The Science of High Intensity Interval Training (HIIT) Tabata and Wingate

To run HIIT intervals requires a longer stride length, so other modes, such as a stationary bike may be more appropriate.

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.

Contents

1 Summary

Here are the high-level conclusions from the available research.

• The preponderance of evidence shows that HIIT provides greater improvements in V̇O₂max than continuous moderate exercise.
• The ideal intensity for HIIT is unclear, but generally believed to be above 90% V̇O₂max.
• 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.

2 Components of HIIT

There are several variables that shape a HIIT workout.

• 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 V̇O₂max. The classic Tabata workout used 1.7x the work at V̇O₂max, though many implementations of this workout simply use "all out." Some consider that the threshold of HIIT to be 90% of the effort of V̇O₂max . In fact, some have suggested that the goal of HIIT is to maximize the time above 90% V̇O₂max. Work intensity is nearly always constant, though there is no reason why the work for a given interval can be a ramp, either increasing or decreasing in intensity during the interval. Likewise, the intensity could vary between intervals, either incrementing or decrementing.
• Work Duration. Given intensities at or above V̇O₂max, the duration needs to be quite short, and it's often 30 seconds or less. Work duration is typically constant throughout the interval training session, but there's no reason why it couldn't change.
• Recovery Intensity. The recovery intensity can be active, but the effort is nearly always fairly low. The work done during the recovery. Can actually help metabolize lactate, and improve recovery in some ways.
• Recovery Duration. The recovery period can be extremely short, which tends to mimic some of the characteristics of a lower intensity, longer duration interval. At the other extreme, the recovery can be long enough to ensure nearly complete recovery, typically taking several minutes.
• Number of Intervals. The number of intervals used can vary dramatically, with one study looking at a single interval, through to studies that use several dozen intervals. There is some evidence that fewer intervals are more effective.

Other variables include the possibility of repeating a series of intervals with a longer break. For instance, it will be possible to do something like 6x(30s+30s recovery), then take a two-minute recovery, then repeat the six intervals again. There are also macro variables, such as the frequency of performing the HIIT workouts.

2.1 Naming Convention

I've not found a naming convention for HIIT, so I've started to use HIIT-[d][r][i]. In my convention, [d] is duration, with S for Short, typically 30 seconds or less, and L is for Long, typically a minute or longer. Then [r] is the recovery, with C for Complete, which is typically several minutes of recovery, and I is for Incomplete, typically less than the duration of the interval itself. The [i] for intensity is based on the intensity of V̇O₂max, so the Tabata 170% would be x170. If only the percent HR_max, that is used instead. Where 'all out' is specified, I've used xAO, as the ratio of Wingate 'all out' power to V̇O₂max appears to be 2-4x. Some examples from the studies reverenced here would be:

• HIIT-SIx90 is 15 seconds at 90-95% HR_max + 15 seconds recoveries.
• HIIT-SCxOA is 30 seconds 'all out' + 4 min recovery.
• HIIT-LCx90 is 4 min run, 2 min rest at 94% HR_max
• A classic Tabata would be HIIT-SIx170

A more precise definition could use a convention based on QWKCODEs used by Golden Cheetah, but allowing for percentages. So "6x30s@90%r30s@50%" would be 30 seconds at 90% with 30 second recovery at 50%. Using power at V̇O₂max would be most precise, though many studies use percentages of Maximum Heart Rate.

2.2 Typical Styles of HIIT

There are several styles of HIIT.

• The first, epitomized by Tabata is HIIT with short, incomplete recovery periods. In this style, 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 more traditional style of interval training is long, complete recoveries, which have recoveries of several minutes, allowing the heart rate to return to a much lower level and producing a distinct saw-toothed heart rate graph. In practice, recovery is not actually "complete", but the athlete should feel reasonably ready to perform at nearly the same level.
• A third style is with the short intervals of Tabata, but with complete recovery periods. This style allows for greater intensity to be achieved for the short periods.

