Test referenceHigh Intensity Interval Training (HIIT) uses repeated short periods of very intense cardiovascular exercise separated by lower intensity recoveries. HIIT has been shown to improve aerobic capacity in untrained and moderately active individuals more quickly than Continuous Moderate Exercise, as well as having potential benefits for highly trained athletes. HIIT has also been shown to reduce body fat more effectively than Continuous Moderate Exercise.=What is HIIT?=High Intensity Interval Training is a form of [[Introduction to Interval Training| Interval Training]] using short intervals of 10 seconds to 5 minutes at an intensity at or above 90% [[VO2max|V̇O<sub>2</sub>max]]<ref name="Boutcher2011"/>. High Intensity Interval Training, abbreviated to HIIT or HIT, is sometimes called High-Intensity Intermittent Exercise<ref name="Trapp-2008"/> or Sprint Training<ref name="Gibala-2006"/>.=Types of HIIT=There are various different protocols for performing HIIT. While the Tabata is probably the most widely recognized name in HIIT, it is one of the least studied and the least used. ==Tabata==The Tabata workout is one of the best known protocols for HIIT and consists of 7-8 repeats of 20 seconds at 170% of [[VO2max|V̇O<sub>2</sub>max]] with 10 seconds rest. The number of repetitions is defined by how long the required intensity can be maintained. An athlete should be able to complete 7-8 intervals; if 9 can be performed, the intensity is increased. However, few people actually follow the Tabata protocol because it requires specialist equipment to measure [[VO2max|V̇O<sub>2</sub>max]], then calculate 1.7x the [[VO2max|V̇O<sub>2</sub>max]] workload that should be used. In practice, most people do what I call the WinTab workout (see below). ==Wingate==This style of HIIT is based around the Wingate test, which is used to measure peak anaerobic power and anaerobic capacity<ref name="Vandewalle-1987"/>. The Wingate HIIT uses 30 seconds of 'all out' intensity, followed by ~4 minutes of recovery, repeated 4-6 times. ==WinTab (Tabata style Wingate)==Most athletes that use the Tabata workout do not use the 170% [[VO2max|V̇O<sub>2</sub>max]] intensity, but rather the Wingate style 'all out' intensity. Because the intensity is not fixed, the WinTab normally has 8 repetitions of 20 seconds all out plus 10 seconds rest, taking 5 minutes (plus [[WarmUp]]), rather than the variable number of repetitions of the original Tabata protocol.==Traditional Anaerobic Intervals==High intensity Interval Training has been in use since before the Tabata study made the term popular. The Jack Daniel's 'R' paced workouts are a classic example of this tradition of anaerobic interval training. The Jack Daniel's 'R' intervals are performed at around mile/1500m pace<ref name="JD"/>, which is generally close to 100% [[VO2max|V̇O<sub>2</sub>max]]. The 'R' workouts are 12-40 repeats of 30-60 seconds with 1-4 min rests<ref name="JD-P132"/>. Jack Daniels mentions once having his athletes perform over 1,000 repetitions of one minute HIIT (4 minute rest) in a 14 day period, with two male athletes averaging 5:00 min/mile pace and therefore covering 250 miles<ref name="JD-P132"/>. =HIIT Training Mode=There are various ways that HIIT could be performed, each with their own pros and cons.{| class="wikitable"! Mode! Pros! Cons|-| Stationary Bike|* The vast majority of studies have used a stationary bike to perform HIIT.* A direct measure of intensity through power is often possible. * Lowest risk of injury. |* The bike does not mimic the running action, using different muscles.|-| Bike|* Should be similar to the stationary bike, but at a lower cost.