A person's anaerobic threshold has been reached when their body is under extreme conditions and the burning of oxygen can not keep up with the body's demands. At this point, the body begins to burn stored fuels, like sugars, instead of oxygen as the primary fuel. The word anaerobic literally means without oxygen..his anaerobic threshold is reached after long periods of activity or during a quick burst of activity such as a runner's sprint. The primary difference between the aerobic and anaerobic systems is in the byproduct produced.