High Intensity Interval Training (HIIT) is a system of organizing cardiorespiratory training which calls for repeated bouts of short duration, high-intensity exercise intervals intermingled with periods of lower intensity intervals of active recovery. On a 1-10 scale of perceived exertion, high intensity can be considered anything over an effort level of 7. When using max heart rate (MHR) as a guide, high intensity can be considered exercising above 80% of MHR. Modes of HIIT can include outdoor activities such as running or cycling, or using equipment such as treadmills, elliptical runners, stair-climbers or stationary bikes. HIIT training calls for challenging work-rates such as sprints (whether on a bicycle or running) for short time frames lasting from thirty seconds to two minutes.What is a typical HIIT session like?A typical HIIT session would call for a warm-up of 5-10 minutes where the intensity gradually increases from a RPE of 3 to a RPE of 5. Once the body is warmed up, it is then time to begin the work intervals. The appropriate work to recovery ratio for HIIT is 1 minute of work to every 2 to 3 minutes of active recovery. Staying active during the recovery period allows the muscles to remove the metabolic waste and produce more energy for the next bout of high intensity exercise. Start with a lower number of work intervals and work up to doing 10-12 high intensity work intervals.An example of outdoor HIIT training would be running at the fastest phealthsite possible on a track for 200 meters, then jogging at a slower phealthsite for 400m (or twice the length of time required to run the 200 meters). An example of indoor HIIT training would be an indoor cycling class where the instructor has the class do hill climbs for two minutes working at a RPE of 7 or 8, followed by four minute flat road intervals working at an RPE of 4 or 5.What are the benefits of this kind of training?One of the major benefits of HIIT fitness is that using the appropriate work-to-recovery intervals can train the body how to become efficient at producing and using energy from the anaerobic energy system. This type of session can also train the body to effectively remove metabolic waste from the muscles between the work intervals. In addition, HIIT also serves as an effective way to increase VO2 max without having to run for long distances or periods of time. Due to the high level of intensity and the amount of time necessary to appropriately recover from the exercise session, it is recommended to do no more than two days of HIIT per week, allowing at least one full day of recovery between training sessions.