Anaerobic metabolism occurs during anaerobic exercise when oxygen uptake is insufficient for aerobic metabolism. Energy production for use is less effective than lighter workouts, yet can be used to increase fitness.
Normally, the energy required for moderate exercise is from glycogen stores in your muscles. Aerobic metabolism is used for as long as possible; supporting you through your longer training runs provided you maintain a comfortable pace.
The glycogen metabolism in an anaerobic state is only 8% as effective as in aerobic glycogen metabolism. Meaning that if you are running hard, the energy available to you is a mere fraction of what would be available at a slower pace. Not only that, high levels of your glycogen stores are consumed and copious amounts of lactic acid produced.
This form of metabolism is triggered when your running intensity exceeds 70% of your maximum oxygen uptake. As you increase your workout intensity or power needed, you breathe harder and your heart rate increases to get enough oxygen for aerobic metabolism.
Eventually, you reach your maximum. At which point this metabolism takes over. Fast speeds can be maintained for short periods. Best used in speed training and for that final kick at the end of a race.