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High 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 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.
=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|V?OV̇O<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"/>.
=HIIT Recommendations=
Below are my recommendations based on my interpretation of the available evidence. These recommendations for incorporating HIIT in your training depend on your current fitness goals.
{{Main|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|V?OV̇O<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|V?OV̇O<sub>2</sub>max]], then calculate 1.7x the [[VO2max|V?OV̇O<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|V?OV̇O<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|V?OV̇O<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.
|}
=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|V?OV̇O<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|V?OV̇O<sub>2</sub>max]].
{| class="wikitable"
! [[VO2max|V?OV̇O<sub>2</sub>max]]
! 35
! 40
| rowspan="4"|
Moderately trained
([[VO2max|V?OV̇O<sub>2</sub>max]] 51-55)
| rowspan="4"|
3 days/week
| '''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|V?OV̇O<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|V?OV̇O<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|V?OV̇O<sub>2</sub>max]] 5.5%
|-
| '''Lactate threshold run'''
24 min at 85% [[Maximum Heart Rate|HR<sub>max</sub>]]
| [[VO2max|V?OV̇O<sub>2</sub>max]] unchanged
|-
| '''Long Slow Distance'''
45 minutes at 70% [[Maximum Heart Rate|HR<sub>max</sub>]]
| [[VO2max|V?OV̇O<sub>2</sub>max]] unchanged
|-
| rowspan="2"|
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|V?OV̇O<sub>2</sub>max]] 36%
| rowspan="2"|
HIIT
| '''Continuous Moderate Exercise'''
47 min at 70% [[Maximum Heart Rate|HR<sub>max</sub>]]
| Raised [[VO2max|V?OV̇O<sub>2</sub>max]] 16%
|-
| rowspan="2"|
8 weeks
| '''HIIT'''
30x 30 sec @ 100% [[VO2max|V?OV̇O<sub>2</sub>max]] + 30 sec rest| Raised [[VO2max|V?OV̇O<sub>2</sub>max]] 9-16%
No change in blood lactate during continuous exercise
| rowspan="2"|
|-
| '''Continuous Moderate Exercise'''
30 minutes at 50% [[VO2max|V?OV̇O<sub>2</sub>max]]| Raised [[VO2max|V?OV̇O<sub>2</sub>max]] 5-7%
Reduced blood lactate during continuous exercise by nearly 50%
|-
Tabata-1996<ref name="Tabata-1996"/>
| rowspan="2"|
14 varsity level collage athletes ([[VO2max|V?OV̇O<sub>2</sub>max]] ~50)
| rowspan="2"|
5 days/week
6 weeks
| '''HIIT'''
4 days/week 7-8x (30 seconds at 170% [[VO2max|V?OV̇O<sub>2</sub>max]] + 10 seconds rest)1 day/week 30 min at 70% [[VO2max|V?OV̇O<sub>2</sub>max]] + 4x (30 seconds at 170% [[VO2max|V?OV̇O<sub>2</sub>max]] + 10 seconds rest)| Raised [[VO2max|V?OV̇O<sub>2</sub>max]] by 14.5%
Increased anaerobic capacity by 28%
| rowspan="2"|
HIIT
| rowspan="2"|
HIIT produced a greater improvement in [[VO2max|V?OV̇O<sub>2</sub>max]] for far less time commitment
|-
| '''Continuous Moderate Exercise'''
60 minutes at 70% [[VO2max|V?OV̇O<sub>2</sub>max]]| Raised [[VO2max|V?OV̇O<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|V?OV̇O<sub>2</sub>max]] from 25 to 51 minutes, despite no change in [[VO2max|V?OV̇O<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|V?OV̇O<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|V?OV̇O<sub>2</sub>max]] by 44%, as well as improved running endurance, with some subjects ending up with a [[VO2max|V?OV̇O<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 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.
| 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|V?OV̇O<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|V?OV̇O<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|V?OV̇O<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|V?OV̇O<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|V?OV̇O<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|V?O<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|V?OV̇O<sub>2</sub>max]] which was not statistically different between group 3 and the controls.
{| class="wikitable"
! Group
! Interval Duration
! Total work time (min)
! Intensity (% [[VO2max|V?OV̇O<sub>2</sub>max]])
! Rest
! Total time
! [[VO2max|V?OV̇O<sub>2</sub>max]] change
! 40K Time Trial Speed improvement
|-
| -1%
|}
* Elite cyclists performed 4 sessions of HIIT (20x 60 sec at [[VO2max|V?OV̇O<sub>2</sub>max]] + 120 sec recovery) that improved peak power by 4% but had no change in [[VO2max|V?OV̇O<sub>2</sub>max]]<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|V?OV̇O<sub>2</sub>max]]. While [[VO2max|V?OV̇O<sub>2</sub>max]] 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.
