High-Intensity Interval Training (HIIT) is a popular form of interval training that alternates between short bursts of high intensity with periods of low-intensity active rest. The high-intensity and low-intensity intervals can range anywhere from 10 seconds to 4 minutes long.

Shorter high-intensity interval ranges (10-30 seconds) engage the anaerobic system for energy. Longer high-intensity interval ranges ranges (more than 30 seconds) will engage the aerobic system for energy.

Generally, a HIIT workout (including warm-up and cool-down) can be completed within 45 minutes based upon the intensity of the workout.