Revision as of 11:40, 27 April 2019

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.

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^[1].

• 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 ^[2][3]. In fact, some have suggested that the goal of HIIT is to maximize the time above 90% V̇O₂max^[1]. 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.

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^[4]. This is based on the observation^[5] 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^[6]. 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^[4]. It's been observed that highly trained runners take at least 60% of T_lim to reach V̇O₂max ^[7]. 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^[8][9]. (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^[10].

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^[11]. 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.

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^[12]. 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^[13]. 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.

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^[14]. 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) ^[15]. 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^[16]. 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^[17].

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^[18], but this seems to be no evidence comparing different levels of recovery intensity.

5 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.

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

This study^[19] 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.

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

This study^[20] 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.

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

This is a similar study^[21] 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.

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

This small, short study^[22] 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^[19]. 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.

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

This is a small study^[23] 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.

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

This is a slightly larger study^[24] 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.

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

This is study^[25] 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.

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

This study^[27] 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.

5.9 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)^[28]. 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.

5.10 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)^[29]. 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.

6 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.

6.1 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^[30], 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.

6.2 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^[31].

7 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^[4]. This is backed up by studies of some of the great endurance athletes, where higher training mileage produced worse rather than better performance^[32]. 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"^[33]. Of course, this is not universally true, and Mike Morton, set the US record holder for 24-hour while training 140-150 miles/week^[34]. 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.

7.1 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^[12]. 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.

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

This is study^[35] 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.

7.3 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)^[36]. 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.

7.4 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^[37].

7.5 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^[38]. 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

7.6 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) ^[39]. 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.

8 HIIT and Weight Loss

8.1 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.

8.2 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^[40]. 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.

8.3 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^[41]. 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.

9 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.

10 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.

