Interval training or high intensity interval training (HIIT) is a great way to train your heart muscle and max out your fat burning metabolism.  When you cross your anaerobic threshold, usually around 80% of your max heart rate, it is because the energy demand of your activity has maxed out the capacity of your aerobic metabolism and needs more energy to continue at that pace.  Your aerobic metabolism doesn’t stop.  It continues at full capacity, breaking down fat, using oxygen, to provide as much energy as it can while your anaerobic metabolism ramps up, adding more stored glycogen to the available energy pool.

This type of training actually stimulates your aerobic energy system to work more efficiently and raises your anaerobic threshold- the level of work at which your bodies requires a majority of its energy to come from stored glycogen.  So both systems are trained with this type of exercise and you can create a lot of energy turnover in a shorter period of time than staying in an aerobic zone of 70-75% of max heart rate for the entire workout.