* It's easier to ride a bike out of the saddle than a stationary bike, which uses more muscles and is closer to the action of running. |* Riding a bike at high intensity adds risk due to impaired mental functioning and coordination.* While it's possible to directly measure power output on a bike, it's expensive.|-| Running|* For runners, the right muscle groups are trained.|* At high Intensity, the movements involved in running become dramatically different, requiring a greater range of motion. This difference creates a significant risk of injury. Running uphill reduces some of this stress.* The rapid acceleration and deceleration of HIIT creates stress that runners are not typically exposed to, again creating a risk of injury. (I suspect the risk of injury during the deceleration is often underestimated, as this is when where a runner's form often deteriorates.)* Even with a [[Footpod]] and a [[Best Running Watch| good running watch]] it's hard to measure pace accurately, and GPS is useless for the short intervals often used in HIIT. Running on a track or other known distance may be slightly easier, but it's still not trivial. (This is not a problem for 'all out' style of HIIT.)|-| Treadmill|* It's possible to run in weather that is too hostile for outside activities. |* Most treadmills will not change pace fast enough for the dramatic changes in pace between interval and recovery used in HIIT.* Matching acceleration and deceleration with the treadmill is not easy, especially when mental functioning is impaired, as it so often is at high intensity* If you cannot maintain the programmed pace, you could be injured by falling off the back of a treadmill. * A treadmill may not be able to go fast enough for HIIT.|}=HIIT Running Paces=I don't generally recommend running HIIT workouts because of the high speeds required. Below is a sampling of the HIIT paces for different fitness levels ([[VO2max|V̇O<sub>2</sub>max]]). For each level, the 5K and marathon times are given, along with the pace that corresponds to 90%, 100% and 170% of [[VO2max|V̇O<sub>2</sub>max]]. {| class="wikitable"! [[VO2max|V̇O<sub>2</sub>max]]! 35! 40! 45! 50! 55! 60! 65! 70! 75! 80! 85|-| 5K| 26:57| 24:05| 21:50| 20:00| 18:28| 17:10| 16:03| 15:05| 14:15| 13:32| 12:51|-| Marathon| 4:15:51| 3:49:27| 3:28:10| 3:10:33| 2:55:49| 2:43:08| 2:32:29| 2:23:04| 2:14:53| 2:07:31| 2:00:58|-| 90% V̇O2max| 9:09/Mile| 8:13/Mile| 7:28/Mile| 6:52/Mile| 6:21/Mile| 5:55/Mile| 5:33/Mile| 5:13/Mile| 4:56/Mile| 4:41/Mile| 4:27/Mile|-| 100% V̇O2max| 8:24/Mile| 7:33/Mile| 6:52/Mile| 6:18/Mile| 5:50/Mile| 5:26/Mile| 5:05/Mile| 4:47/Mile| 4:32/Mile| 4:18/Mile| 4:06/Mile|-| 170% V̇O2max| 5:28/Mile| 4:54/Mile| 4:27/Mile| 4:06/Mile| 3:48/Mile| 3:33/Mile| 3:19/Mile| 3:08/Mile| 2:58/Mile| 2:49/Mile| 2:41/Mile|}=HIIT Recommendations=The recommendations for incorporating HIIT in your training depend on your current fitness goals.* For sedentary people, HIIT is probably more effective in building initial fitness than Moderate Continuous Exercise. Three sessions per week of Wingate HIIT on a stationary bike should provide an improvement in fitness for a modest time commitment. Using WinTab style HIIT may produce similar benefits for a lower time commitment. * For people looking to lose weight, HIIT will probably produce a greater reduction in body fat than Moderate Continuous Exercise. Three sessions per week of Wingate HIIT on a stationary bike should help with weight loss and appetite control. The HIIT could be combined with other forms of exercise on the other days, which may further improve weight loss. * Recreationally active people looking for rounded fitness may benefit from including HIIT in their overall training program. Adding 1-3 sessions per week of Wingate or WinTab HIIT on a stationary bike should provide an improvement in fitness.