3 Optimizing High Intensity Intervals

There is remarkably little scientific basis for optimizing HIIT. Different studies use various combinations of intensity, duration, and recovery, and I found no attempt at finding the optimal combination. Below you'll find a few studies that will do some coarse comparisons, such as 15sec/15sec compared with 4 min/4min, but nothing more detailed. One hypothesis is that the intensity should be above the intensity that it generates V̇O₂max. This is based on the observation that V̇O₂max is limited by the ability of the cardiovascular system to deliver oxygen to the working muscles. This seems to be a remarkably tenuous basis, but there's not much else to go on so far. There is some evidence that if you want to train at or above V̇O₂max, then intermittent training allows athletes to maintain the V̇O₂max intensity for much longer (3x) than continuous exercise. If you accept this approach to intensity, then the duration of the intervals can be defined in terms of T_lim, where T_lim is the time to exhaustion at 100% V̇O2max. (Obviously you can't train for longer than this.) Examples of interval structure that use T_lim have durations of 50-75% of T_lim, and recovery periods are commonly the same as the interval, or twice the interval duration. It's been observed that highly trained runners take at least 60% of T_lim to reach V̇O₂max . This has been taken as an indication that intervals should be at least that long, but of course that observation only holds true for a single bout, not a repeated set of intervals. In fact, there's some evidence that exercise above Lactate Threshold but below the intensity V̇O₂max can increase oxygen consumption to the V̇O₂max level. (This is often referred to as the |V̇O₂ slow component.) One study found that 30 second intervals with 30 second recoveries allows for more time at or above 95% V̇O2max than 3 minute intervals with 3 minute recoveries.

3.1 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. 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.

Group	Subjects	Number of intervals	Interval duration (min)	Total work time (min)	Intensity (% peak power)	Rest (min)	Cumulative Intervals	Duration * intensity	Total Time	Improvement in 40K Time Trial Speed	Improvement in Peak Power
1	4	12	0.5	6	175%	4.5	6 min	10.5	60 min	2.0%	0.5%
2	3	12	1	12	100%	4.0	12 min	12	60 min	0.0%	0.5%
3	4	12	2	24	90%	3.0	24 min	21.6	60 min	1.5%	1.5%
4	4	8	4	32	85%	1.5	32 min	27.2	44 min	2.5%	2.0%
5	4	4	8	32	80%	1.0	32 min	25.6	36 min	0.0%	1.0%

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 V̇O2max of group 1 produced results that varied from no improvement through to more than 4% improvement in TT, while the slightly longer V̇O2max intensity intervals of group 2 produced a little improvement in TT. It might be tempting to conclude that HIIT needs to be greater V̇O₂max, 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.

3.2 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. 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. The training was 6-9x (5min @ 80% V̇O₂max + 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.

Days	Sessions	Power
Baseline	0	404
12	4	419
24	8	421
36	12	424

3.3 Training Frequency

Most studies seem to train their subjects 3-4 times per week, and while the protocol seems intuitively reasonable, there's little supporting evidence. A study of 10 healthy volunteers compared to training protocols that were identical other than the frequency of training. Both protocols used 30 seconds of maximal cycling with 12-minute recovery periods (unusually long.) The number of repetitions was increased during the study as the subjects became fitter. In one group, the subjects trained every day, while the other group had to day rest periods between each training session. The subjects without rest days did not improve either average or peak power during a 30-second Sprint test, while the subjects that did have rest days improved both of values. This is weak evidence that rest days between HIIT sessions are good idea.

3.4 Recovery Duration

Most studies tend to use a simple ratio of interval to recovery period of 2:1, 1:1, 1:2. The famous Tabata compared his usual regime of 6-7x (20seconds @ 170% V̇O₂max + 10 seconds recovery) with 4-5x(30 seconds @ 200% V̇O₂max + 120 seconds recovery) . Not surprisingly, the shorter recovery times taxed both aerobic and anaerobic systems far more than the longer recovery times, but there was no evaluation of any performance benefit from the two protocols. A study of seven physical education students tested recovery periods of 30, 60, & 120 seconds between fifteen 40 m sprints. The Sprint times were only ~5-6 seconds long, and for the 120 seconds recovery, the Sprint times remained fairly constant, for the 60 second recovery, the lost three sprints were slower, and for the 30 second recovery there was a progressive slow down for each interval. Blood lactate levels were surprisingly similar for all recovery periods, only becoming different after the session had completed, with the longer recovery is having lower blood lactate that dropped more quickly. One study found that not surprisingly the difficulty (RPE) varied dramatically with the recovery duration, with the shorter recoveries being harder.

4 Recovery Intensity

Most studies use active recovery, where the athlete continues to exercise between intervals. This activity may speed recovery by metabolizing the lactic acid faster than complete rest, but this seems to be no evidence comparing different levels of recovery intensity.

5 Number of Intervals

A meta-analysis of 38 trails over 34 studies found that fewer intervals was more effective than more. They found that improvements in V̇O₂max was on average 1.2% less for each two intervals. While this seems counterintuitive, it suggests that not the intensity that can be maintained over multiple intervals drops, but even the initial intensity might be lower. From a "central governor" perspective, this would make sense, as effort is reduced in the face of a longer effort. (My personal experience is that for "all out", two intervals, with the second slightly easier than the first provides the highest effort.)

The number of intervals against change in V̇O₂max.

6 HIIT Comparisons on Untrained or Moderately Active Subjects

This section 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. By comparing two different types of exercise, we get the most useful information.

