Changes

HIIT can be highly effective training, but involves a risk of injury. 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 in untrained people more effectively than Continuous Moderate Exercise. However, there is no evidence to suggest that HIIT can replace other forms of training for endurance races. High Intensity Interval Training (HIIT) uses repeated short periods of very intense cardiovascular exercise separated by lower intensity recoveries. (I've found the [[Stryd]] estimate of power output is the best approach to gauging effort during HIIT, though [[Moxy]] can provide some interesting insight from [[Muscle Oxygen Saturation]].)

This section looks at the scientific evidence, divided into three sections. Studies that compare HIIT with other modes For details 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. ==science around 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" style="margin-left: auto; margin-right: auto; border: none;"! Study! Subjects! Study length! Protocol! Outcome! Best Result! Notes|-| rowspan="4" rowspan="4"| Helgerud-2007<ref name="Helgerud-2007"/>| rowspan="4" rowspan="4"| Moderately trained(see [[VO2max|V̇O₂max]] 51-55)| rowspan="4" rowspan="4"| 3 days/week8 weeks| '''Short HIIT'''47x 15 seconds at 90-95% [[Maximum Heart Rate|HR_max]] + 15 seconds at 70% [[Maximum Heart Rate|HR_max]] | Raised [[VO2max|V̇O₂max]] 7.2%| rowspan="4" rowspan="4"| Short HIIT| rowspan="4" rowspan="4"| All groups improved economy, with no differences, and [[Lactate Threshold]] unchanged as a percentage The Science of [[VO2max|V̇O₂max]]|-| '''Long HIIT'''4x 4 min, 90-95% [[Maximum Heart Rate|HR_max]] + 3 min at 70%max| Raised [[VO2max|V̇O₂max]] 5.5%|-| '''[[Lactate Threshold]] run'''24 min at 85% [[Maximum Heart Rate|HR_max]] | [[VO2max|V̇O₂max]] unchanged|-| '''Long Slow Distance'''45 minutes at 70% [[Maximum Heart Rate|HR_max]]| [[VO2max|V̇O₂max]] unchanged|-| rowspan="2" rowspan="2"| TjonnaLee-2008<ref name="TjonnaLee2008"/>| rowspan="2" rowspan="2"| Untrained, metabolic syndrome patients| rowspan="2" rowspan="2"| 3 days/week16 weeks| '''HIIT'''4x 4 min at 90% [[Maximum Heart Rate|HR_max]] + 3 min 70% [[Maximum Heart Rate|HR_max]]total 40 min, | Raised [[VO2max|V̇O₂max]] 36%| rowspan="2" rowspan="2"| HIIT| rowspan="2" 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_max]] | Raised [[VO2max|V̇O₂max]] 16%|-| rowspan="2" rowspan="2"| Gibala-2006<ref name="Gibala-2006"/> | rowspan="2" rowspan="2"| Recreationally active| rowspan="2" rowspan="2"| 2 weeks| '''HIIT'''4-6x 30 seconds 'all out' + 4 min recoveryTotals for two weeks, 135 minutes and 950 Kj| rowspan="2" rowspan="2"| Same improvement in laboratory time trials| rowspan="2" rowspan="2"| HIIT| rowspan="2" rowspan="2"| Same improvement, but only 22% of the time commitment|-| '''Continuous Moderate Exercise'''90-120 min at 65% [[VO2max|V̇O₂peak]]Totals for two weeks, 630 minutes and 6500 Kj|-| rowspan="2" rowspan="2"| Gorostiaga-1991<ref name="Gorostiaga-1991"/> | rowspan="2" rowspan="2"| Sedentary| rowspan="2" rowspan="2"| 3 days/week8 weeks| '''HIIT'''30x 30 sec @ 100% [[VO2max|V̇O₂max]] + 30 sec rest| Raised [[VO2max|V̇O₂max]] 9-16%No change in blood [[Lactate]] during continuous exercise | rowspan="2" rowspan="2"| HIIT| rowspan="2" rowspan="2"| Same average work in each group|-| '''Continuous Moderate Exercise'''30 minutes at 50% [[VO2max|V̇O₂max]]| Raised [[VO2max|V̇O₂max]] 5-7%Reduced blood [[Lactate]] during continuous exercise by nearly 50%|-| rowspan="3" rowspan="3"| Franch-1998<ref name="Franch-1998"/>| rowspan="3" rowspan="3"| 36 recreational runners| rowspan="3" rowspan="3"| 3 days/week at high intensityPlus 3 runs/week <= 65% [[Maximum Heart Rate|HR_max]]6 weeks| '''Short HIIT'''30-40x 15 sec run, 15 sec restAvg ~3.0 Km/workout92% [[Maximum Heart Rate|HR_max]]| Time to exhaustion increased 65%[[Running Economy]] improved 0.9%| rowspan="3" rowspan="3"| Continuous High Intensity| rowspan="3" 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_max]]| Time to exhaustion increased 67%[[Running Economy]] improved 3.0%|-| '''Continuous High