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. HIIT 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). The method thereby relies on "the anaerobic energy releasing system almost maximally". 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 reductions in the fat mass of the whole-body in young women comparable to prolonged moderate-intensity continuous training (MICT). 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. Sprint interval training (SIT) is an exercise conducted in a similar way to HIIT, but instead of using "near maximal" effort for the high-intensity periods, "supramaximal" or "all-out" efforts are used in shorter bursts. In physiological terms, "near maximal" means reaching 80–100% HRmax, while "supramaximal" means a pace that exceeds what would elicit VO2 peak. SIT regimens generally include a lower volume of total exercise compared with HIIT ones as well as longer, lower activity recovery periods and creates a greater homeostatic disturbance. Both HIIT and SIT fall into the larger class of interval training. Distinction between the two is not always maintained, even in academia: for example, Tabata describes his 170% VO2 max regimen as "supermaximal", but does not use the term SIT.
Procedure HIIT exercise sessions generally consist of a warm-up period followed by repetitions of high-intensity exercises separated by medium intensity exercises for recovery, then a cool-down period. The high-intensity exercise should be done at near maximum intensity. The medium exercise should be about 50% intensity. The number of repetitions and length of each depends on the exercise, but may be as little as three repetitions with just 20 seconds of intense exercise. The specific exercises performed during the high-intensity portions vary. Most of the research on HIIT has been done using a cycling ergometer, but other exercises like a rowing ergometer, running, stair climbing and uphill walking can also be effective. There is no specific formula for HIIT. Depending on one's level of cardiovascular development, the moderate-level intensity can be as slow as walking. A typical HIIT session uses a 2:1 work-to-rest ratio, for example, 30–40 seconds of hard sprinting alternated with 15–20 seconds of jogging or walking, repeated to failure. The entire HIIT session may last between four and 30 minutes, meaning that it is considered to be an excellent way to maximize a workout limited by time constraints. Use of a clock or timer is recommended to keep accurate times, the number of rounds, and intensity. An alternative form of HIIT, designed for heart rate training, involves a 30-minute period of cardio followed by 30 minutes of full-body resistance training to help maximize calorie burning. The idea is to combine aerobic exercise with intense weight and resistance training to achieve a high level heart rate for an extended period of time, in order to maximize strength and endurance.
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Peter Coe regimen In the 1970s a type of high-intensity interval training with short recovery periods was used by the athletics coach Peter Coe when setting sessions for his son Sebastian Coe. Inspired by the principles propounded by the German coach and university professor Woldemar Gerschler and the Swedish physiologist Per-Olof Åstrand, Coe set sessions involving repeated fast 200 meter runs with only 30 seconds recovery between each fast run.
Tabata regimen A version of HIIT is 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. The steady state group had a higher VO2max at the end (from 52 to 57 mL/(kg•min)). The Tabata group had achieved comparable aerobic improvements but only exercised 4 minutes per day on their 4 HIIT days compared to 60 minutes for the aerobic group. The Tabata group also started lower and gained more overall (from 48 to 55 mL/(kg•min)). Also, only the Tabata group had gained anaerobic capacity benefits (As Tabata's protocol is "supermaximal", his regimen technically falls into the scope of SIT).
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