High-intensity interval training (HIIT), also called high-intensity intermittent exercise (HIIE) or sprint interval training (SIT), is a form of interval training, a cardiovascular exercise strategy alternating short periods of intense anaerobic exercise with less intense recovery periods, until too exhausted to continue. Though there is no universal HIIT session duration, these intense workouts typically last under 30 minutes, with times varying based on a participant’s current fitness level. HIIT workouts provide improved athletic capacity and condition as well as improved glucose metabolism. Compared with 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. 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.
HIIT exercise sessions generally consist of a warm up period, then several repetitions of high-intensity exercise separated by medium intensity exercise 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 running, stair climbing and uphill walking can also be effective. There is no specific formula to HIIT. Depending on one’s level of cardiovascular development, the moderate-level intensity can be as slow as walking. A common formula involves a 2:1 ratio of work to recovery periods, 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 thirty minutes, meaning that it is considered to be an excellent way to maximize a workout that is limited on time. Use of a clock or timer is recommended to keep accurate times, the number of rounds, and intensity.
A type of high-intensity interval training with short recovery periods was used in the 1970s 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 metre runs with only 30 seconds recovery between each fast run.
A version of HIIT was based on a 1996 study by 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 VO 2 max) 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% VO 2 max) 5 times per week. The steady state group had a higher VO 2 max at the end (from 52 to 57 mL/(kg•min)), but the Tabata group had started lower and gained more overall (from 48 to 55 mL/(kg•min)). Also, only the Tabata group had gained anaerobic capacity benefits. In the original study from 1996, participants were disqualified if they could not keep a steady cycling pace of 85RPM for the full 20 seconds of work. In popular culture, “Tabata training” has now come to refer to a wide variety of HIIT protocols and exercise regimens that may or may not have similar benefits to those found in Tabata’s original study.
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 VO 2 max) followed by 75 seconds of rest, repeated for 8–12 cycles (sometimes referred to as “The Little Method”). Subjects using this method training 3 times per week obtained gains similar to what would be expected from subjects who did steady state (50–70% VO 2 max) training five times per week. While still a demanding form of training, this exercise protocol could be used by the general public with nothing more than an average exercise bike. Gibala’s group published a less intense version of their regimen in a 2011 paper in Medicine & Science in Sports & Exercise. This was intended as a gentler option for sedentary people who had done no exercise for over a year. It included 3 minutes of warm-up, 10 repetitions of 60-second bursts at 60% peak power (80–95% of heart rate reserve) each followed by 60 seconds of recovery, and then a 5-minute cool-down.
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 VO 2 max followed by 30 seconds of rest allowed for the highest VO 2 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. Zuniga’s protocol has been implemented to great success by his students participating in Creighton’s Army ROTC program. Cadets completing the protocol twice a week saw greater improvements in APFT scores than in years past.