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* 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.
=Components of HIIT=
There are several variables that shape a HIIT workout<ref name="BuchheitLaursen2013"/>.
This study<ref name="EarnestTjønna2013"/> 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 [[VO2max|V̇O<sub>2</sub>max]] for the two groups was a little different, with the single repetition group having a [[VO2max|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 [[VO2max|V̇O<sub>2</sub>max]] 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.
==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)<ref name="MetcalfeBabraj2011"/>. 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 [[VO2max|V̇O<sub>2</sub>max]] 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. [[VO2max|V̇O<sub>2</sub>max]] 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.
==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)<ref name="RuffinoSongsorn2017"/>. This study used 16 men with type II diabetes and morbid obesity (BMI >35), who were otherwise reasonably healthy. Their [[VO2max|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 [[VO2max|V̇O<sub>2</sub>max]]. The REHIT resulted in 7% [[VO2max|V̇O<sub>2</sub>max]] 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.
=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.
==High-intensity interval exercise induces 24-h energy expenditure similar to traditional endurance exercise despite reduced time commitment (SkellyAndrews2014)==
Nine men with [[VO2max|V̇O<sub>2</sub>max]] of ~46 (3:25 marathon equivalent) were tested three times to evaluate their 24-hour energy expenditure<ref name="SkellyAndrews2014"/>. 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.
=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]].
=Limitations of the HIIT science=
There are some important limitations of the HIIT science.
<ref name="SkellyAndrews2014">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 [http://www.worldcat.org/issn/1715-5312 1715-5312], doi [http://dx.doi.org/10.1139/apnm-2013-0562 10.1139/apnm-2013-0562]</ref>
<ref name="ZunigaBerg2011">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 [http://www.worldcat.org/issn/1064-8011 1064-8011], doi [http://dx.doi.org/10.1519/JSC.0b013e3181d681b6 10.1519/JSC.0b013e3181d681b6]</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>
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