Study	Subjects	Study length	Protocol	Outcome	Best Result	Notes
Tabata-1996	14 varsity level collage athletes (V̇O2max ~50)	5 days/week 6 weeks	HIIT (HIIT-SIx170) 4 days/week 7-8x (20 seconds at 170% V̇O2max + 10 seconds rest) 1 day/week 30 min at 70% V̇O2max + 4x (30 seconds at 170% V̇O2max + 10 seconds rest)	Raised V̇O2max by 14.5% Increased anaerobic capacity by 28%	HIIT	HIIT produced a greater improvement in V̇O2max for far less time commitment
			Continuous Moderate Exercise 60 minutes at 70% V̇O2max	Raised V̇O2max by 9.5% No change in anaerobic capacity
Helgerud-2007	Moderately trained (V̇O2max 51-55)	3 days/week 8 weeks	Short HIIT (HIIT-SIx90) 47x 15 seconds at 90-95% HRmax + 15 seconds at 70% HRmax	Raised V̇O2max 5.5%	Short and Long HIIT (no statistical difference)	All groups improved economy, with no differences, and Lactate Threshold unchanged as a percentage of V̇O2max
			Long HIIT (HIIT-LCx90) 4x 4 min, 90-95% HRmax + 3 min at 70%max	Raised V̇O2max 7.2%
			Lactate Threshold run 24 min at 85% HRmax	V̇O2max unchanged
			Long Slow Distance 45 minutes at 70% HRmax	V̇O2max unchanged
TjonnaLee-2008	Untrained, metabolic syndrome patients	3 days/week 16 weeks	HIIT (HIIT-LCx90) 4x 4 min at 90% HRmax + 3 min 70% HRmax total 40 min,	Raised V̇O2max 36%	HIIT	Same calories burned in each group Both groups had an equal reduction in body weight and blood pressure
			Continuous Moderate Exercise 47 min at 70% HRmax	Raised V̇O2max 16%
Gibala-2006	Recreationally active	2 weeks	HIIT (HIIT-SCx260) 4-6x 30 seconds 'all out' + 4 min recovery (260%) Totals for two weeks, 135 minutes and 950 Kj	Same improvement in laboratory time trials	HIIT	Same improvement, but only 22% of the time commitment
			Continuous Moderate Exercise 90-120 min at 65% V̇O2peak Totals for two weeks, 630 minutes and 6500 Kj
Gorostiaga-1991	Sedentary	3 days/week 8 weeks	HIIT (HIIT-SIx100) 30x 30 sec @ 100% V̇O2max + 30 sec rest	Raised V̇O2max 9-16%	HIIT	Same average work in each group
			Continuous Moderate Exercise 30 minutes at 50% V̇O2max	Raised V̇O2max 5-7%
Franch-1998	36 recreational runners	3 days/week at high intensity Plus 3 runs/week <= 65% HRmax 6 weeks	Short HIIT (HIIT-SIx90) 30-40x 15 sec run, 15 sec rest Avg ~3.0 Km/workout 92% HRmax	Time to exhaustion increased 65% Running Economy improved 0.9%	Continuous High Intensity	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 (HIIT-LCx90) 4-6x 4 min run, 2 min rest Avg ~5.6 Km/workout 94% HRmax	Time to exhaustion increased 67% Running Economy improved 3.0%
			Continuous High Intensity 20-30 minutes Avg ~6.4 Km/workout 93% HRmax	Time to exhaustion increased 94% Running Economy improved 3.1%
BurgomasterHowarth-2007	20 Untrained	HIIT 3x week Continuous 5x week 6 weeks	HIIT (HIIT-SCxAO) 4-6x 30 seconds 'all out', 4.5 min rest 1.5 hours/week ~225 Kj/week	Both increased V̇O2peak by ~5%	HIIT	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% V̇O2peak 4.5 hours/week 2250 Kj/week
Trapp-2008	34 sedentary women	45 workouts over 15 weeks	HIIT (HIIT-SIxAO) 60x 8 seconds 'all out', 12 seconds rest (5 min Warmup, 20 min conditioning, 5 min Cooldown)	Increased V̇O2peak 24% 5 pound/2.5 Kg reduction in body fat Significant 31% reduction in fasting insulin Significant reduction in Leptin	HIIT	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% V̇O2peak	Increased V̇O2peak 19% 1 pound/0.5 Kg gain in body fat Non-significant 9% reduction in fasting insulin No change in Leptin
EarnestTjønna-2013	26 healthy overweight men (BMI 25-30)	3 days/week 10 weeks	4x HIIT (HIIT-LCx90) 10 min Warmup 4x 4 min at 90% HRmax + 3 min 70% HRmax 5 min Cooldown total 40 min	Raised V̇O2max by 13% Work economy improved by 13% Systolic blood pressure decreased 3.2 mmHg Diastolic blood pressure decreased 6.3 mmHg	Similar results with both protocols	This study showed remarkable results using a single high intensity bout of exercise.
			1x HIIT (HIIT-LCx90) 10 min Warmup 4 min at 90% HRmax 5 min Cooldown total 19 min	Raised V̇O2max by 10% Work economy improved by 14% Systolic blood pressure decreased 6.2 mmHg Diastolic blood pressure decreased 7.7 mmHg

