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. HIIT is the concept where one performs a short burst of high-intensity (or max-intensity) exercise followed by a brief low-intensity activity, repeatedly, 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 successfully 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.
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High-intensity interval training can be described as an exercise session composed entirely of HIIT techniques, or as a component of an exercise plan. 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.
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Peter Coe regimen
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.
Tabata regimen
A version of HIIT was based on a 1996 study by Professor Izumi Tabata (???) 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, 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/(kgomin)), but the Tabata group had started lower and gained more overall (from 48 to 55 mL/(kgomin)). Also, only the Tabata group had gained anaerobic capacity benefits. It is important to note that 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.
Gibala regimen
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 (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% VO2max) 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.
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 maximum power output 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.
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.
Vollaard regimen
Dr Niels Vollaard at the University of Stirling proposed that when high-intensity intervals are done at 'all-out' intensities, associated health benefits plateau after performing 2 or 3 sprint repetitions. This led to the development of a 10-minute exercise routine consisting of easy pedalling interspersed with two 20-second 'all-out' cycling sprints. In a 2017 meta-analysis, Vollaard indeed showed that common protocols with as many as 6 to 10 repetitions of 30-second 'all-out' sprints do not improve aerobic fitness more than the '2x20-s' protocol. It is claimed that this short protocol may remove many of the drawbacks that make other high-intensity interval training protocols unsuitable for the general population.
In a BBC Horizon programme in February 2012, Jamie Timmons, professor of systems biology at the University of Loughborough, put Michael Mosley through this exercise bike regimen, but with three sprints instead of two. This was done three times a week for a total of 30 minutes of exercise per week (3 minutes of intense exercise), plus warm-up and recovery time.
Regimen comparison
Wood et al. compared High-intensity interval training of eight 1-minute bouts at 85% Wmax interspersed with a 1-minute active recovery at 25% Wmax v Sprint interval training of eight 30-second bouts at 130% Wmax interspersed with 90-second active recovery at 25% Wmax. (Total time matched at 24 mins including warm up & cool down). Their conclusion was "HIIT is the recommended routine" but "the magnitude of differences in various parameters between regimens was small; therefore, preference for either modality may be up to the individual".
Health effects
Cardiovascular fitness
A 2015 systematic review and meta-analysis of randomized controlled trials found that HIIT training and traditional endurance training both lead to significantly improved cardiovascular fitness in healthy adults ages 18-45 but greater improvements in VO2 max were seen in those participating in the HIIT exercise regimen. Another analysis also found that HIIT regimens of one month or longer effectively improve cardiovascular fitness in adolescents and lead to moderate improvements in body composition. Furthermore, a separate systematic review and meta-analysis of seven small randomized controlled trials found that HIIT (defined as four intervals of four minutes at 85-95% of max heart rate with three-minute intervals at 60-70% of max heart rate) was more effective than moderate-intensity continuous training at improving blood vessel function and markers of blood vessel health.
Cardiovascular disease
A 2015 meta-analysis comparing HIIT to moderate intensity continuous training (MICT) in people with coronary artery disease found that HIIT leads to greater improvements in VO2 max but that MICT leads to greater reductions in body weight and heart rate. A 2014 meta-analysis found that the cardiorespiratory fitness, as measured by VO2 max, of individuals with lifestyle-induced chronic cardiovascular or metabolic diseases (including high blood pressure, obesity, heart failure, coronary artery disease, or metabolic syndrome) who completed a HIIT exercise program was nearly double that of individuals who completed a MICT exercise program.
Metabolic effects
HIIT significantly lowers insulin resistance compared to continuous training or control conditions and leads to modestly decreased fasting blood glucose levels and increased weight loss compared to those who do not undergo a physical activity intervention. Another study found that HIIT was more effective than moderate-intensity continuous training at fasting insulin levels (31% decrease and 9% decrease, respectively).
Fat oxidation
A 2007 study examined HIIT's physiological effects on fat oxidation in moderately active women. The participants in the study performed HIIT (defined as ten sets of 4-minute cycling bursts at an intensity of 90% VO2max separated by 2 minutes of rest) every other day over a 2-week period. The study found that seven sessions of HIIT over a 2-week period improved whole body fat oxidation and the capacity for skeletal muscle to oxidize fat in moderately active women. A 2010 systematic review of HIIT summarized the results of HIIT on fat loss and stated that HIIT can result in modest reductions of subcutaneous fat in young and healthy individuals, but greater reductions for overweight individuals.
Brain power
A 2017 study examined the effect of HIIT on cognitive performance among a group of children (N=318). The authors show that HIIT is beneficial to cognitive control and working memory capacity when compared against "a blend of board games, computer games, and trivia quizzes" and that this effect is mediated by the BDNF polymorphism. They conclude that the study "suggests a promising alternative to enhance cognition, via short and potent exercise regimens".
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