What is a byproduct of energy production in the body that at high levels causes muscle fatigue?

The byproduct of energy production in the body that at high levels causes muscle fatigue is lactic acid. Lactic acid is produced when the body does not have enough oxygen to meet its energy needs. This can happen during intense exercise or when the body is under stress. Lactic acid can cause muscle fatigue because it interferes with the body's ability to produce energy. It can also cause muscle pain and soreness.