6.1 Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max (Tabata-1996)

This study is arguably the origin of HIIT, and is the most famous name, Tabata. Izumu Tabata was the coach that tested a training protocol and compared it with the more conventional continuous exercise. The study is small, with just 14 male physical education students divided into two groups, and it's reasonably short at 5 days/week for only six weeks. The students had a V̇O₂max of ~50, which would translate to a marathon time of about 3:10. (I have a more intuitive grasp of someone's fitness level from their hypothetical marathon time than their raw V̇O₂max.) The small number of subjects weren't quite as closely matched as I would have liked, with the continuous exercise having a V̇O₂max of ~53 and the HIIT group having ~48. Both training protocols used a stationary bike, and both protocols increased the training intensity to maintain the required percentage of V̇O₂max. The continuous exercise protocol was 60 minutes at 70% V̇O₂max, and it raised V̇O₂max by 9.5%. The study also used a measure of anaerobic capacity by evaluating the oxygen debt, and found no change for the continuous exercise protocol. The HIIT protocol used 7-8x repeats of 20 seconds at 170% V̇O₂max + 10 seconds rest (HIIT-SIx170). It also included one day/week of 30 min at 70% V̇O₂max + 4x (30 seconds at 170% V̇O₂max + 10 seconds rest). It's unclear why there is this mixture of protocols for the HIIT. The HIIT included a 10-minute warmup at 50% V̇O2max. The HIIT training raised V̇O₂max by 14.5%, and increased anaerobic capacity by 28%. Conclusion: This study suggests that HIIT-SIx170 can increase V̇O₂max more than continuous moderate exercise, as well as improving anaerobic capacity in a way that doesn't occur with continuous moderate exercise.

6.2 Aerobic High-Intensity Intervals Improve VO2Max More Than Moderate Training (Helgerud-2007)

This study was on 40 male university students, all physically active and with V̇O₂max of 55-60, which is translates to marathon times of 2:55 to 2:43, so they're pretty fit. The four running based training interventions were designed to have the same amount of total work, which makes this study a little unusual as often HIIT requires far less work than other approaches. Each program included 3 workouts per week and lasted for 8 weeks. The training is described in the table above, and all performed on a treadmill at 5.3% incline. I noticed that the LSD is somewhat higher intensity 70% Maximum Heart Rate, and shorter 45 min than I'd expect for typical LSD training. The Lactate Threshold was defined as 1.5 mmol/l above resting, which is slightly odd, but no worse than most protocols that don't use the gold standard of Maximum Lactate Steady State. The LSD and LT runs decreased the speed of the treadmill as the heart rate rose due to drift. The short interval protocol was based around 15 seconds at 90-95% Maximum Heart Rate with 15 seconds recovery, which produced a fairly steady state heart rate. They did 47 repetitions, which is quite a stunning number, and far more than I'd have expected 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.

Heart rate for the four interventions, top left to bottom right: LSD, Lactate Threshold, 47x15+15, 4x4+3.

The results were that V̇O₂max went down fractionally for LSD and up fractionally for Lactate Threshold, though neither was a significant change from baseline. The two interval training approaches raised V̇O₂max with no significant difference, though the 4x4 was slightly better than the 47x15. Running Economy and Lactate Threshold were unchanged for all groups.

Conclusion: this study suggests that high intensity training can improve the aerobic capacity of relatively fit subjects, while LSD and lactate threshold training is ineffective. However, the structure of the short intervals is rather unusual, and atypical, and I would not classify the intensity as sufficient to quality as HIIT.

6.3 Aerobic Interval Training Versus Continuous Moderate Exercise as a Treatment for the Metabolic Syndrome (TjonnaLee2008)

This is a similar study to the one above, both from "Norwegian University of Science and Technology", though the researchers appear different. The subjects are certainly different, as they are 32 patients with metabolic syndrome, average V̇O₂max is 34, which translates to about a 4:20 marathon. This study used just two training programs; 47 minutes at 70% of Maximum Heart Rate or four intervals of 4 minutes at 70% of Maximum Heart Rate with 3-minute recoveries. Each program included 3 workouts per week and lasted for 16 weeks, consisting of walking/running on an incline treadmill. The intervals resulted in a greater improvement in V̇O₂max, and better mitigation of the risk factors associated with metabolic syndrome.
Conclusion: this extends the previous study's finding to less fit subjects who have medical issues.