Intensity'''20-30 minutesAvg ~6.4 Km/workout93% [[Maximum Heart Rate|HR_max]]| Time to exhaustion increased 94%[[Running Economy]] improved 3.1%|-| rowspan="2" rowspan="2"| BurgomasterHowarth-2007<ref name="BurgomasterHowarth2007"/>| rowspan="2" rowspan="2"| 20 Untrained| rowspan="2" 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" rowspan="2"| Both increased [[VO2max|V̇O₂peak]] by ~5%| rowspan="2" rowspan="2"| HIIT| rowspan="2" 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|V̇O₂peak]]4.5 hours/week2250 Kj/week|-| rowspan="2" rowspan="2"| Trapp-2008<ref name="Trapp-2008"/>| rowspan="2" rowspan="2"| 34 sedentary women| rowspan="2" 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 [[VO2max|V̇O₂peak]] 24%5 pound/2.5 Kg reduction in body fatSignificant 31% reduction in fasting insulinSignificant reduction in Leptin| rowspan="2" rowspan="2"| HIIT| rowspan="2" 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|V̇O₂peak]]| Increased [[VO2max|V̇O₂peak]] 19%1 pound/0.5 Kg gain in body fatNon-significant 9% reduction in fasting insulinNo change in Leptin|-| rowspan="2" rowspan="2"| Tabata-1996<ref name="Tabata-1996"/>| rowspan="2" rowspan="2"| 14 varsity level collage athletes ([[VO2max|V̇O₂max]] ~50)| rowspan="2" rowspan="2"| 5 days/week6 weeks | '''HIIT'''4 days/week 7-8x (30 seconds at 170% [[VO2max|V̇O₂max]] + 10 seconds rest)1 day/week 30 min at 70% [[VO2max|V̇O₂max]] + 4x (30 seconds at 170% [[VO2max|V̇O₂max]] + 10 seconds rest)| Raised [[VO2max|V̇O₂max]] by 14.5%Increased anaerobic capacity by 28%| rowspan="2" rowspan="2"| HIIT| rowspan="2" rowspan="2"| HIIT produced a greater improvement in [[VO2max|V̇O₂max]] for far less time commitment|-| '''Continuous Moderate Exercise'''60 minutes at 70% [[VO2max|V̇O₂max]]| Raised [[VO2max|V̇O₂max]] by 9.5%No change in anaerobic capacity |-| rowspan="2" rowspan="2"| EarnestTjønna-2013<ref name="EarnestTjønna2013"/>| rowspan="2" rowspan="2"| 26 healthy overweight men (BMI 25-30)| rowspan="2" rowspan="2"| 3 days/week10 weeks| '''4x HIIT'''10 min [[Warmup]]4x 4 min at 90% [[Maximum Heart Rate|HR_max]] + 3 min 70% [[Maximum Heart Rate|HR_max]]5 min [[Cooldown]]total 40 min| Raised [[VO2max|V̇O₂max]] by 13%Work economy improved by 13%Systolic blood pressure decreased 3.2 mmHg Diastolic blood pressure decreased 6.3 mmHg  | rowspan="2" rowspan="2"| Similar results with both protocols| rowspan="2" rowspan="2"| This study showed remarkable results using a single high intensity bout of exercise. |-| '''1x HIIT'''10 min [[Warmup]]4 min at 90% [[Maximum Heart Rate|HR_max]]5 min [[Cooldown]]total 19 min| Raised [[VO2max|V̇O₂max]] by 10%Work economy improved by 14%Systolic blood pressure decreased 6.2 mmHg Diastolic blood pressure decreased 7.7 mmHg  |}==HIIT Studies on Untrained or Moderately Active Subjects without Controls==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|V̇O₂max]] from 25 to 51 minutes, despite no change in [[VO2max|V̇O₂max]]<ref name="Burgomaster-2005"/>, a remarkable improvement. In another study, the combination of Continuous Moderate Exercise and moderate intensity intervals (60-70% [[VO2max|V̇O₂max]]) 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|V̇O₂max]] by 44%, as well as improved running endurance, with some subjects ending up with a [[VO2max|V̇O₂max]] 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 has been suggested that elite athletes do not benefit from further increases in volume, and should instead look to HIIT for performance improvements<ref name="Laursen-1-2002"/>. This is backed up by studies of some of the great endurance athletes, where higher training mileage produced worse rather than better performance<ref name="elitemileage"/>. In the Lore of Running, Tim Noakes said that elite runners perform best "when they train between 75-125 miles (120-200 km) per week, with an increasing likelihood that they will perform indifferently when they train more than 125 miles (200 km) per week"<ref name="LoR-P477"/>. Of course this is not universally true, and Mike Morton, set the US record holder for 24 hour while training 140-150 miles/week<ref name="militarytimes.com"/>. However, the evaluation of HIIT on elite athletes is not as easy as lessor folk. 