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[[File:Sprinter.jpg|right|thumb|400px|To run HIIT intervals requires a longer stride length, so other modes, such as a stationary bike may be more appropriate.]]
{{DISPLAYTITLE:The Science of High Intensity Interval Training (HIIT) Tabata and Wingate}}
This page looks at the scientific evidence on [[High Intensity Interval Training]] (HIIT), divided into three sections. Studies that compare HIIT with other modes of training are the most interesting, though they often don't cover highly trained athletes. I've included a few other studies that are not comparative because they have some particularly dramatic results. The third section looks at the studies that have looked at HIIT for highly trained athletes. For an introduction to HIIT, see [[High Intensity Interval Training]].
=Components of HIIT=
There are three main variable that shape a HIIT workout.
* '''Intensity'''. I'd argue that to be considered "high intensity" the workout should be higher than Lactate Threshold, and really, I'd consider it to be at least 100% of the effort at [[VO2max|VO<sub>2</sub>max]]. The classic Tabata workout used 1.7x the work at [[VO2max|VO<sub>2</sub>max]], though many implementations of this workout simply use "all out."
* '''Duration'''. Given intensities at or above [[VO2max|VO<sub>2</sub>max]], the duration needs to be quite short, and it's often 30 seconds or less.
* '''Recovery'''. The recovery period can be extremely short, which tends 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. The recovery intensity can be active, but the effort is nearly always fairly low.
=HIIT Comparisons on Untrained or Moderately Active Subjects=
The table below looks at studies that have compared HIIT with other types of training, often Continuous Moderate Exercise (CME). These studies on untrained or moderately trained subjects generally show a greater improvement in fitness measure compared with other forms of training, or similar improvements for far less training time.
{{:The Science of High Intensity Interval Training-table}}
==Aerobic High-Intensity Intervals Improve VO2Max More Than Moderate Training==
This study<ref name="Helgerud-2007"/> was on 40 male university students, all physically active and with [[VO2max|VO<sub>2</sub>max]] 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 seems to translate to 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.
[[File:Helgerud-2007-HR.jpg|center|thumb|600px|Heart rate for the four interventions, top left to bottom right: LSD, Lactate Threshold, 47x15+15, 4x4+3.]]
The results were that [[VO2max|VO<sub>2</sub>max]] 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 [[VO2max|VO<sub>2</sub>max]] with no significant difference, though the 4x4 was slightly better than the 47x15. [[Running Economy]] and [[Lactate Threshold]] were unchanged for all groups.
[[File:Helgerud-2007-Vo2max.jpg|center|thumb|300px|]]
'''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'm not sure I would consider either protocol truly "HIIT".
==Aerobic Interval Training Versus Continuous Moderate Exercise as a Treatment for the Metabolic Syndrome==
This is a similar study<ref name="TjonnaLee2008"/> 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 [[VO2max|VO<sub>2</sub>max]] 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 [[VO2max|VO<sub>2</sub>max]], and better mitigation of the risk factors associated with metabolic syndrome. <br/>
'''Conclusion''': this extends the previous study's finding to less fit subjects who have medical issues.
==Short-term sprint interval versus traditional endurance training==
=HIIT Studies on Untrained or Moderately Active Subjects without Controls=
While studies that compare HIIT with other forms of training are the most useful, there are a few other studies on untrained or moderately active people that are noteworthy. For instance, six sessions of HIIT over two weeks doubled the endurance of untrained subjects at 80% [[VO2max|V̇O<sub>2</sub>max]] from 25 to 51 minutes, despite no change in [[VO2max|V̇O<sub>2</sub>max]]<ref name="Burgomaster-2005"/>, a remarkable improvement. In another study, the combination of Continuous Moderate Exercise and moderate intensity intervals (60-70% [[VO2max|V̇O<sub>2</sub>max]]) reduced body fat by 15%, which was nine times more than Continuous Moderate Exercise alone, even though the Continuous Moderate Exercise burned over twice the calories<ref name="Tremblay-1994"/>. Another study also used a combination of HIIT on 3 day/week plus running as far as possible in 40 min on another 3 days/week , resulting in an increase in [[VO2max|V̇O<sub>2</sub>max]] by 44%, as well as improved running endurance, with some subjects ending up with a [[VO2max|V̇O<sub>2</sub>max]] exceeding 60 ml/kg per min, which is remarkably high for 10 weeks of training<ref name="Hickson-1977"/>.