6.4 Short-term sprint interval versus traditional endurance training (Gibala-2006)

This small, short study used a group of 16 active men, with V̇O₂max of ~50, which is translates to about a 3:10 marathon time. They were divided into two groups, both groups training on a stationary bike six times over two weeks. The HIIT group did "all out" intervals of 30 seconds with 4-minute recoveries, repeated 4-6 times. The study noted that "all-out" was about 700 watts, which is quite intense. The endurance group did 90-120 minutes at 65% of V̇O₂max, which was noted at about 175 watts. Given the two groups had similar V̇O₂max values, that means the HIIT group were exercising at 260% of the effort for V̇O₂max, far higher than the 170% of that Tabata used. I'd classify the intervals as HIIT-SCx260. On average the HIIT exercised for 135 minutes, burning ~230 Calories while the endurance group exercised for 630 minutes and burned ~1550 Calories. Both groups improved their time trail performance by similar amounts. The study looked at lots of factors, but the small group size made many of the differences non-statistical.

The changes in time trial for HIIT (SIT) and endurance (ET).

Conclusion: The HIIT-SCx260 produced similar performance improvements for less than a quarter the time.

6.5 Uniqueness of interval and continuous training at the same maintained exercise intensity (Gorostiaga-1991)

This is a small study of 12 subjects for 8 weeks, with an initial V̇O₂max of ~36, which is translates to a 4:10 marathon. Half the subjects cycled for 30 minutes at 50% the effort of V̇O₂max, the other half did 30 seconds at 100% the effort for V̇O₂max with 30 seconds recovery, repeated 30 times. This roughly equates to the same training stress for both groups. The interval training was short, Incomplete-recovery at 100%, or HIIT-SIx100. The HIIT group improved V̇O₂max by 9-16% while the continuous training group improved by 5-7%. As fitness improved, the training load increased to maintain the same percentage of V̇O₂max. Because of this, the training load in the HIIT group rose by 10% compared with 5% for the continuous training group. There were changes in Lactate levels after training, but these are tricky to interpret. Both groups had their Lactate levels checked during intermittent and continuous exercise, before training, and again after the training at both the same absolute workload and the same relative workload. I'm hesitant to draw any conclusions from the lactate levels as there are too many variables to consider. The HIIT training is slightly unusual, as 30 repetitions is far higher than most HIIT approaches. Conclusion: The HIIT-SIx100 improved V̇O₂max more than continuous exercise, but the training used more repetitions at a lower intensity than is typical of HIIT.

6.6 Improved running economy following intensified training correlates with reduced ventilatory demands (Franch-1998)

This is a slightly larger study of 36 runners with a pre-training V̇O₂max of ~55, which translates to a 2:55 marathon. All are established recreational runners, and were divided into three groups. Four weeks before the test, the runners' training was evaluated and on average they ran for 2.2 hours/week at about 65% of max heart rate. The study then used three intense training runs per week for six weeks, plus 1-3 of their normal training runs per week. Three different intense training programs were used. Short HIIT was 30-40x 15 sec run with 15 sec rest (SIx90), Long HIIT was 4-6x 4 min run with 2 min rest (LCx90), and Distance Training (DT) was 20-30 minutes. The intensity and repetitions were designed to so the runners reached exhaustion, with average paces of 6:25 min/mile for DT, 5:49 for Long HIIT, and 4:43 for Short HIIT. My interpretation is that the DT training was well above Lactate Threshold, though it's hard to be sure from the details provided. The distance covered was 4miles/6.4Km for DT, 3.5miles/5.6Km for Long HIIT, 1.9miles/3Km for Short HIIT. While the average training load was 2.7 hours/week, it varied between 1.8 and 5.2 hours, and it's not clear if the distribution was even between the interventions. The average heart rate was reported as 92-94% of max HR for all interventions. V̇O₂max improved for all groups, with DT=5.9%, Long HIIT=6.0%, and Short HIIT=3.6%. The time to exhaustion increased dramatically, with DT=94%, Long HIIT=67%, and Short HIIT=65%. [Running Economy]] improved in both DT and Long HIIT, but not Short HIIT, while heart rate and lactate were comparably lower for all interventions. Conclusion: This study indicates that a high volume (30-40) of short, moderately high intensity intervals is not as effective as either longer intervals or continuous running at a similar intensity.

