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{{DISPLAYTITLE:The Science of High Intensity Interval Training (HIIT) Tabata and Wingate}}
This page looks at the scientific evidence on [[High Intensity Interval Training]] (HIIT), divided into three sections. Studies that compare HIIT with other modes of training are the most interesting, though they often 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. For an introduction to HIIT, see [[High Intensity Interval Training]].
=Summary=
Here are the high-level conclusions from the available research.
* The preponderance of evidence shows that HIIT provides greater improvements in [[VO2max|V̇O<sub>2</sub>max]] than continuous moderate exercise.
* The ideal intensity for HIIT is unclear, but generally believed to be above 90% [[VO2max|V̇O<sub>2</sub>max]].
* There is some evidence that HIIT results in greater body fat reductions than continuous moderate exercise in untrained or moderately trained subjects.
* There is some evidence that HIIT improves race performance in short events (<60 minutes), but no evidence for longer events.
* There is some evidence that even very low doses of HIIT (1-2 intervals) can improve fitness and insulin sensitivity in an untrained population.
=Components of HIIT=
There are three main variable several variables that shape a HIIT workout<ref name="BuchheitLaursen2013"/>. * '''Work Intensity'''. I'd argue that to be considered "high intensity" the workout should be higher than Lactate Threshold, and really, I'd consider it to be at least 100% of the effort at [[VO2max|VOV̇O<sub>2</sub>max]]. The classic Tabata workout used 1.7x the work at [[VO2max|VOV̇O<sub>2</sub>max]], though many implementations of this workout simply use "all out."* '''Duration'''. Given intensities at or above Some consider that the threshold of HIIT to be 90% of the effort of [[VO2max|VOV̇O<sub>2</sub>max]]<ref name="Billat-2001"/><ref name="Midgley-2006"/>. In fact, the duration needs to be quite short, and it's often 30 seconds or less.* '''Recovery'''. The recovery period can be extremely short, which tends some have suggested that the goal of HIIT is to mimic some of maximize the characteristics of a lower intensity, longer duration interval. At the other extremetime above 90% [[VO2max|V̇O<sub>2</sub>max]]<ref name="BuchheitLaursen2013"/>. Work intensity is nearly always constant, though there is no reason why the recovery work for a given interval can be long enough to ensure nearly complete recoverya ramp, typically taking several minuteseither increasing or decreasing in intensity during the interval. The recovery intensity can be activeLikewise, but the effort is nearly always fairly lowintensity could vary between intervals, either incrementing or decrementing.=HIIT Comparisons on Untrained * '''Work Duration'''. Given intensities at or Moderately Active Subjects=The table below looks at studies that have compared HIIT with other types of trainingabove [[VO2max| V̇O<sub>2</sub>max]], the duration needs to be quite short, and it's often Continuous Moderate Exercise (CME)30 seconds or less. These studies on untrained or moderately trained subjects generally show a greater improvement in fitness measure compared with other forms of Work duration is typically constant throughout the interval trainingsession, or similar improvements for far less training timebut there's no reason why it couldn't change. {{:* '''Recovery Intensity'''. The Science of High Intensity Interval Training-table}}==Aerobic High-Intensity Intervals Improve VO2Max More Than Moderate Training==This study<ref name="Helgerud-2007"/> was on 40 male university studentsrecovery intensity can be active, but the effort is nearly always fairly low. The work done during the recovery. Can actually help metabolize lactate, all physically active and with [[VO2max|VO<sub>2</sub>max]] of 55-60improve recovery in some ways.