=HIIT and Highly Trained Athletes=
It has been suggested that elite athletes do not benefit from further increases in volume, and should instead look to HIIT for performance improvements<ref name="Laursen-1-2002"/>. This is backed up by studies of some of the great endurance athletes, where higher training mileage produced worse rather than better performance<ref name="elitemileage"/>. In the Lore of Running, Tim Noakes said that elite runners perform best "when they train between 75-125 miles (120-200 km) per week, with an increasing likelihood that they will perform indifferently when they train more than 125 miles (200 km) per week"<ref name="LoR-P477"/>. Of course this is not universally true, and Mike Morton, set the US record holder for 24 hour while training 140-150 miles/week<ref name="militarytimes.com"/>. However, the evaluation of HIIT on elite athletes is not as easy as lessor folk. It's not practical to compare the effect of HIIT with other forms of exercise in highly trained athletes as they are typically already performing large volumes of Continuous Moderate Exercise. Instead, studies of highly trained athletes look at how HIIT impacts their fitness compared with a baseline taken beforehand.
* HIIT improved peak power output and 40 Km time trial in elite cyclists<ref name="Lindsay-1996"/><ref name="Westgarth-Taylor-1997"/>
* A study of elite cyclists used various HIIT workouts as shown in the table below, with the best results seen group 4 or group 1<ref name="Stepto-1999"/>. Group 4 trained at 85% peak power, which corresponds to the intensity normally seen in the 40K time trial, which takes ~60 minutes for an elite cyclist. Not surprisingly, this intensity is commonly used for cyclists training for 40K time trials. However, the higher intensity of group 1 is more intriguing; the time trial performance improved without an improvement in peak power, suggesting that a different mechanism may be responsible. This raises the possibility that the benefits of the different intensities might be combined. Note that there were only four athletes in each group, and responses tended to vary, so caution should be used in interpreting the results.
{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
! Group
! Number of intervals
! Interval duration (min)
! Total work time (min)
! Intensity (% peak power)
! Rest (min)
! Total Time
! Improvement in 40K Time Trial Speed
! Improvement in Peak Power
|-
| 1
| 12
| 0.5
| 6
| 175%
| 4.5
| 60 min
| 2.0%
| 0.5%
|-
| 2
| 12
| 1
| 12
| 100%
| 4.0
| 60 min
| 0.0%
| 0.5%
|-
| 3
| 12
| 2
| 24
| 90%
| 3.0
| 60 min
| 1.5%
| 1.5%
|-
| 4
| 8
| 4
| 32
| 85%
| 1.5
| 44 min
| 2.5%
| 2.0%
|-
| 5
| 4
| 8
| 32
| 80%
| 1.0
| 36 min
| 0.0%
| 1.0%
|}
* One approach to optimizing the length of the intervals in highly trained athletes is to use a percentage of [[vVO2max|T<sub>lim</sub>]] , where [[vVO2max|T<sub>lim</sub>]] is the time to exhaustion at 100% [[VO2max|V̇O<sub>2</sub>max]]<ref name="Laursen-1-2002"/>.
* 5 state level middle distance runners that underwent 4 weeks of HIIT training reduced their 3K time by 2.8% (10:16 to 9:59) and [[VO2max|V̇O<sub>2</sub>max]] by 4.9% (61 to 64)<ref name="Smith-1999"/>. The HIIT training consisted of 2 sessions per week of 6 intervals at 100% [[VO2max|V̇O<sub>2</sub>max]] with time varying between 60-75% [[vVO2max|T<sub>lim</sub>]], plus one weekly run of 30 min at 60% v[[VO2max|V̇O<sub>2</sub>max]]. For these runners, [[vVO2max|T<sub>lim</sub>]] averaged 225 seconds, so the intervals were between 135 and 170 seconds.
* Well trained, competitive runners trained twice a week for four weeks with intervals at 100% [[VO2max|V̇O<sub>2</sub>max]] for either 6x 60% [[vVO2max|T<sub>lim</sub>]] (133 sec) or 5x 70% (154 sec) [[vVO2max|T<sub>lim</sub>]], resting for twice the interval time. Their 3K time improved by 17.6 sec (60% [[vVO2max|T<sub>lim</sub>]]) or 6.3 sec (70% [[vVO2max|T<sub>lim</sub>]]), but there was no change in their 5K time<ref name="Smith-2003"/>.