6.7 Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans (BurgomasterHowarth2007)

This is study of 20 subjects with a pre-training V̇O₂max of ~41, which translates to a 3:45 marathon. The training consisted of 3 cycling workouts per week for six weeks. The Endurance group hich cycled at 65% V̇O₂max for an hour, while the HIIT group cycled did 4-6 repeats of 30 seconds "all out" with 4.5 minute recoveries. The "all out" intensity was about 500w, which equates to about 220% of the effort for V̇O₂max, so this is HIIT-SCx220. The training effort was 1.5 hours/week for the HIIT and 4.5 hours/week for the endurance, and the Calories burned were roughly ten times higher in the endurance group. Both groups improved V̇O₂max by similar amounts (41 to 45), and had similar changes in enzyme activity. Conclusion: Similar V̇O₂max improvement for far less time with HIIT than continuous training.

6.8 Low- and High-Volume of Intensive Endurance Training Significantly Improves Maximal Oxygen Uptake after 10-Weeks of Training in Healthy Men (EarnestTjønna2013)

This study was to evaluate the incremental benefits of additional repetitions in overweight, sedentary men. The subjects were 26 healthy overweight men (BMI 25-30), who had not exercised regularly for at least two years. The V̇O₂max for the two groups was a little different, with the single repetition group having a V̇O2max of 40 while the four-repetition group was ~45. That translates to marathon time of 3:50 and 3:28 respectively. The four-repetition group was also about 20Lb/10kg lighter. These differences could influence the outcome. The study had the subjects exercise on a treadmill for 3 days/week for 10 weeks. After the training, both groups improved, but the study provided no statistical analysis as to which differences were statistically significant. The four-repetition group improved V̇O₂max by 13% and work economy by 13%, while the single repetition group improved by 10% and 14% respectively. There were drops in blood pressure, but without statistical significance calculations it's hard to interpret those drops. Conclusion: A single repetition of four minutes high intensity produced broadly similar improvements in fitness as a single repetition for sedentary, overweight, but otherwise healthy men.

6.9 Towards the minimal amount of exercise for improving metabolic health (MetcalfeBabraj2011)

This is another study that uses a minimalist approach to HIIT, something they call reduced-exertion HIT (REHIT). The focus of the study is prevention of type II diabetes, and general health, rather than athletic improvement. The study compared 15 healthy subjects that underwent HIIT with 14 matched controls that remained sedentary. Their BMI was normal (~22-25), with starting V̇O₂max of ~37 (~4:05 marathon equiv.) for men and ~33 for women (~4:28 marathon equiv.) The protocol was minimalistic, with the intervention subjects exercising for 10 minutes, 3 times a week, for 6 weeks. The intervention subjects exercised for 10 minutes at a very low intensity (60 W). In the first week the training included 1x 10-second "all out sprint", weeks 2-3 had 2x 10-second sprints, then weeks 4-9 increased the duration from 10 seconds to 15, and weeks 10-18 increased the generations to 20-seconds. In weeks 2-18, the intervals occurred roughly at the 3 and 7-minute marks. V̇O₂max increased by 12% in the women and 15% in the men, a remarkable improvement for such a low time commitment. Only the trained men had a significant change in insulin sensitivity. I suspect this is due to the small group size. Conclusion: Just two 20 second "all-out" intervals produced remarkable improvements in untrained subjects.

6.10 A comparison of the health benefits of reduced-exertion high-intensity interval training (REHIT) and moderate-intensity walking in type 2 diabetes patients (RuffinoSongsorn2017)

This is another study looking at reduced-exertion HIT (REHIT). This study used 16 men with type II diabetes and morbid obesity (BMI >35), who were otherwise reasonably healthy. Their V̇O2max was ~27, which is too low to really have a marathon equivalent. The subjects performed either REHIT (3 days/week) or 30 minutes walking (5 days/week) for 8 weeks, then had an 8 week washout period, then performed the other training intervention for a final 8 weeks, in a crossover, counterbalance design. The REHIT training was similar to the above study, with intervals increasing from 10 seconds to 20 seconds over the intervention. The study noted that the all-out cycling sprints were at 350% of V̇O₂max. The REHIT resulted in 7% V̇O₂max improvement compared with just 1% for walking, with both interventions reducing blood pressure to a similar extent. Neither intervention improved insulin sensitivity or glycemic control. 12 of the 16 subjects preferred the interval training over walking. Conclusion: As above, just 2 "all-out" intervals produced better results than continuous moderate exercise lasting 5 times as long.

7 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.

7.1 Six sessions of sprint interval training increases muscle oxidative potential and cycle endurance capacity in humans (Burgomaster-2005)

Six sessions of HIIT over two weeks doubled the endurance of untrained subjects at 80% V̇O₂max from 25 to 51 minutes, despite no change in V̇O₂max, a remarkable improvement. 16 subjects were used with 8 acting as controls, and their V̇O2max was ~45 (3:30 marathon equivalent.) Improvements in endurance aren't always seen with HIIT (or maybe just not looked for). The HIIT also increased peak power and glycogen storage.

