respiration which uses pyruvate to play the role of oxygen as the final electron acceptor. In the process, pyruvate accepts an electron and is reduced to lactic acid in the muscle tissues.