* 41 elite ([[VO2max|V̇O<sub>2</sub>peak]] ~65) cyclists and triathletes were split into four groups, with three groups using the HIIT described below and the fourth acting as a control that followed only low to moderate intensity training<ref name="Laursen-3-2002"/>. Note that groups 1 and 2 vary only in their rest time, which is based on [[Heart Rate]] dropping to 65% of [[Maximum Heart Rate|HR<sub>max</sub>]] in group 2 (averaging around 180 seconds). This was a demanding regime, as the subjects reached exhaustion on nearly every HIIT training session, with '''only 64% of the dictated intervals actually completed'''. Note that like comparison of different workouts above, the shorter HIIT produced a similar improvement in 40K performance without the accompanying rise in [[VO2max|V̇O<sub>2</sub>max]] which was not statistically different between group 3 and the controls.
{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
! Group
! Intended number of intervals
! Interval Duration
! Total work time (min)
! Intensity (% [[VO2max|V̇O<sub>2</sub>max]])
! Rest
! Total time
! [[VO2max|V̇O<sub>2</sub>max]] change
! 40K Time Trial Speed improvement
|-
| 1
| 8
| 60% [[vVO2max|T<sub>lim</sub>]]<sub></sub>(~150 sec)
| 20
| 100%
| 120% [[vVO2max|T<sub>lim</sub>]] (~290 sec)
| 58 min
| 5.2%
| 5.2%
|-
| 2
| 8
| 60% [[vVO2max|T<sub>lim</sub>]]<sub></sub>(~150 sec)
| 20
| 100%
| 65% [[Maximum Heart Rate|HR<sub>max</sub>]] (~180 sec)
| Varies
| 8.0%
| 5.6%
|-
| 3
| 12
| 30 seconds
| 6
| 175%
| 4.5 min
| 60 min
| 3.1%
| 4.3%
|-
| Control
| N/A
| N/A
| N/A
| N/A
| N/A
| N/A
| 0.8%
| -1%
|}
* Elite cyclists performed 4 sessions of HIIT (20x 60 sec at [[VO2max|V̇O<sub>2</sub>max]] + 120 sec recovery) that improved peak power by 4% but had no change in [[VO2max|V̇O<sub>2</sub>max]]<ref name="Laursen-2-2002"/>.
=Limitations of the HIIT science=
There are some important limitations of the HIIT science.
* The majority of studies are on sedentary or recreationally active people, not trained runners.
* Few studies use real world measures of improvement, relying instead on indirect metrics such as [[VO2max|V̇O<sub>2</sub>max]]. While [[VO2max|V̇O<sub>2</sub>max]] is linked to improved performance, there are other important factors involved.
* Studies that do look at the effect of HIIT on real world performance tend to focus on shorter events, such as 3K or 5K running, or 40K cycling.
* Most studies are short duration, looking at the effects of HIIT over just a few weeks.
