Oxygen is not present with anaerobic exercise. When we exercise anaerobically glycogen is used as fuel. Once all the glycogen has been depleted (usually in about two hours) you can expect to hit the proverbial wall. Endurance athletes avoid this performance buster with carbo loading before exercise (which when converted to sugar gives more energy) and supplements during exercise to sustain energy..

Exercise requires energy. When we exercise aerobically our bodies use glycogen and fat as fuel. This low to moderate level of exertion can be sustained over long periods. As you breathe more heavily with exertion carbon dioxide is expelled from your body. Lactic acid is not produced as it is with anaerobic exercise.