Anaerobic means lack of oxygen. Your muscles are working in an oxygen deprived state. This causes your body to produce sugar-burning enzymes. You are expending energy faster than your body can replace it by metabolizing oxygen.

An anaerobic exercise is any activity that fails to meet the above 4 requirements. An aerobic activity can become anaerobic if the heart rate is elevated above the training zone for a long period of time. Any start/stop activities would qualify.

Examples: tennis, football, sprinting, skiing, weight training.