=References=
<references>
<ref name="Tabata-1996">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, volume 28, issue 10, pages 1327-30, Oct 1996, PMID [http://www.ncbi.nlm.nih.gov/pubmed/8897392 8897392]</ref>
<ref name="Smith-2003"> 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, volume 89, issue 3-4, pages 337-43, May 2003, doi [http://dx.doi.org/10.1007/s00421-003-0806-6 10.1007/s00421-003-0806-6], PMID [http://www.ncbi.nlm.nih.gov/pubmed/12736843 12736843]</ref>
<ref name="Smith-1999"> TP. Smith, LR. McNaughton, KJ. Marshall, Effects of 4-wk training using Vmax/Tlim on VO2max and performance in athletes., Med Sci Sports Exerc, volume 31, issue 6, pages 892-6, Jun 1999, PMID [http://www.ncbi.nlm.nih.gov/pubmed/10378918 10378918]</ref>
<ref name="Stepto-1999"> NK. Stepto, JA. Hawley, SC. Dennis, WG. Hopkins, Effects of different interval-training programs on cycling time-trial performance., Med Sci Sports Exerc, volume 31, issue 5, pages 736-41, May 1999, PMID [http://www.ncbi.nlm.nih.gov/pubmed/10331896 10331896]</ref>
<ref name="Westgarth-Taylor-1997"> 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, volume 75, issue 4, pages 298-304, 1997, PMID [http://www.ncbi.nlm.nih.gov/pubmed/9134360 9134360]</ref>
<ref name="Lindsay-1996"> 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, volume 28, issue 11, pages 1427-34, Nov 1996, PMID [http://www.ncbi.nlm.nih.gov/pubmed/8933495 8933495]</ref>
<ref name="Franch-1998"> J. Franch, K. Madsen, MS. Djurhuus, PK. Pedersen, Improved running economy following intensified training correlates with reduced ventilatory demands., Med Sci Sports Exerc, volume 30, issue 8, pages 1250-6, Aug 1998, PMID [http://www.ncbi.nlm.nih.gov/pubmed/9710865 9710865]</ref>
<ref name="Gorostiaga-1991"> 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, volume 63, issue 2, pages 101-7, 1991, PMID [http://www.ncbi.nlm.nih.gov/pubmed/1748098 1748098]</ref>
<ref name="Hickson-1977"> RC. Hickson, HA. Bomze, JO. Holloszy, Linear increase in aerobic power induced by a strenuous program of endurance exercise., J Appl Physiol, volume 42, issue 3, pages 372-6, Mar 1977, PMID [http://www.ncbi.nlm.nih.gov/pubmed/838658 838658]</ref>
<ref name="Helgerud-2007"> 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, volume 39, issue 4, pages 665-71, Apr 2007, doi [http://dx.doi.org/10.1249/mss.0b013e3180304570 10.1249/mss.0b013e3180304570], PMID [http://www.ncbi.nlm.nih.gov/pubmed/17414804 17414804]</ref>
<ref name="BurgomasterHowarth2007">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, volume 586, issue 1, 2007, pages 151–160, ISSN [http://www.worldcat.org/issn/0022-3751 0022-3751], doi [http://dx.doi.org/10.1113/jphysiol.2007.142109 10.1113/jphysiol.2007.142109]</ref>
<ref name="Trapp-2008"> 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), volume 32, issue 4, pages 684-91, Apr 2008, doi [http://dx.doi.org/10.1038/sj.ijo.0803781 10.1038/sj.ijo.0803781], PMID [http://www.ncbi.nlm.nih.gov/pubmed/18197184 18197184]</ref>
<ref name="Tremblay-1994"> A. Tremblay, JA. Simoneau, C. Bouchard, Impact of exercise intensity on body fatness and skeletal muscle metabolism., Metabolism, volume 43, issue 7, pages 814-8, Jul 1994, PMID [http://www.ncbi.nlm.nih.gov/pubmed/8028502 8028502]</ref>
<ref name="Laursen-1-2002"> 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, volume 32, issue 1, pages 53-73, 2002, PMID [http://www.ncbi.nlm.nih.gov/pubmed/11772161 11772161]</ref>
<ref name="TjonnaLee2008">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, volume 118, issue 4, 2008, pages 346–354, ISSN [http://www.worldcat.org/issn/0009-7322 0009-7322], doi [http://dx.doi.org/10.1161/CIRCULATIONAHA.108.772822 10.1161/CIRCULATIONAHA.108.772822]</ref>
<ref name="Burgomaster-2005"> 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, volume 98, issue 6, pages 1985-90, Jun 2005, doi [http://dx.doi.org/10.1152/japplphysiol.01095.2004 10.1152/japplphysiol.01095.2004], PMID [http://www.ncbi.nlm.nih.gov/pubmed/15705728 15705728]</ref>
<ref name="Gibala-2006"> 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, volume 575, issue Pt 3, pages 901-11, Sep 2006, doi [http://dx.doi.org/10.1113/jphysiol.2006.112094 10.1113/jphysiol.2006.112094], PMID [http://www.ncbi.nlm.nih.gov/pubmed/16825308 16825308]</ref>
<ref name="Laursen-2-2002"> 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, volume 27, issue 4, pages 336-48, Aug 2002, PMID [http://www.ncbi.nlm.nih.gov/pubmed/12442351 12442351]</ref>