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" style="margin-left: auto; margin-right: auto; border: none;"! 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_lim]] , where [[vVO2max|T_limTraining]] is the time to exhaustion at 100% [[VO2max|V̇O₂max]]<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|V̇O₂max]] 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|V̇O₂max]] with time varying between 60-75% [[vVO2max|T_lim]], plus one weekly run of 30 min at 60% v[[VO2max|V̇O₂max]]. For these runners, [[vVO2max|T_lim]] 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|V̇O₂max]] for either 6x 60% [[vVO2max|T_lim]] (133 sec) or 5x 70% (154 sec) [[vVO2max|T_lim]], resting for twice the interval time. Their 3K time improved by 17.6 sec (60% [[vVO2max|T_lim]]) or 6.3 sec (70% [[vVO2max|T_lim]]), but there was no change in their 5K time<ref name="Smith-2003"/>.* 41 elite ([[VO2max|V̇O₂peak]] ~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_max]] 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|V̇O₂max]] which was not statistically different between group 3 and the controls. {| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"! Group! Intended number of intervals! Interval Duration! Total work time (min)! Intensity (% [[VO2max|V̇O₂max]])! Rest ! Total time! [[VO2max|V̇O₂max]] change! 40K Time Trial Speed improvement|-| 1| 8| 60% [[vVO2max|T_lim]](~150 sec)| 20| 100%| 120% [[vVO2max|T_lim]] (~290 sec)| 58 min| 5.2%| 5.2%|-| 2| 8| 60% [[vVO2max|T_lim]](~150 sec)| 20| 100%| 65% [[Maximum Heart Rate|HR_max]] (~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|V̇O₂max]] + 120 sec recovery) that improved peak power by 4% but had no change in [[VO2max|V̇O₂max]]<ref name="Laursen-2-2002"/>.==Limitations of the HIIT science==There are some important limitations of the HIIT science. * The majority of studies are on sedentary or recreationally active people, not trained runners. * Few studies use real world measures of improvement, relying instead on indirect metrics such as [[VO2max|V̇O₂max]]. While [[VO2max|V̇O₂max]] is linked to improved performance, there are other important factors involved. * 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.

Crossfit Endurance is a training approach that reduces the normal endurance training volumes while increasing the training intensity<ref name="www.crossfitendurance.com-about"/>. The reduction in volume is quite dramatic compared with other training plans; for instance, the Crossfit Endurance marathon training plan has a 10 mile time trial as its longest run, which is combined with weight training and interval training<ref name="crossfitmarathon"/>. Crossfit Endurance claims that eight 100 meter intervals "accomplishes everything you would by jogging 20 miles but doesn't put the same level of stress and damage on the body." There is the caveat that "some kind of stamina work that lost more than the 70 seconds is required to "dial in technique, just rhythm, and formulate pace strategy." While there are some testimonials to the Crossfit Endurance methodology<ref name="www.crossfitendurance.com-test"/>, their approach is controversial<ref name="SoRCf"/>. The science that is used to support Crossfit Endurance does not support replacing traditional Continuous Moderate Exercise with HIIT for endurance events, only using HIIT as a supplementary form of training<ref name="metamorphitness.wordpress.com"/>. Most of the anecdotal reports suggest that people have set personal records at 5K and 10K distances and completed half marathons using the Crossfit Endurance approach, but there are few reports of longer races. The anecdotal reports are hard to interpret objectively without knowing the individuals prior training methodology. (I have some other concerns with CrossFit, such as their emphasis on forefoot landing, then approach to hydration that includes the overly simplistic "sweat rate test", but that's outside of the scope of this article that focuses on interval training.)