11 References

1. ↑ ^{1.0 1.1} Martin Buchheit, Paul B. Laursen, High-Intensity Interval Training, Solutions to the Programming Puzzle, Sports Medicine, volume 43, issue 5, 2013, pages 313–338, ISSN 0112-1642, doi 10.1007/s40279-013-0029-x
2. ↑ LV. Billat, Interval training for performance: a scientific and empirical practice. Special recommendations for middle- and long-distance running. Part II: anaerobic interval training., Sports Med, volume 31, issue 2, pages 75-90, Feb 2001, PMID 11227980
3. ↑ AW. Midgley, LR. McNaughton, M. Wilkinson, Is there an optimal training intensity for enhancing the maximal oxygen uptake of distance runners?: empirical research findings, current opinions, physiological rationale and practical recommendations., Sports Med, volume 36, issue 2, pages 117-32, 2006, PMID 16464121
4. ↑ ^{4.0 4.1 4.2} PB. Laursen, DG. Jenkins, The scientific basis for high-intensity interval training: optimising training programmes and maximising performance in highly trained endurance athletes., Sports Med, 32 !!V̇olume!!, issue 1, pages 53-73, 2002, PMID 11772161
5. ↑ David R. Bassett, Limiting factors for maximum oxygen uptake and determinants of endurance performance, Medicine & Science in Sports & Exercise, 2000, pages 70, ISSN 0195-9131, doi 10.1097/00005768-200001000-00012
6. ↑ VL. Billat, J. Slawinski, V. Bocquet, A. Demarle, L. Lafitte, P. Chassaing, JP. Koralsztein, Intermittent runs at the velocity associated with maximal oxygen uptake enables subjects to remain at maximal oxygen uptake for a longer time than intense but submaximal runs., Eur J Appl Physiol, volume 81, issue 3, pages 188-96, Feb 2000, doi 10.1007/s004210050029, PMID 10638376
7. ↑ DW. Hill, AL. Rowell, Responses to exercise at the velocity associated with VO2max., Med Sci Sports Exerc, volume 29, issue 1, pages 113-6, Jan 1997, PMID 9000163
8. ↑ A. Lucia, The slow component of VO2 in professional cyclists, British Journal of Sports Medicine, volume 34, issue 5, 2000, pages 367–374, ISSN 03063674, doi 10.1136/bjsm.34.5.367
9. ↑ VL. Billat, L. Mille-Hamard, B. Petit, JP. Koralsztein, The role of cadence on the VO2 slow component in cycling and running in triathletes., Int J Sports Med, volume 20, issue 7, pages 429-37, Oct 1999, doi 10.1055/s-1999-8825, PMID 10551337
10. ↑ Jorge M Zuniga, Kris Berg, John Noble, Jeanette Harder, Morgan E Chaffin, Vidya S Hanumanthu, Physiological Responses during Interval Training with Different Intensities and Duration of Exercise, Journal of Strength and Conditioning Research, volume 25, issue 5, 2011, pages 1279–1284, ISSN 1064-8011, doi 10.1519/JSC.0b013e3181d681b6
11. ↑ NK. Stepto, JA. Hawley, SC. Dennis, WG. Hopkins, Effects of different interval-training programs on cycling time-trial performance., Med Sci Sports Exerc, 31 !!V̇olume!!, issue 5, pages 736-41, May 1999, PMID 10331896
12. ↑ ^{12.0 12.1} FH. Lindsay, JA. Hawley, KH. Myburgh, HH. Schomer, TD. Noakes, SC. Dennis, Improved athletic performance in highly trained cyclists after interval training., Med Sci Sports Exerc, 28 !!V̇olume!!, issue 11, pages 1427-34, Nov 1996, PMID 8933495
13. ↑ Christopher Westgarth-Taylor, John A. Hawley, Scott Rickard, Kathryn H. Myburgh, Timothy D. Noakes, Steven C. Dennis, Metabolic and performance adaptations to interval training in endurance-trained cyclists, European Journal of Applied Physiology, volume 75, issue 4, 1997, pages 298–304, ISSN 1439-6319, doi 10.1007/s004210050164
14. ↑ J. Parra, JA. Cadefau, G. Rodas, N. Amigó, R. Cussó, The distribution of rest periods affects performance and adaptations of energy metabolism induced by high-intensity training in human muscle., Acta Physiol Scand, volume 169, issue 2, pages 157-65, Jun 2000, doi 10.1046/j.1365-201x.2000.00730.x, PMID 10848646