<ref name="Laursen-3-2002"> PB. Laursen, CM. Shing, JM. Peake, JS. Coombes, DG. Jenkins, Interval training program optimization in highly trained endurance cyclists., Med Sci Sports Exerc, volume 34, issue 11, pages 1801-7, Nov 2002, doi [http://dx.doi.org/10.1249/01.MSS.0000036691.95035.7D 10.1249/01.MSS.0000036691.95035.7D], PMID [http://www.ncbi.nlm.nih.gov/pubmed/12439086 12439086]</ref>
<ref name="elitemileage"> An Interesting Analysis of Some Elites’ Training History, Accessed on 26 February 2013</ref>
<ref name="LoR-P477">Timothy Noakes, Lore of runnin, date 2003, publisher Human Kinetics, location Champaign, IL, isbn 0-87322-959-2, pages 447*448</ref>
<ref name="militarytimes.com"> 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</ref>
<ref name="EarnestTjønna2013">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, volume 8, issue 5, 2013, pages e65382, ISSN [http://www.worldcat.org/issn/1932-6203 1932-6203], doi [http://dx.doi.org/10.1371/journal.pone.0065382 10.1371/journal.pone.0065382]</ref>
</references>
{{DISPLAYTITLE:The Science of High Intensity Interval Training (HIIT) Tabata and Wingate}}
This page looks at the scientific evidence on [[High Intensity Interval Training]] (HIIT), divided into three sections. Studies that compare HIIT with other modes of training are the most interesting, though they often don't cover highly trained athletes. I've included a few other studies that are not comparative because they have some particularly dramatic results. The third section looks at the studies that have looked at HIIT for highly trained athletes. For an introduction to HIIT, see [[High Intensity Interval Training]].
=Components of HIIT=
There are three main variable that shape a HIIT workout.
* '''Intensity'''. I'd argue that to be considered "high intensity" the workout should be higher than Lactate Threshold, and really, I'd consider it to be at least 100% of the effort at [[VO2max|VO<sub>2</sub>max]]. The classic Tabata workout used 1.7x the work at [[VO2max|VO<sub>2</sub>max]], though many implementations of this workout simply use "all out."
* '''Duration'''. Given intensities at or above [[VO2max|VO<sub>2</sub>max]], the duration needs to be quite short, and it's often 30 seconds or less.
* '''Recovery'''. The recovery period can be extremely short, which tends 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. The recovery intensity can be active, but the effort is nearly always fairly low.
=HIIT Comparisons on Untrained or Moderately Active Subjects=
The table below looks at studies that have compared HIIT with other types of training, often Continuous Moderate Exercise (CME). These studies on untrained or moderately trained subjects generally show a greater improvement in fitness measure compared with other forms of training, or similar improvements for far less training time.
{{:The Science of High Intensity Interval Training-table}}
==Aerobic High-Intensity Intervals Improve VO2Max More Than Moderate Training==
This study<ref name="Helgerud-2007"/> was on 40 male university students, all physically active and with [[VO2max|VO<sub>2</sub>max]] 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 seems to translate to 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.
[[File:Helgerud-2007-HR.jpg|center|thumb|600px|Heart rate for the four interventions, top left to bottom right: LSD, Lactate Threshold, 47x15+15, 4x4+3.]]
The results were that [[VO2max|VO<sub>2</sub>max]] 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 [[VO2max|VO<sub>2</sub>max]] with no significant difference, though the 4x4 was slightly better than the 47x15. [[Running Economy]] and [[Lactate Threshold]] were unchanged for all groups.
[[File:Helgerud-2007-Vo2max.jpg|center|thumb|300px|]]
'''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'm not sure I would consider either protocol truly "HIIT".
==Aerobic Interval Training Versus Continuous Moderate Exercise as a Treatment for the Metabolic Syndrome==
This is a similar study<ref name="TjonnaLee2008"/> 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 [[VO2max|VO<sub>2</sub>max]] 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 [[VO2max|VO<sub>2</sub>max]], and better mitigation of the risk factors associated with metabolic syndrome. <br/>
'''Conclusion''': this extends the previous study's finding to less fit subjects who have medical issues.