* Athletes focused on improving their performance in endurance races lasting less than an hour will probably benefit from replacing some of their training load with a combination of both traditional HIIT and shorter Wingate or WinTab style HIIT. Up to one traditional HIIT or 1-3 shorter HIIT sessions could be included in a weekly training routine. Care should be taken to increase [[TRIMP| Training Load]] slowly with the additional HIIT and try to avoid increasing [[Training Monotony]]. The traditional HIIT should be performed by running on a track or other outdoor location. For the shorter HIIT, a stationary bike probably has a lower injury risk, but the additional benefits of outdoor running may outweigh the additional injury risk. However, the fast paces of the shorter HIIT should be introduced gradually, building up the pace over a number of workouts. * There is no direct evidence to indicate if HIIT will help athletes focused on improving their performance in events lasting longer than an hour. However, it seems reasonable that HIIT would provide some benefits. Including 1 or 2 Wingate or WinTab HIIT sessions per week may be appropriate. As noted in the prior bullet point for shorter duration athletes, care should be taken to monitor [[TRIMP| Training Load]] and [[Training Monotony]]. As with athletes focusing on events lasting less than an hour, the stationary bike has the lowest injury risk, but there may be benefits to other training modes. However, athletes competing in longer duration events typically don't have the same need for the very high paces that are sometimes seen at the end of shorter events.* There is no evidence to suggest that HIIT can be used to replace the Long Run that core part of endurance training. =The HIIT Science=This section looks at the scientific evidence, divided into three sections. Studies that compare HIIT with other modes of training are the most interesting, though they don't cover highly trained athletes. I've included a few other studies that are not comparative because they have some particularly dramatic results. The third section looks at the studies that have looked at HIIT for highly trained athletes. ==HIIT Comparisons on Untrained or Moderately Active Subjects==The table below looks at studies that have compared HIIT with other types of training, often Continuous Moderate Exercise (CME). These studies on untrained or moderately trained subjects generally show a greater improvement in fitness measure compared with other forms of training, or similar improvements for far less training time. {| class="wikitable"! Study! Subjects! Study length! Protocol! Outcome! Best Result! Notes|-| rowspan="4"| Helgerud-2007<ref name="Helgerud-2007"/>| rowspan="4"| Moderately trained([[VO2max|V̇O<sub>2</sub>max]] 51-55)| rowspan="4"| 3 days/week8 weeks| '''Short HIIT'''47x 15 seconds at 90-95% [[Maximum Heart Rate|HR<sub>max</sub>]] + 15 seconds at 70% [[Maximum Heart Rate|HR<sub>max</sub>]] | Raised [[VO2max|V̇O<sub>2</sub>max]] 7.2%| rowspan="4"| Short HIIT| rowspan="4"| All groups improved economy, with no differences, and Lactate Threshold unchanged as a percentage of [[VO2max|V̇O<sub>2</sub>max]]|-| '''Long HIIT'''4x 4 min, 90-95% [[Maximum Heart Rate|HR<sub>max</sub>]] + 3 min at 70%max| Raised [[VO2max|V̇O<sub>2</sub>max]] 5.5%|-| '''Lactate threshold run'''24 min at 85% [[Maximum Heart Rate|HR<sub>max</sub>]] | [[VO2max|V̇O<sub>2</sub>max]] unchanged|-| '''Long Slow Distance'''45 minutes at 70% [[Maximum Heart Rate|HR<sub>max</sub>]]| [[VO2max|V̇O<sub>2</sub>max]] unchanged|-| rowspan="2"| TjonnaLee-2008<ref name="TjonnaLee2008"/>| rowspan="2"| Untrained, metabolic syndrome patients| rowspan="2"| 3 days/week16 weeks| '''HIIT'''4x 4 min at 90% [[Maximum Heart