7.2 Linear increase in aerobic power induced by a strenuous program of endurance exercise (Hickson-1977)

The 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 V̇O₂max by 44%, as well as improved running endurance, with some subjects ending up with a V̇O₂max exceeding 60 (2:43 marathon equivalent), which is remarkably high for 10 weeks of training.

8 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. This is backed up by studies of some of the great endurance athletes, where higher training mileage produced worse rather than better performance. 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". Of course, this is not universally true, and Mike Morton, set the US record holder for 24-hour while training 140-150 miles/week. 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.

8.1 Improved athletic performance in highly trained cyclists after interval training (Lindsay-1996)

HIIT improved peak power output and 40 Km time trial in elite cyclists. The subjects were 12 competitive male cyclists who'd been training for at least four years, but had not performed any interval training in the last three months. Their average V̇O₂max was 65 (2:33 marathon equivalent), and their power at V̇O₂max was 416w. The HIIT used 6-8x(5 min @ 80% V̇O₂max with 1 min at 100w recoveries.) The HIIT was performed over 6 sessions spread out over 4 weeks.

8.2 Metabolic and performance adaptations to interval training in endurance-trained cyclists (Westgarth-Taylor-1997)

This is study is from the same university and has a common subset of researchers as the above study (Lindsay-1996). It's not clear if this study used a subset of the subjects above, but there are noteworthy similarities. Instead of 12 cyclists, this study used 8, with a similar fitness levels, and a similar HIIT protocol. Instead of 6-8 repeats, this study used 6-9, with 12 sessions over 6 weeks. The results are broadly similar to the previous study.

8.3 Effects of 4-wk training using Vmax/Tlim on VO2max and performance in athletes (Smith-1999)

Five 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 V̇O₂max by 4.9% (61 to 64). That V̇O2max improvement represents a marathon equivalent of 2:41 to 2:35.) The HIIT training consisted of 2 sessions per week of 6 intervals at 100% V̇O₂max with time varying between 60-75% T_lim, plus one weekly run of 30 min at 60% vV̇O₂max. For these runners, T_lim averaged 225 seconds, so the intervals were between 135 and 170 seconds.

8.4 Optimising high-intensity treadmill training using the running speed at maximal O(2) uptake and the time for which this can be maintained (Smith-2003)

Well trained, competitive runners trained twice a week for four weeks with intervals at 100% V̇O₂max for either 6x 60% T_lim (133 sec) or 5x 70% (154 sec) T_lim, resting for twice the interval time. Their 3K time improved by 17.6 sec (60% T_lim) or 6.3 sec (70% T_lim), but there was no change in their 5K time.

8.5 Interval training program optimization in highly trained endurance cyclists (Laursen-2002)

41 elite (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. Note that groups 1 and 2 vary only in their rest time, which is based on Heart Rate dropping to 65% of 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 V̇O₂max which was not statistically different between group 3 and the controls. The Science of High Intensity Interval Training-table- Laursen-3-2002

8.6 Acute high-intensity interval training improves Tvent and peak power output in highly trained males (Laursen-2-2002)

This study used 14highly trained cyclists, V̇O₂max of 60 (marathon equivalent of 2:43) . They had only performed low intensity training over the previous 2 months. Half the subjects performed 4 sessions over 2 weeks of HIIT (20x 60 sec at V̇O₂max + 120 sec recovery @ 50w). The HIIT group improved their peak power by 4% but had no change in V̇O₂max.

9 HIIT and Weight Loss

9.1 The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women (Trapp-2008)

This is a rather different study, focusing on 45 young women who are sedentary and on the upper end of the normal BMI scale (~22-24 Kg/m²), but healthy. Their V̇O₂max was ~30, which would translate to about a 5-hour marathon. The subjects were split into three groups, interval training, continuous training, and a control group (few studies on HIIT include a control group.) The interval group did up to 60x 8 seconds 'all out' cycling, 12 seconds rest (HIIT-SIxAO). The continuous group did up to 40 minutes cycling at 60% V̇O₂peak. The interval group started with as little as 5 minutes of intervals, building up to 20 minutes, at which point the resistance was increased. By the end of 2 weeks all interval subjects were completing the 20 minutes. Likewise, the continuous training group started with 10-20 minutes and gradually built up to the full 40 minutes. V̇O₂max increased by ~24% for the interval training group and ~19% for the continuous group. Perhaps more interestingly, the weight of the continuous training group didn't change, but the interval training group lost weight. The interval training group lost body fat (~2.5Kg/5lb), while the continuous training group actually gained some body fat (~0.5Kg/1lb). This study used an unusually large number of unusually short intervals for the HIIT group. Sadly, I couldn't work out what intensity constituted "all out." This is a study that used a much higher number of repetitions than I see most runners performing.