==Short-term sprint interval versus traditional endurance training==
=HIIT Studies on Untrained or Moderately Active Subjects without Controls=
While studies that compare HIIT with other forms of training are the most useful, there are a few other studies on untrained or moderately active people that are noteworthy. For instance, six sessions of HIIT over two weeks doubled the endurance of untrained subjects at 80% [[VO2max|V̇O<sub>2</sub>max]] from 25 to 51 minutes, despite no change in [[VO2max|V̇O<sub>2</sub>max]]<ref name="Burgomaster-2005"/>, a remarkable improvement. In another study, the combination of Continuous Moderate Exercise and moderate intensity intervals (60-70% [[VO2max|V̇O<sub>2</sub>max]]) reduced body fat by 15%, which was nine times more than Continuous Moderate Exercise alone, even though the Continuous Moderate Exercise burned over twice the calories<ref name="Tremblay-1994"/>. Another study also used a combination of HIIT on 3 day/week plus running as far as possible in 40 min on another 3 days/week , resulting in an increase in [[VO2max|V̇O<sub>2</sub>max]] by 44%, as well as improved running endurance, with some subjects ending up with a [[VO2max|V̇O<sub>2</sub>max]] exceeding 60 ml/kg per min, which is remarkably high for 10 weeks of training<ref name="Hickson-1977"/>.
=HIIT and Highly Trained Athletes=
It has been suggested that elite athletes do not benefit from further increases in volume, and should instead look to HIIT for performance improvements<ref name="Laursen-1-2002"/>. This is backed up by studies of some of the great endurance athletes, where higher training mileage produced worse rather than better performance<ref name="elitemileage"/>. In the Lore of Running, Tim Noakes said that elite runners perform best "when they train between 75-125 miles (120-200 km) per week, with an increasing likelihood that they will perform indifferently when they train more than 125 miles (200 km) per week"<ref name="LoR-P477"/>. Of course this is not universally true, and Mike Morton, set the US record holder for 24 hour while training 140-150 miles/week<ref name="militarytimes.com"/>. However, the evaluation of HIIT on elite athletes is not as easy as lessor folk. It's not practical to compare the effect of HIIT with other forms of exercise in highly trained athletes as they are typically already performing large volumes of Continuous Moderate Exercise. Instead, studies of highly trained athletes look at how HIIT impacts their fitness compared with a baseline taken beforehand.
* HIIT improved peak power output and 40 Km time trial in elite cyclists<ref name="Lindsay-1996"/><ref name="Westgarth-Taylor-1997"/>
* A study of elite cyclists used various HIIT workouts as shown in the table below, with the best results seen group 4 or group 1<ref name="Stepto-1999"/>. Group 4 trained at 85% peak power, which corresponds to the intensity normally seen in the 40K time trial, which takes ~60 minutes for an elite cyclist. Not surprisingly, this intensity is commonly used for cyclists training for 40K time trials. However, the higher intensity of group 1 is more intriguing; the time trial performance improved without an improvement in peak power, suggesting that a different mechanism may be responsible. This raises the possibility that the benefits of the different intensities might be combined. Note that there were only four athletes in each group, and responses tended to vary, so caution should be used in interpreting the results.
{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
! Group
! Number of intervals
! Interval duration (min)
! Total work time (min)
! Intensity (% peak power)
! Rest (min)
! Total Time
! Improvement in 40K Time Trial Speed
! Improvement in Peak Power
|-
| 1
| 12
| 0.5
| 6
| 175%
| 4.5
| 60 min
| 2.0%
| 0.5%
|-
| 2
| 12
| 1
| 12
| 100%
| 4.0
| 60 min
| 0.0%
| 0.5%
|-
| 3
| 12
| 2
| 24
| 90%
| 3.0
| 60 min
| 1.5%
| 1.5%
|-
| 4
| 8
| 4
| 32
| 85%
| 1.5
| 44 min
| 2.5%
| 2.0%
|-
| 5
| 4
| 8
| 32
| 80%
| 1.0
| 36 min
| 0.0%
| 1.0%
|}
* One approach to optimizing the length of the intervals in highly trained athletes is to use a percentage of [[vVO2max|T<sub>lim</sub>]] , where [[vVO2max|T<sub>lim</sub>]] is the time to exhaustion at 100% [[VO2max|V̇O<sub>2</sub>max]]<ref name="Laursen-1-2002"/>.