Rate|HR<sub>max</sub>]] + 3 min 70% [[Maximum Heart Rate|HR<sub>max</sub>]]total 40 min, | Raised [[VO2max|V̇O<sub>2</sub>max]] 36%| rowspan="2"| HIIT| rowspan="2"| Same calories burned in each groupBoth groups had an equal reduction in body weight and blood pressure|-| '''Continuous Moderate Exercise'''47 min at 70% [[Maximum Heart Rate|HR<sub>max</sub>]] | Raised [[VO2max|V̇O<sub>2</sub>max]] 16%|-| rowspan="2"| Gibala-2006<ref name="PasmanGibala-2006"/> | rowspan="2"| Recreationally active| rowspan="2"| 2 weeks| '''HIIT'''4-6x 30 seconds 'all out' + 4 min recoveryTotals for two weeks, 135 minutes and 950 Kj| rowspan="2"| Same improvement in laboratory time trials| rowspan="2"| HIIT| rowspan="2"| Same improvement, but only 22% of the time commitment|-| '''Continuous Moderate Exercise'''90-120 min at 65% [[VO2max|V̇O<sub>2</sub>peak]]Totals for two weeks, 630 minutes and 6500 Kj|-| rowspan="2"| Gorostiaga-1991<ref name="Gorostiaga-1991"/> | rowspan="2"| Sedentary| rowspan="2"| 3 days/week8 weeks| '''HIIT'''30x 30 sec @ 100% [[VO2max|V̇O<sub>2</sub>max]] + 30 sec rest| Raised [[VO2max|V̇O<sub>2</sub>max]] 9-16%No change in blood lactate during continuous exercise | rowspan="2"| HIIT| rowspan="2"| Same average work in each group|-| '''Continuous Moderate Exercise'''30 minutes at 50% [[VO2max|V̇O<sub>2</sub>max]]| Raised [[VO2max|V̇O<sub>2</sub>max]] 5-7%Reduced blood lactate during continuous exercise by nearly 50%|-| rowspan="3"| Franch-1998<ref name="Franch-1998"/>| rowspan="3"| 36 recreational runners| rowspan="3"| 3 days/week at high intensityPlus 3 runs/week <= 65% [[Maximum Heart Rate|HR<sub>max</sub>]]6 weeks| '''Short HIIT'''30-199540x 15 sec run, 15 sec restAvg ~3.0 Km/workout92% [[Maximum Heart Rate|HR<sub>max</sub>]]| Time to exhaustion increased 65%Running economy improved 0.9%| rowspan="3"| Continuous High Intensity| rowspan="3"| Better improvements from continuous training than HIIT, but the continuous training is at an unusually high intensity that is probably close to a 10K race, three times a week. |-| '''Long HIIT'''4-6x 4 min run, 2 min restAvg ~5.6 Km/workout94% [[Maximum Heart Rate|HR<sub>max</sub>]]| Time to exhaustion increased 67%Running economy improved 3.0%|-| '''Continuous High Intensity'''20-30 minutesAvg ~6.4 Km/workout93% [[Maximum Heart Rate|HR<sub>max</sub>]]| Time to exhaustion increased 94%Running economy improved 3.1%|-| rowspan="2"| BurgomasterHowarth-2007<ref name="MacfarlaneBurgomasterHowarth2007"/>| rowspan="2"| 20 Untrained| rowspan="2"| HIIT 3x weekContinuous 5x week6 weeks| '''HIIT'''4-6x 30 seconds 'all out', 4.5 min rest1.5 hours/week~225 Kj/week| rowspan="2"| Both increased [[VO2max|V̇O<sub>2</sub>peak]] by ~5%| rowspan="2"| HIIT| rowspan="2"| Similar changes in HIIT for 10% of the workload and 30% of the time of continuous training.|-| '''Continuous Moderate Exercise'''40-60 min at 65% [[VO2max|V̇O<sub>2</sub>peak]]4.5 hours/week2250 Kj/week|-| rowspan="2"| Trapp-2008<ref name="ShishikuraKhokhar2005Trapp-2008"/>| rowspan="2"| 34 sedentary women| rowspan="2"| 45 workouts over 15 weeks| '''HIIT'''60x 8 seconds 'all out', 12 seconds rest (5 min warmup, 20 min conditioning, 5 min cooldown)| Increased [[Delayed Onset Muscle SorenessVO2max| DOMSV̇O<sub>2</sub>peak]]24%5 pound/2.5 Kg reduction in body fatSignificant 31% reduction in fasting insulinSignificant reduction in [http://testen.wikipedia.org/wiki/Leptin Leptin]| rowspan="2"| HIIT| rowspan="2"| HIIT produced similar improvements in fitness for a lower time commitment, as well as a reduction in body fat that was not seen with continuous exercise. |-| '''Continuous Moderate Exercise'''40 minutes at 60% [[VO2max|V̇O<sub>2</sub>peak]]| Increased [[VO2max|V̇O<sub>2</sub>peak]] 19%1 pound/0.5 Kg gain in body fatNon-significant 