* '''Recovery Duration'''. The recovery period can be extremely short, which is translates tends to marathon times of 2:55 to 2:43, so they're pretty fit. The four running based training interventions were designed to have mimic some of the same amount characteristics of total worka lower intensity, which makes this study a little unusual as often HIIT requires far less work than other approacheslonger duration interval. Each program included 3 workouts per week and lasted for 8 weeks. The training is described in At the table aboveother extreme, and all performed on a treadmill at 5.3% incline. I noticed that the LSD is somewhat higher intensity 70% [[Maximum Heart Rate]]recovery can be long enough to ensure nearly complete recovery, and shorter 45 min than Itypically taking several minutes. * '''Number of Intervals'''d expect for typical LSD training. The [[Lactate Threshold]] was defined as 1.5 mmol/l above restingThe number of intervals used can vary dramatically, which is slightly oddwith one study looking at a single interval, but no worse than most protocols through to studies that don't use several dozen intervals. There is some evidence that fewer intervals are more effective. Other variables include the gold standard possibility of Maximum Lactate Steady Staterepeating a series of intervals with a longer break. The LSD and LT runs decreased the speed of the treadmill as the heart rate rose due For instance, it will be possible to drift. The short interval protocol was based around 15 seconds at 90do something like 6x(30s+30s recovery), then take a two-95% [[Maximum Heart Rate]] with 15 seconds minute recovery, which seems to translate to a fairly steady state heart ratethen repeat the six intervals again. They did 47 repetitionsThere are also macro variables, which is quite a stunning number, and far more than such as the frequency of performing the HIIT workouts. ==Naming Convention==I've not found a naming convention for HIIT, so I'd have expected ve started to be doable. By comparison, the four repeats of 4 minutes with 3-minute recoveries is rather more mainstream. All four protocols burned similar levels of oxygen.use HIIT-[d][r][File:Helgerud-2007-HRi].jpg|center|thumb|600px|Heart rate In my convention, [d] is duration, with S for the four interventionsShort, top left to bottom right: LSDtypically 30 seconds or less, Lactate Thresholdand L is for Long, 47x15+15, 4x4+3typically a minute or longer.Then [r]]The results were that [[VO2max|VO<sub>2</sub>max]] went down fractionally for LSD is the recovery, with C for Complete, which is typically several minutes of recovery, and up fractionally I is for Lactate ThresholdIncomplete, though neither was a significant change from baselinetypically less than the duration of the interval itself. The two interval training approaches raised [i] for intensity is based on the intensity of [[VO2max|VOV̇O<sub>2</sub>max]] with no significant difference, though so the 4x4 was slightly better than Tabata 170% would be x170. If only the 47x15. percent [[Running EconomyMaximum Heart Rate|HR<sub>max</sub>]] and , that is used instead. Where 'all out' is specified, I've used xAO, as the ratio of Wingate 'all out' power to [[Lactate ThresholdVO2max| V̇O<sub>2</sub>max]] were unchanged for all groupsappears to be 2-4x. Some examples from the studies reverenced here would be:[[File:Helgerud* HIIT-2007SIx90 is 15 seconds at 90-Vo2max.jpg|center|thumb|300px95% [[Maximum Heart Rate|HR<sub>max</sub>]]+ 15 seconds recoveries.* HIIT-SCxOA is 30 seconds 'all out''Conclusion''': this study suggests that high intensity training can improve the aerobic capacity of relatively fit subjects, while LSD and lactate threshold training + 4 min recovery.* HIIT-LCx90 is ineffective. However4 min run, the structure of the short intervals is rather unusual, and atypical, and I'm not sure I would consider either protocol truly "2 min rest at 94% [[Maximum Heart Rate|HR<sub>max</sub>]]* A classic Tabata would be HIIT".-SIx170==Aerobic Interval Training Versus Continuous Moderate Exercise as A more precise definition could use a Treatment for the Metabolic Syndrome==This is a similar study<ref name=convention based on QWKCODEs used by Golden Cheetah, but allowing for percentages. So "TjonnaLee20086x30s@90%r30s@50%"/> to the one above, both from "Norwegian University of Science and Technology", though the researchers appear differentwould be 30 seconds at 90% with 30 second recovery at 50%. The subjects are certainly different, as they are 32 patients with metabolic syndrome, average Using power at [[VO2max|VOV̇O<sub>2</sub>max]] is 34would be most precise, which translates to about a 4:20 marathon. This study used just two training programs; 47 minutes at 70% though many studies use percentages of [[Maximum Heart Rate]] or four intervals . ==Typical Styles of 4 minutes at 70% HIIT==There are several styles of [[Maximum Heart Rate]] HIIT. * The first, epitomized by Tabata is HIIT with 3-minute recoveriesshort, incomplete recovery periods. Each program included 3 workouts per week and lasted for 16 weeksIn this style, consisting of walking/running on an incline