* 5 state level middle distance runners that underwent 4 weeks of HIIT training reduced their 3K time by 2.8% (10:16 to 9:59) and [[VO2max|V̇O<sub>2</sub>max]] by 4.9% (61 to 64)<ref name="Smith-1999"/>. The HIIT training consisted of 2 sessions per week of 6 intervals at 100% [[VO2max|V̇O<sub>2</sub>max]] with time varying between 60-75% [[vVO2max|T<sub>lim</sub>]], plus one weekly run of 30 min at 60% v[[VO2max|V̇O<sub>2</sub>max]]. For these runners, [[vVO2max|T<sub>lim</sub>]] averaged 225 seconds, so the intervals were between 135 and 170 seconds.
* Well trained, competitive runners trained twice a week for four weeks with intervals at 100% [[VO2max|V̇O<sub>2</sub>max]] for either 6x 60% [[vVO2max|T<sub>lim</sub>]] (133 sec) or 5x 70% (154 sec) [[vVO2max|T<sub>lim</sub>]], resting for twice the interval time. Their 3K time improved by 17.6 sec (60% [[vVO2max|T<sub>lim</sub>]]) or 6.3 sec (70% [[vVO2max|T<sub>lim</sub>]]), but there was no change in their 5K time<ref name="Smith-2003"/>.
* 41 elite ([[VO2max|V̇O<sub>2</sub>peak]] ~65) cyclists and triathletes were split into four groups, with three groups using the HIIT described below and the fourth acting as a control that followed only low to moderate intensity training<ref name="Laursen-3-2002"/>. Note that groups 1 and 2 vary only in their rest time, which is based on [[Heart Rate]] dropping to 65% of [[Maximum Heart Rate|HR<sub>max</sub>]] in group 2 (averaging around 180 seconds). This was a demanding regime, as the subjects reached exhaustion on nearly every HIIT training session, with '''only 64% of the dictated intervals actually completed'''. Note that like comparison of different workouts above, the shorter HIIT produced a similar improvement in 40K performance without the accompanying rise in [[VO2max|V̇O<sub>2</sub>max]] which was not statistically different between group 3 and the controls.
{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
! Group
! Intended number of intervals
! Interval Duration
! Total work time (min)
! Intensity (% [[VO2max|V̇O<sub>2</sub>max]])
! Rest
! Total time
! [[VO2max|V̇O<sub>2</sub>max]] change
! 40K Time Trial Speed improvement
|-
| 1
| 8
| 60% [[vVO2max|T<sub>lim</sub>]]<sub></sub>(~150 sec)
| 20
| 100%
| 120% [[vVO2max|T<sub>lim</sub>]] (~290 sec)
| 58 min
| 5.2%
| 5.2%
|-
| 2
| 8
| 60% [[vVO2max|T<sub>lim</sub>]]<sub></sub>(~150 sec)
| 20
| 100%
| 65% [[Maximum Heart Rate|HR<sub>max</sub>]] (~180 sec)
| Varies
| 8.0%
| 5.6%
|-
| 3
| 12
| 30 seconds
| 6
| 175%
| 4.5 min
| 60 min
| 3.1%
| 4.3%
|-
| Control
| N/A
| N/A
| N/A
| N/A
| N/A
| N/A
| 0.8%
| -1%
|}
* Elite cyclists performed 4 sessions of HIIT (20x 60 sec at [[VO2max|V̇O<sub>2</sub>max]] + 120 sec recovery) that improved peak power by 4% but had no change in [[VO2max|V̇O<sub>2</sub>max]]<ref name="Laursen-2-2002"/>.
=Limitations of the HIIT science=
There are some important limitations of the HIIT science.
* The majority of studies are on sedentary or recreationally active people, not trained runners.
* Few studies use real world measures of improvement, relying instead on indirect metrics such as [[VO2max|V̇O<sub>2</sub>max]]. While [[VO2max|V̇O<sub>2</sub>max]] is linked to improved performance, there are other important factors involved.
* Studies that do look at the effect of HIIT on real world performance tend to focus on shorter events, such as 3K or 5K running, or 40K cycling.
* Most studies are short duration, looking at the effects of HIIT over just a few weeks.
=References=
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