9% reduction in fasting insulinNo change in [http://en.wikipedia.org/wiki/Leptin Leptin]|-| rowspan="2"| Tabata-1996<ref name="Tabata-1996"/>| rowspan="2"| 14 varsity level collage athletes ([[VO2max|V̇O<sub>2</sub>max]] ~50)| rowspan="2"| 5 days/week6 weeks| '''HIIT'''4 days/week 7-8x (30 seconds at 170% [[VO2max|V̇O<sub>2</sub>max]] + 10 seconds rest)1 day/week 30 min at 70% [[VO2max|V̇O<sub>2</sub>max]] + 4x (30 seconds at 170% [[VO2max|V̇O<sub>2</sub>max]] + 10 seconds rest)| Raised [[VO2max|V̇O<sub>2</sub>max]] by 14.5%Increased anaerobic capacity by 28%| rowspan="2"| HIIT| rowspan="2"| HIIT produced a greater improvement in [[VO2max|V̇O<sub>2</sub>max]] for far less time commitment|-| '''Continuous Moderate Exercise'''60 minutes at 70% [[VO2max|V̇O<sub>2</sub>max]]| Raised [[VO2max|V̇O<sub>2</sub>max]] by 9.5%No change in anaerobic capacity |}==HIIT Studies on Untrained or Moderately Active Subjects==While studies that compare HIIT with other forms of training are the most useful, there are a few other studies on untrained or moderately active people that are noteworthy. For instance, six sessions of HIIT over two weeks doubled the endurance of untrained subjects at 80% [[VO2max|V̇O<sub>2</sub>max]] from 25 to 51 minutes, despite no change in [[VO2max|V̇O<sub>2</sub>max]]<ref name="Burgomaster-2005"/>, a remarkable improvement. In another study, the combination of Continuous Moderate Exercise and moderate intensity intervals (60-70% [[VO2max|V̇O<sub>2</sub>max]]) reduced body fat by 15%, which was nine times more than Continuous Moderate Exercise alone, even though the Continuous Moderate Exercise burned over twice the calories<ref name="Tremblay-1994"/>. Another study also used a combination of HIIT on 3 day/week plus running as far as possible in 40 min on another 3 days/week , resulting in an increase in [[VO2max|V̇O<sub>2</sub>max]] by 44%, as well as improved running endurance, with some subjects ending up with a [[VO2max|V̇O<sub>2</sub>max]] exceeding 60 ml/kg per min, which is remarkably high for 10 weeks of training<ref name="Hickson-1977"/>. ==HIIT and Highly Trained Athletes==It's not practical to compare the effect of HIIT with other forms of exercise in highly trained athletes as they are typically already performing large volumes of Continuous Moderate Exercise. Instead, studies of highly trained athletes look at how HIIT impacts their fitness compared with a baseline taken beforehand. * HIIT improved peak power output and 40 Km time trial in elite cyclists<ref name="Lindsay-1996"/><ref name="Westgarth-Taylor-1997"/>* A study of elite cyclists used various HIIT workouts as shown in the table below, with the best results seen group 4 or group 1<ref name="Stepto-1999"/>. Group 4 trained at 85% peak power, which corresponds to the intensity normally seen in the 40K time trial, which takes ~60 minutes for an elite cyclist. Not surprisingly, this intensity is commonly used for cyclists training for 40K time trials. However, the higher intensity of group 1 is more intriguing; the time trial performance improved without an improvement in peak power, suggesting that a different mechanism may be responsible. This raises the possibility that the benefits of the different intensities might be combined. Note that there were only four athletes in each group, and responses tended to vary, so caution should be used in interpreting the results. {| class="wikitable"! Group! Number of intervals! Interval duration (min)! Total work time (min)! Intensity (% peak power)! Rest (min)! Total Time! Improvement in 40K Time Trial Speed! Improvement in Peak Power|-| 1| 12| 0.5| 6| 175%| 4.5| 60 min| 2.0%| 0.5%|-| 2| 12| 1| 12| 100%| 4.0| 60 min| 0.0%| 0.5%|-| 3| 12| 2| 24| 90%| 3.0| 60 min| 1.5%| 1.5%|-| 4| 8| 4| 32| 85%| 1.5| 44 min| 2.5%| 2.0%|-| 5| 4| 8| 32| 80%| 1.0| 36 min| 0.0%| 1.0%|}* One approach to optimizing the length of the intervals in highly trained athletes is to use a percentage of [[vVO2max|T<sub>lim</sub>]] , where [[vVO2max|T<sub>lim</sub>]] is the time to exhaustion at 100% [[VO2max|V̇O<sub>2</sub>max]]<ref name="Laursen-1-2002"/>.