treadmill. the recovery periods are often 50-100% the duration of the interval, and the heart rate for this style of HIIT will often appear close to steady state. * The intervals resulted in a greater improvement in [[VO2max|VO<sub>2</sub>max]]more traditional style of interval training is long, and better mitigation complete recoveries, which have recoveries of several minutes, allowing the risk factors associated with metabolic syndromeheart rate to return to a much lower level and producing a distinct saw-toothed heart rate graph. <br/>'''Conclusion''': this extends In practice, recovery is not actually "complete", but the previous study's finding athlete should feel reasonably ready to less fit subjects who have medical issuesperform at nearly the same level. ==Short-term sprint interval versus traditional endurance training===HIIT Studies on Untrained or Moderately Active Subjects without Controls=While studies that compare HIIT * A third style is with other forms the short intervals of training are the most usefulTabata, there are a few other studies on untrained or moderately active people that are noteworthybut with complete recovery periods. For instance, six sessions of This style allows for greater intensity to be achieved for the short periods. =Optimizing High Intensity Intervals =There is remarkably little scientific basis for optimizing HIIT over two weeks doubled the endurance . Different studies use various combinations of untrained subjects intensity, duration, and recovery, and I found no attempt at 80% [[VO2max|V̇O<sub>2<finding the optimal combination. Below you'll find a few studies that will do some coarse comparisons, such as 15sec/sub>max]] from 25 to 51 minutes15sec compared with 4 min/4min, despite no change in but nothing more detailed. One hypothesis is that the intensity should be above the intensity that it generates [[VO2max|V̇O<sub>2</sub>max]]<ref name="BurgomasterLaursen-20051-2002"/>, a remarkable improvement. In another study, This is based on the combination of Continuous Moderate Exercise and moderate intensity intervals (60-70% observation<ref name="Bassett2000"/> that [[VO2max|V̇O<sub>2</sub>max]]) reduced body fat is limited by 15%, which was nine times more than Continuous Moderate Exercise alone, even though the Continuous Moderate Exercise burned over twice ability of the calories<ref name="Tremblay-1994"/>cardiovascular system to deliver oxygen to the working muscles. Another study also used This seems to be a combination of HIIT on 3 day/week plus running as remarkably tenuous basis, but there's not much else to go on so far as possible in 40 min on another 3 days/week , resulting in an increase in [. There is some evidence that if you want to train at or above [[VO2max|V̇O<sub>2</sub>max]] by 44%, as well as improved running endurance, with some subjects ending up with a then intermittent training allows athletes to maintain the [[VO2max|V̇O<sub>2</sub>max]] exceeding 60 ml/kg per min, which is remarkably high for 10 weeks of trainingintensity for much longer (3x) than continuous exercise<ref name="HicksonBillat-19772000"/>. =HIIT and Highly Trained Athletes=It has been suggested that elite athletes do not benefit from further increases If you accept this approach to intensity, then the duration of the intervals can be defined in volumeterms of [[vVO2max|T<sub>lim</sub>]], and should instead look to HIIT for performance improvementswhere [[vVO2max|T<sub>lim<ref name="Laursen-1-2002"/sub>. This ]] is backed up by studies the time to exhaustion at 100% [[VO2max|V̇O2max]]. (Obviously you can't train for longer than this.) Examples of some of the great endurance athletes, where higher training mileage produced worse rather than better performanceinterval structure that use [[vVO2max|T<sub>lim<ref name="elitemileage"/sub>. In the Lore ]] have durations of Running, Tim Noakes said that elite runners perform best "when they train between 50-75-125 miles (120-200 km) per week% of [[vVO2max|T<sub>lim</sub>]], with an increasing likelihood that they will perform indifferently when they train more than 125 miles (200 km) per week"and recovery periods are commonly the same as the interval, or twice the interval duration<ref name="LoRLaursen-P477"/>. Of course this is not universally true, and Mike Morton, set the US record holder for 24 hour while training 1401-150 miles/week<ref name="militarytimes.com2002"/>. However, the evaluation of HIIT on elite athletes is not as easy as lessor folk. ItIt's not practical to compare the effect