The answer is lactate. Muscle produce lactic acid during intense exercise. Its a metabolic byproduct that makes no contribution to exercise performance, it causes muscle fatigue and post-exercise muscle soreness. During intense exercise, there may not be enough oxygen available to complete the process, so a substance called lactate is made. The body converts this lactate to energy without using oxygen, But this lactate or lactic acid can build up in your bloodstream faster than you can burn it off.