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Interval training (or high-intensity interval training or HIIT) involves performing repeated bouts of high and low intensity runs. The length and duration of the high intensity work bout will depend on what you are trying to accomplish (ie. your goals).

For instance, a longer work interval requires greater involvement of aerobic energy production. This is occurs because longer intervals need to slightly lower in intensity (or speed) than shorter ones. After all, for how long do you think you can possibly sprint? Not much longer than maybe 30 seconds.

On the other hand, shorter, more intense intervals, such as sprinting, involve a greater amount of anaerobic energy production. This is the energy system that gives you a lot of ATP (or energy) in a short amount of time but it is also an energy pathway that fatigues quickly and produces lactic acid in your muscles. That’s why higher intensities can only be sustained for a short amount of time.