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.