Fasting insulin also dropped far more for the interval training group than the other groups.

Conclusion: For sedentary women, HIIT improved V̇O₂max and fasting insulin more than continuous exercise, and HIIT resulted in weight loss that did not occur in continuous exercise.

9.2 Impact of exercise intensity on body fatness and skeletal muscle metabolism (Tremblay-1994)

The combination of Continuous Moderate Exercise and moderate intensity intervals (60-70% V̇O₂max) reduced body fat more than Continuous Moderate Exercise alone, even though the Continuous Moderate Exercise burned over twice the calories. This study doesn't have the details I would've liked, and it uses skinfold thickness to estimate body fat. The study "corrects" the reduction in body fat based on the calories burned in each training regime, something that seems dubious at best. Neither group lost any weight, but the HIIT group lost an average of 13.9 mm from the sum of six skinfold sites, compared with 4.5mm for the controls. However, the HIIT group had an average of 15 mm extra body fat at the start.

9.3 High-intensity interval exercise induces 24-h energy expenditure similar to traditional endurance exercise despite reduced time commitment (SkellyAndrews2014)

Nine men with V̇O₂max of ~46 (3:25 marathon equivalent) were tested three times to evaluate their 24-hour energy expenditure. The trails were endurance, HIIT, and no exercise as the control. HIIT was cycling 10x (60 seconds @ ~90% Max HR with 60 second recoveries at 50w). The endurance was 70% max HR for 50 minutes. The 24-hour energy expenditure was similar for both HIIT and endurance, even though HIIT was less than half the time and required less than half the mechanical work than the endurance protocol. This suggests that the post-exercise energy increase of HIIT might be one of the mechanisms behind improved weight loss.

10 HIIT and Glycogen Depletion

It's estimated that an "average" 150-pound runner will have about 400g of muscle Glycogen and about 100g of liver Glycogen, which provides about 2,000 calories under aerobic conditions. However, when exercising anaerobically, you're only getting 1/15 the energy from glycogen, so the 2,000 calories you'd typically have would only give 133 calories! This means that HIIT can deplete glycogen stores quite rapidly, and while these stores are restored fairly quickly afterward, it seems like much of that may come from breaking down muscle protein. (Glucose gives 2 ATP anaerobic rather than 30 ATP aerobic.) For the research behind this, see Glycogen Depletion and HIIT.

11 HIIT and Altitude (RSH)

There has been some research on performing HIIT at (simulated) altitude, often called "Repeated-sprint training in hypoxia" or RSH. A 2017 meta-analysis concluded that RSH produced greater benefits than similar training in normal air for repeated sprint performance and a trivial benefit for V̇O₂max. The protocols used are typically Tabata style short (6-10 second) "all out" with 20-30 second recoveries, repeated 6-10 times with a hypoxia of 3000m or 13% O₂.

12 HIIT and Muscle Fiber Types

One concern with HIIT for endurance athletes is the possibility that it may result in a shift from slow twitch to fast twitch, thus impairing endurance. ( click here for more on muscle fiber types.) Unfortunately, the research is limited and somewhat conflicting.

• A study of 24 subjects and 10 controls that were healthy but untrained found that HIIT resulted in an increase in the size and percentage of slow twitch and a reduction in size and percentage of the fast twitch fibers. The HIIT used 4-5 sessions per week for 15 weeks, consisting of either 10-15x 15-30 seconds or 4-5x 60-90 seconds, each with recoveries allowing the heart rate to drop to 120-130 BPM. The intensity increased over the course of the training from 60% to 80% V̇O2max for the short intervals and 70%-90% for the long. The slow twitch (type I) increased by 6% and their area increased by 7%. Fast twitch type IIa area was reduced by 3% with no change in percent, and IIb were reduced by 6% and area by 4%.
• A study of physical education students used 4-6 weeks of Wingate training (30sec "all out") found that there was a change from slow twitch to fast twitch fibers. The slow twitch decreased from 57% to 48% while fast twitch (IIa) went from 32% to 38%. Interesting there was no performance improvement due to the training.

There's plenty of evidence that HIIT improves oxidative capacity in untrained subjects, but there's less research in highly trained subjects. One study found that oxidative capacity didn't change in well trained cyclists with HIIT, though their 40Km Time Trial performance improved, and performance improvements with HIIT in well trained subjects seems well supported.

13 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 V̇O₂max. While 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.