of HIIT with other forms of exercise in been observed that highly trained athletes as they are typically already performing large volumes runners take at least 60% 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[[vVO2max|T<sub>lim</sub>]] to reach [[VO2max|V̇O<sub>2<ref name="Lindsay-1996"/sub>max]] <ref name="Westgarth-TaylorHill-1997"/>* A study of elite cyclists used various HIIT workouts . This has been taken as shown in the table belowan indication that intervals should be at least that long, but of course that observation only holds true for a single bout, with the best results seen group 4 or group 1<ref name="Stepto-1999"/>not a repeated set of intervals. Group 4 trained at 85% peak powerIn fact, which corresponds to there's some evidence that exercise above [[Lactate Threshold]] but below 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[[VO2max|V̇O<sub>2</sub>max]] can increase oxygen consumption to the [[VO2max|V̇O<sub>2</sub>max]] level<ref name="Lucia2000"/><ref name="Billat-1999"/>. (This raises is often referred to as the possibility |V̇O<sub>2</sub> slow component.) One study found 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 30 second intervals with 30 second recoveries allows for more time at or above 95% [[VO2max|V̇O2max]] than 3 minute intervals with 3 minute recoveries<ref name="ZunigaBerg2011"/>. ==Effects of different interval-training programs on cycling time-trial performance (Stepto-1999)==This is one of the few studies that compares different types of interval training<ref name="Stepto-1999"/>. Unfortunately, this study only had 20 cyclists, divided into five groups, which produced a group size of only four athletes (one cyclist dropped out giving one group only three athletes.) The study varies the intensity and duration inversely from 175%/0.5 minute to 80%/8 minutes. The study measured 40Km time trial performance, peak power output, and Sprint performance. {{:The Science of High Intensity Interval Training-table-Stepto1999}}Three of the protocols improved the time trial performance, but there was a lot of variability between the individual athletes. The study attempts to link the different protocols using a cubic trend, which seems a little bizarre to me. The intervals above [[VO2max|V̇O2max]] of group 1 produced results that varied from no improvement through to more than 4% improvement in TT, while the slightly longer [[VO2max|V̇O2max]] intensity intervals of group 2 produced a little improvement in TT. It might be tempting to conclude that HIIT needs to be greater [[VO2max|V̇O<sub>2</sub>max]], but this could easily be skewed by the small sample size. The longer intervals that are closer to a typical aerobic interval training session produce more consistent improvements in TT, but again, a small sample size makes it hard to be confident.[[File:Stepo1999.jpg|center|thumb|300px|]]==Time To Benefit==[[Modeling Human Performance]] is based on the premise that any training stress will produce a short-term impairment in performance, followed by a longer-term benefit. This is seen in every day training, where a hard training session results in degraded performance the next day. The time between the training stress and the benefit seems to vary based on multiple factors, including training status, type of training, and individuality. HIIT with endurance trained cyclists resulted in no improvement after two weeks, but it did after four weeks<ref name="Lindsay-1996"/>. A study of eight endurance trained competitive cyclists who had not performed any interval training for at least three months suggests the bulk of the improvement occurs within 12 days<ref name="Westgarth-TaylorHawley1997"/>. The training was 6-9x (5min @ 80% [[VO2max|V̇O<sub>2</sub>max]] + 1 min recovery). This is not really intensive enough to be HIIT, and is closer to traditional aerobic intervals, so caution is needed in interpreting the results.
{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
! GroupDays! Number of intervalsSessions! Interval duration (min)! Total work time (min)! Intensity (% peak power)! Rest (min)! Total Time! Improvement in 40K Time Trial Speed! Improvement in Peak Power
|-
| 1Baseline| 12| 0.5| 6| 175%| 4.5| 60 min| 2.0%| 0.5%404
|-
| 12
|-
| 24
| 8
| 4| 32| 85%| 1.5| 44 min| 2.5%| 2.0%421
|-
| 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%424
|}
=Limitations of the HIIT science=
There are some important limitations of the HIIT science.
=References=
<references>
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