Muscles rely on oxygen for energy, according to Men's Health. The energy oxygen provides is necessary for muscles to contract efficiently during work. With inefficient contracting, muscles are not as effective at performing physical work.
During periods of heavy lifting or extreme muscle strain, muscles do not get oxygen quickly enough to provide energy. In these cases, the body must convert a substance called pyruvate into a substance called lactate, reports Scientific American. Too much reliance on lactate can lead to high levels of acidity in muscles and disruption of metabolites. When muscles have adequate oxygen, a person performs aerobic exercise for extended periods without trouble. Production of lactate only generates enough energy for one to three minutes of anaerobic activity.