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* '''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.
==Naming Convention==
=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<ref name="Belcastro-1975"/>, but this seems to be no evidence comparing different levels of recovery intensity.
=Number of Intervals=
A meta-analysis of 38 trails over 34 studies found that fewer intervals was more effective than more<ref name="VollaardMetcalfe2017"/>. They found that improvements in [[VO2max|V̇O<sub>2</sub>max]] 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.)
[[File:Number of intervals.jpg|center|thumb|300px|The number of intervals against change in [[VO2max|V̇O<sub>2</sub>max]].]]
=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. {{:The Science of High Intensity Interval Training-table}}
=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#Glycogen_Depletion_and_HIIT| Glycogen Depletion and HIIT]].
=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 [[VO2max|V̇O<sub>2</sub>max]]<ref name="BrocherieGirard2017"/>. 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% O<sub>2</sub>.
=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. ([[Muscle| 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<ref name="SimoneauLortie1985"/>. 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% [[VO2max|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<ref name="JanssonEsbjörnsson1990"/>. 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<ref name="PerryHeigenhauser2008"/><ref name="HoodLittle2011"/>, 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<ref name="WestonMyburgh1996"/>, and performance improvements with HIIT in well trained subjects seems well supported<ref name="LaursenShing2005"/><ref name="Westgarth-TaylorHawley1997"/>.
=Limitations of the HIIT science=
There are some important limitations of the HIIT science.
=References=
<references>
<ref name="BrocherieGirard2017">Franck Brocherie, Olivier Girard, Raphaël Faiss, Grégoire P. Millet, Effects of Repeated-Sprint Training in Hypoxia on Sea-Level Performance: A Meta-Analysis, Sports Medicine, volume 47, issue 8, 2017, pages 1651–1660, ISSN [http://www.worldcat.org/issn/0112-1642 0112-1642], doi [http://dx.doi.org/10.1007/s40279-017-0685-3 10.1007/s40279-017-0685-3]</ref>
<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, 28 !!V̇olume!!, 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, 89 !!V̇olume!!, 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="MetcalfeBabraj2011">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 [http://www.worldcat.org/issn/1439-6319 1439-6319], doi [http://dx.doi.org/10.1007/s00421-011-2254-z 10.1007/s00421-011-2254-z]</ref>
<ref name="RuffinoSongsorn2017">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 [http://www.worldcat.org/issn/1715-5312 1715-5312], doi [http://dx.doi.org/10.1139/apnm-2016-0497 10.1139/apnm-2016-0497]</ref>
<ref name="PerryHeigenhauser2008">Christopher G.R. Perry, George J.F. Heigenhauser, Arend Bonen, Lawrence L. Spriet, High-intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle, Applied Physiology, Nutrition, and Metabolism, volume 33, issue 6, 2008, pages 1112–1123, ISSN [http://www.worldcat.org/issn/1715-5312 1715-5312], doi [http://dx.doi.org/10.1139/H08-097 10.1139/H08-097]</ref>
<ref name="HoodLittle2011">Melanie S. Hood, Jonathan P. Little, Mark A. Tarnopolsky, Frank Myslik, Martin J. Gibala, Low-Volume Interval Training Improves Muscle Oxidative Capacity in Sedentary Adults, Medicine & Science in Sports & Exercise, volume 43, issue 10, 2011, pages 1849–1856, ISSN [http://www.worldcat.org/issn/0195-9131 0195-9131], doi [http://dx.doi.org/10.1249/MSS.0b013e3182199834 10.1249/MSS.0b013e3182199834]</ref>
<ref name="Westgarth-TaylorHawley1997">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 [http://www.worldcat.org/issn/1439-6319 1439-6319], doi [http://dx.doi.org/10.1007/s004210050164 10.1007/s004210050164]</ref>
<ref name="LaursenShing2005">Paul B. Laursen, Cecilia M. Shing, Jonathan M. Peake, Jeff S. Coombes, David G. Jenkins, Influence of High-Intensity Interval Training on Adaptations in Well-Trained Cyclists, The Journal of Strength and Conditioning Research, volume 19, issue 3, 2005, pages 527, ISSN [http://www.worldcat.org/issn/1064-8011 1064-8011], doi [http://dx.doi.org/10.1519/15964.1 10.1519/15964.1]</ref>
<ref name="WestonMyburgh1996">Adle R. Weston, K. H. Myburgh, F. H. Lindsay, Steven C. Dennis, Timothy D. Noakes, J. A. Hawley, Skeletal muscle buffering capacity and endurance performance after high-intensity interval training by well-trained cyclists, European Journal of Applied Physiology, volume 75, issue 1, 1996, pages 7–13, ISSN [http://www.worldcat.org/issn/1439-6319 1439-6319], doi [http://dx.doi.org/10.1007/s004210050119 10.1007/s004210050119]</ref>
<ref name="JanssonEsbjörnsson1990">E. Jansson, M. Esbjörnsson, I. Holm, I. Jacobs, Increase in the proportion of fast-twitch muscle fibres by sprint training in males, Acta Physiologica Scandinavica, volume 140, issue 3, 1990, pages 359–363, ISSN [http://www.worldcat.org/issn/00016772 00016772], doi [http://dx.doi.org/10.1111/j.1748-1716.1990.tb09010.x 10.1111/j.1748-1716.1990.tb09010.x]</ref>
<ref name="SimoneauLortie1985">J. -A. Simoneau, G. Lortie, M. R. Boulay, M. Marcotte, M. -C. Thibault, C. Bouchard, Human skeletal muscle fiber type alteration with high-intensity intermittent training, European Journal of Applied Physiology and Occupational Physiology, volume 54, issue 3, 1985, pages 250–253, ISSN [http://www.worldcat.org/issn/0301-5548 0301-5548], doi [http://dx.doi.org/10.1007/BF00426141 10.1007/BF00426141]</ref>
<ref name="VollaardMetcalfe2017">Niels B. J. Vollaard, Richard S. Metcalfe, Sean Williams, Effect of Number of Sprints in an SIT Session on Change in V˙O2max, Medicine & Science in Sports & Exercise, volume 49, issue 6, 2017, pages 1147–1156, ISSN [http://www.worldcat.org/issn/0195-9131 0195-9131], doi [http://dx.doi.org/10.1249/MSS.0000000000001204 10.1249/MSS.0000000000001204]</ref>
</references>