15. ↑ I. Tabata, K. Irisawa, M. Kouzaki, K. Nishimura, F. Ogita, M. Miyachi, Metabolic profile of high intensity intermittent exercises., Med Sci Sports Exerc, volume 29, issue 3, pages 390-5, Mar 1997, PMID 9139179
16. ↑ PD. Balsom, JY. Seger, B. Sjödin, B. Ekblom, Maximal-intensity intermittent exercise: effect of recovery duration., Int J Sports Med, volume 13, issue 7, pages 528-33, Oct 1992, doi 10.1055/s-2007-1021311, PMID 1459748
17. ↑ Gerald S. Zavorsky, David L. Montgomery, David J. Pearsall, Effect of intense interval workouts on running economy using three recovery durations, European Journal of Applied Physiology, volume 77, issue 3, 1998, pages 224–230, ISSN 1439-6319, doi 10.1007/s004210050326
18. ↑ AN. Belcastro, A. Bonen, Lactic acid removal rates during controlled and uncontrolled recovery exercise., J Appl Physiol, volume 39, issue 6, pages 932-6, Dec 1975, doi 10.1152/jappl.1975.39.6.932, PMID 765313
19. ↑ ^{19.0 19.1 19.2} I. Tabata, K. Nishimura, M. Kouzaki, Y. Hirai, F. Ogita, M. Miyachi, K. Yamamoto, Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max., Med Sci Sports Exerc, 28 !!V̇olume!!, issue 10, pages 1327-30, Oct 1996, PMID 8897392
20. ↑ ^{20.0 20.1} J. Helgerud, K. Høydal, E. Wang, T. Karlsen, P. Berg, M. Bjerkaas, T. Simonsen, C. Helgesen, N. Hjorth, Aerobic high-intensity intervals improve VO2max more than moderate training., Med Sci Sports Exerc, 39 !!V̇olume!!, issue 4, pages 665-71, Apr 2007, doi 10.1249/mss.0b013e3180304570, PMID 17414804
21. ↑ ^{21.0 21.1} A. E. Tjonna, S. J. Lee, O. Rognmo, T. O. Stolen, A. Bye, P. M. Haram, J. P. Loennechen, Q. Y. Al-Share, E. Skogvoll, S. A. Slordahl, O. J. Kemi, S. M. Najjar, U. Wisloff, Aerobic Interval Training Versus Continuous Moderate Exercise as a Treatment for the Metabolic Syndrome: A Pilot Study, Circulation, 118 !!V̇olume!!, issue 4, 2008, pages 346–354, ISSN 0009-7322, doi 10.1161/CIRCULATIONAHA.108.772822
22. ↑ ^{22.0 22.1} MJ. Gibala, JP. Little, M. van Essen, GP. Wilkin, KA. Burgomaster, A. Safdar, S. Raha, MA. Tarnopolsky, Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance., J Physiol, 575 !!V̇olume!!, issue Pt 3, pages 901-11, Sep 2006, doi 10.1113/jphysiol.2006.112094, PMID 16825308
23. ↑ ^{23.0 23.1} EM. Gorostiaga, CB. Walter, C. Foster, RC. Hickson, Uniqueness of interval and continuous training at the same maintained exercise intensity., Eur J Appl Physiol Occup Physiol, 63 !!V̇olume!!, issue 2, pages 101-7, 1991, PMID 1748098
24. ↑ ^{24.0 24.1} J. Franch, K. Madsen, MS. Djurhuus, PK. Pedersen, Improved running economy following intensified training correlates with reduced ventilatory demands., Med Sci Sports Exerc, 30 !!V̇olume!!, issue 8, pages 1250-6, Aug 1998, PMID 9710865
25. ↑ ^{25.0 25.1} K. A. Burgomaster, K. R. Howarth, S. M. Phillips, M. Rakobowchuk, M. J. MacDonald, S. L. McGee, M. J. Gibala, Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans, The Journal of Physiology, 586 !!V̇olume!!, issue 1, 2007, pages 151–160, ISSN 0022-3751, doi 10.1113/jphysiol.2007.142109
26. ↑ EG. Trapp, DJ. Chisholm, J. Freund, SH. Boutcher, The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women., Int J Obes (Lond), 32 !!V̇olume!!, issue 4, pages 684-91, Apr 2008, doi 10.1038/sj.ijo.0803781, PMID 18197184
27. ↑ ^{27.0 27.1} Conrad P. Earnest, Arnt Erik Tjønna, Ingeborg Megaard Leinan, Anette Thoresen Bartnes, Bjørn M. Jenssen, Martin J. Gibala, Richard A. Winett, Ulrik Wisløff, Low- and High-Volume of Intensive Endurance Training Significantly Improves Maximal Oxygen Uptake after 10-Weeks of Training in Healthy Men, PLoS ONE, 8 !!V̇olume!!, issue 5, 2013, pages e65382, ISSN 1932-6203, doi 10.1371/journal.pone.0065382
