High-intensity interval training (HIIT) is a training protocol alternating short periods of intense or explosive anaerobic exercise with brief recovery periods until the point of exhaustion, which thereby relies on "the anaerobic energy releasing system almost maximally." The method involves exercises performed in repeated quick bursts at maximum or near maximal effort with periods of rest or low activity between bouts. The very high level of intensity, the interval duration and number of bouts distinguish it from aerobic (cardiovascular) activity, because the body significantly recruits anaerobic energy systems (although not completely to the exclusion of aerobic pathways). Although there are varying forms of HIIT-style workouts which may involve exercises associated with both cardiovascular activity and also resistance training, HIIT's crucial features of maximal effort, duration, and short rest periods (thereby triggering the anaerobic pathways of energy production) materially differentiate it from being considered a form of cardiovascular exercise. Though there is no universal HIIT session duration, a HIIT workout typically lasts under 30 minutes in total as it uses the anaerobic energy systems which are typically used for short, sharp bursts. The times vary, based on a participant's current fitness level. Traditional HIIT initially had been designed to be no longer than 20 seconds on with no more than 10 seconds off; however, intervals of exercise effort tend to range from 20 to 45 seconds but no longer than 75 seconds, at which point the aerobic system would then kick in.

HIIT workouts provide improved athletic capacity and condition as well as improved glucose metabolism. Compared with longer sessions typical of other regimens, HIIT may not be as effective for treating hyperlipidemia and obesity, or improving muscle and bone mass. However, research has shown that HIIT regimens produced significant reductions in the fat mass of the whole-body in young women. Some researchers also note that HIIT requires "an extremely high level of subject motivation" and question whether the general population could safely or practically tolerate the extreme nature of the exercise regimen.

TABATA REGIMEN:

A version of HIIT was based on a 1996 study by Ritsumeikan University Professor Izumi Tabata (田畑泉) et al. initially involving Olympic speedskaters. The study used 20 seconds of ultra-intense exercise (at an intensity of about 170% of VO2max) followed by 10 seconds of rest, repeated continuously for 4 minutes (8 cycles). The exercise was performed on a mechanically braked cycle ergometer. Tabata called this the IE1 protocol. In the original study, athletes using this method trained 4 times per week, plus another day of steady-state training, for 6 weeks and obtained gains similar to a group of athletes who did steady state training (70% VO2max) 5 times per week.

GIBALA EGIMEN

Professor Martin Gibala and his team at McMaster University in Canada have been researching high-intensity exercise for several years. Their 2010 study on students uses 3 minutes for warming up, then 60 seconds of intense exercise (at 95% of VO2max) followed by 75 seconds of rest, repeated for 8–12 cycles.

ZUNIGA REGIMEN

Jorge Zuniga, assistant professor of exercise science at Creighton University, set out to determine how to fit the highest volume of work and oxygen consumption into the smallest amount of time. He found that intervals of 30 seconds at 90% of power output at VO2 max followed by 30 seconds of rest allowed for the highest VO2 consumption and the longest workout duration at specified intensity. Alternative protocols considered included 100% of maximum power output on the same interval schedule, similar to the Coe regimen, and 90% of maximum power output for three minutes, similar to traditional interval training.