* 5 state level middle distance runners that underwent 4 weeks of HIIT training reduced their 3K time by 2.8% (10:16 to 9:59) and [[VO2max|V̇O<sub>2</sub>max]] by 4.9% (61 to 64)<ref name="Smith-1999"/>. The HIIT training consisted of 2 sessions per week of 6 intervals at 100% [[VO2max|V̇O<sub>2</sub>max]] with time varying between 60-75% [[vVO2max|T<sub>lim</sub>]], plus one weekly run of 30 min at 60% v[[VO2max|V̇O<sub>2</sub>max]]. For these runners, [[vVO2max|T<sub>lim</sub>]] averaged 225 seconds, so the intervals were between 135 and 170 seconds.* Well trained, competitive runners trained twice a week for four weeks with intervals at 100% [[VO2max|V̇O<sub>2</sub>max]] for either 6x 60% [[vVO2max|T<sub>lim</sub>]] (133 sec) or 5x 70% (154 sec) [[vVO2max|T<sub>lim</sub>]], resting for twice the interval time. Their 3K time improved by 17.6 sec (60% [[vVO2max|T<sub>lim</sub>]]) or 6.3 sec (70% [[vVO2max|T<sub>lim</sub>]]), but there was no change in their 5K time<ref name="Smith-2003"/>.* 41 elite ([[VO2max|V̇O<sub>2</sub>peak]] ~65) cyclists and triathletes were split into four groups, with three groups using the HIIT described below and the fourth acting as a control that followed only low to moderate intensity training<ref name="Laursen-3-2002"/>. Note that groups 1 and 2 vary only in their rest time, which is based on heart rate dropping to 65% of [[Maximum Heart Rate|HR<sub>max</sub>]] in group 2 (averaging around 180 seconds). This was a demanding regime, as the subjects reached exhaustion on nearly every HIIT training session, with '''only 64% of the dictated intervals actually completed'''. Note that like comparison of different workouts above, the shorter HIIT produced a similar improvement in 40K performance without the accompanying rise in [[VO2max|V̇O<sub>2</sub>max]] which was not statistically different between group 3 and the controls. {| class="wikitable"! Group! Intended number of intervals! Interval Duration! Total work time (min)! Intensity (% [[VO2max|V̇O<sub>2</sub>max]])! Rest ! Total time! [[VO2max|V̇O<sub>2</sub>max]] change! 40K Time Trial Speed improvement|-| 1| 8| 60% [[vVO2max|T<sub>lim</sub>]]<sub></sub>(~150 sec)| 20| 100%| 120% [[vVO2max|T<sub>lim</sub>]] (~290 sec)| 58 min| 5.2%| 5.2%|-| 2| 8| 60% [[vVO2max|T<sub>lim</sub>]]<sub></sub>(~150 sec)| 20| 100%| 65% [[Maximum Heart Rate|HR<sub>max</sub>]] (~180 sec)| Varies| 8.0%| 5.6%|-| 3| 12| 30 seconds| 6| 175%| 4.5 min| 60 min| 3.1%| 4.3%|-| Control| N/A| N/A| N/A| N/A| N/A| N/A| 0.8%| -1%|}* Elite cyclists performed 4 sessions of HIIT (20x 60 sec at [[VO2max|V̇O<sub>2</sub>max]] + 120 sec recovery) that improved peak power by 4% but had no change in [[VO2max|V̇O<sub>2</sub>max]]<ref name="Laursen-2-2002"/>.==Limitations of the HIIT science==There are some important limitations of the HIIT science.skimlinks* The majority of studies are on sedentary or recreationally active people, not trained runners.com* Few studies use real world measures of improvement, relying instead on indirect metrics such as [[VO2max|V̇O<sub>2</ Skimlinks testsub>max]]. * Studies that do look at the effect of HIIT on real world performance tend to focus on shorter events, such as 3K or 5K running, or 40K cycling.* Most studies are short duration, looking at the effects of HIIT over just a few weeks.=referencesReferences=
<references>
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