28. ↑ Richard S. Metcalfe, John A. Babraj, Samantha G. Fawkner, Niels B. J. Vollaard, Towards the minimal amount of exercise for improving metabolic health: beneficial effects of reduced-exertion high-intensity interval training, European Journal of Applied Physiology, volume 112, issue 7, 2011, pages 2767–2775, ISSN 1439-6319, doi 10.1007/s00421-011-2254-z
29. ↑ José S. Ruffino, Preeyaphorn Songsorn, Malindi Haggett, Daniel Edmonds, Anthony M. Robinson, Dylan Thompson, Niels B.J. Vollaard, A comparison of the health benefits of reduced-exertion high-intensity interval training (REHIT) and moderate-intensity walking in type 2 diabetes patients, Applied Physiology, Nutrition, and Metabolism, volume 42, issue 2, 2017, pages 202–208, ISSN 1715-5312, doi 10.1139/apnm-2016-0497
30. ↑ KA. Burgomaster, SC. Hughes, GJ. Heigenhauser, SN. Bradwell, MJ. Gibala, Six sessions of sprint interval training increases muscle oxidative potential and cycle endurance capacity in humans., J Appl Physiol, 98 !!V̇olume!!, issue 6, pages 1985-90, Jun 2005, doi 10.1152/japplphysiol.01095.2004, PMID 15705728
31. ↑ RC. Hickson, HA. Bomze, JO. Holloszy, Linear increase in aerobic power induced by a strenuous program of endurance exercise., J Appl Physiol, 42 !!V̇olume!!, issue 3, pages 372-6, Mar 1977, PMID 838658
32. ↑ An Interesting Analysis of Some Elites’ Training History, Accessed on 26 February 2013
33. ↑ Timothy Noakes, Lore of runnin, date 2003, publisher Human Kinetics, location Champaign, IL, isbn 0-87322-959-2, pages 447*448
34. ↑ http://militarytimes.com/blogs/pt365/2012/05/07/army-master-sgt-mike-morton-and-his-14-year-journey-to-the-badwater-135-ultramarathon/, http://militarytimes.com/blogs/pt365/2012/05/07/army-master-sgt-mike-morton-and-his-14-year-journey-to-the-badwater-135-ultramarathon/, Accessed on 26 February 2013
35. ↑ C. Westgarth-Taylor, JA. Hawley, S. Rickard, KH. Myburgh, TD. Noakes, SC. Dennis, Metabolic and performance adaptations to interval training in endurance-trained cyclists., Eur J Appl Physiol Occup Physiol, 75 !!V̇olume!!, issue 4, pages 298-304, 1997, PMID 9134360
36. ↑ TP. Smith, LR. McNaughton, KJ. Marshall, Effects of 4-wk training using Vmax/Tlim on VO2max and performance in athletes., Med Sci Sports Exerc, 31 !!V̇olume!!, issue 6, pages 892-6, Jun 1999, PMID 10378918
37. ↑ TP. Smith, JS. Coombes, DP. Geraghty, Optimising high-intensity treadmill training using the running speed at maximal O(2) uptake and the time for which this can be maintained., Eur J Appl Physiol, 89 !!V̇olume!!, issue 3-4, pages 337-43, May 2003, doi 10.1007/s00421-003-0806-6, PMID 12736843
38. ↑ PB. Laursen, CM. Shing, JM. Peake, JS. Coombes, DG. Jenkins, Interval training program optimization in highly trained endurance cyclists., Med Sci Sports Exerc, 34 !!V̇olume!!, issue 11, pages 1801-7, Nov 2002, doi 10.1249/01.MSS.0000036691.95035.7D, PMID 12439086
39. ↑ PB. Laursen, MA. Blanchard, DG. Jenkins, Acute high-intensity interval training improves Tvent and peak power output in highly trained males., Can J Appl Physiol, 27 !!V̇olume!!, issue 4, pages 336-48, Aug 2002, PMID 12442351
40. ↑ A. Tremblay, JA. Simoneau, C. Bouchard, Impact of exercise intensity on body fatness and skeletal muscle metabolism., Metabolism, 43 !!V̇olume!!, issue 7, pages 814-8, Jul 1994, PMID 8028502
41. ↑ Lauren E. Skelly, Patricia C. Andrews, Jenna B. Gillen, Brian J. Martin, Michael E. Percival, Martin J. Gibala, High-intensity interval exercise induces 24-h energy expenditure similar to traditional endurance exercise despite reduced time commitment, Applied Physiology, Nutrition, and Metabolism, volume 39, issue 7, 2014, pages 845–848, ISSN 1715-5312, doi 10.1139/apnm-2013-0562