“I don’t have time!” is one of the top reasons for not exercising, as many traditional workouts push a commitment of about an hour. High intensity interval training, or HIIT, challenges this barrier by incorporating an effective workout in half that time. In our time-pressured culture, HIIT has claimed a spot in the top 10 fitness trends since 2014 as surveyed by the American College of Sports Medicine.  In about 30 minutes, it is considered a complete workout that combines both aerobic and strength (resistance) training. In order to achieve similar benefits to a longer workout, the intensity is vigorous.
HIIT is a type of interval training exercise. It incorporates several rounds that alternate between several minutes of high intensity movements to significantly increase the heart rate to at least 80% of one’s maximum heart rate, followed by short periods of lower intensity movements. Interval training was first introduced in the 1950s as a higher intensity form called sprint interval training, which reached 100% maximum heart rate and was used to improve the performance of elite Olympic athletes.
Body weight can be used as the main form of resistance so that additional equipment is not needed. HIIT workouts also generally do not require a large amount of space, making the format ideal for a home workout. HIIT workouts can be integrated into various exercise formats, such as running (outdoors or on a treadmill), dancing, rowing machines, stationary bicycles, or stair climbers.  The interval durations can be timed by using one to five-minute music tracks.
Other terms that are used interchangeably with HIIT are Tabata and circuit training. Tabata is a form of HIIT that was created by Professor Izumi Tabata in 1996 involving Olympic speedskaters. His exercise intervals were extremely high intensity, followed by very brief rest periods. Fitness centers and gyms that offer Tabata classes are typically 20-30 minutes and encourage participants to reach their highest intensity ability, but they can self-regulate their workouts. Circuit training involves 8-12 exercise stations that target different muscle groups. Participants rotate through each station, completing one exercise that lasts several minutes. The difference with circuit training is that the intensity is variable, whereas HIIT encourages maximum effort by reaching 80-90% maximum heart rate.
HIIT can help to decrease body fat, increase strength and endurance, and improve health outcomes, but it is not necessarily better than other exercise formats. Its main appeal is that it can achieve similar fitness and health benefits in a shorter duration, and that it includes periods of rest.
People who are deconditioned, recovering from injury, elderly, have overweight, or have medical conditions should be followed and monitored closely by their physician and an exercise professional, because of the higher intensity achieved with HIIT. It has been observed that for deconditioned individuals, the intensity of HIIT is comparable to what they may encounter during activities of daily living. The American College of Sports Medicine provides screening tools that can be used to identify risk factors using the HIIT format, to lower the risk of adverse events.  These checklists include medical conditions that are contraindicated to performing HIIT workouts (e.g., uncontrolled heart rate such as with arrhythmias, uncontrolled diabetes, retinopathy), and symptoms to watch for to end a HIIT workout early, such as a significant rise or drop in blood pressure during the workout. 
HIIT workouts should be tailored to the individual’s fitness level and medical conditions. Research has generally found HIIT to be a safe and enjoyable exercise for a range of ages and medical conditions. A meta-review of HIIT compared with control groups found HIIT to be safe (no acute injury reports or serious cardiovascular events) in controlled supervised settings, with mean compliance rates in completing the program reaching >80%. 
Example of a beginner HIIT workout
- 30 seconds of side lunges, alternating right to left
- 15 seconds of slow marches in place
- 30 seconds of squats (variation for higher intensity: jump squats)
- 15 seconds of slow marches in place
- 30 seconds of push-ups on the floor (modification: at a 45-degree angle on a sturdy chair, or against the wall)
- 15 seconds of slow marches in place
- 30 seconds of jumping jacks (modification: alternate right and left tapping toes to the sides while bringing arms overhead as you would a jumping jack)
- 15 seconds of slow marches in place
- 30 seconds of triceps dips using a sturdy chair or bed
- 15 seconds of slow marches in place
- 30 seconds of alternating high knees (variation for higher intensity: jogging high knees)
- 15 seconds of slow marches in place
- 30 seconds of sit-ups (modification: sit-ups on a stability ball, or abdominal crunches on the floor)
REST FOR 60 SECONDS AND REPEAT THE SESSION 2 MORE TIMES
HIIT and Health
HIIT is a well-researched exercise format, showing benefits for a range of medical conditions across a broad age range, from adolescents to older adults.  In research studies, HIIT is typically compared with moderate intensity continuous training (MICT), which incorporates lower intensity movements at a constant pace without interval breaks. Whereas HIIT causes individuals to reach 80-85% of their maximum heart rate, MICT reaches about 55-70% of their maximum heart rate.
When energy expenditure remains the same for HIIT versus MICT workouts, some studies show a greater benefit with HIIT because it achieves greater aerobic capacity (the body’s ability to use more oxygen).  Although initially applied to athletes to improve their performance, HIIT is now included as a potential exercise option for individuals with chronic diseases. It can help to improve their physical functioning, exercise tolerance, and quality of life.
Although the higher intensities reached with HIIT formats may appear too difficult for people with chronic diseases, research has shown that the intermittent rest intervals and overall shorter duration of the exercise make HIIT a potentially feasible and safe option for even more serious conditions like lung disease, heart disease, and chronic kidney disease under close supervision.  Modifications may be implemented, such as using specific exercises or incorporating longer warm-up and cool-down periods for people with heart disease or on blood pressure medications.  Some research in these populations found HIIT to be rated more enjoyable and with longer-term compliance than MICT.  However, larger and additional studies need to be performed with these at-risk groups before routinely prescribing HIIT. HIIT has also been found to improve depression more than MICT in people with major depression or bipolar disorders, without reported adverse events. 
Most research on HIIT and chronic disease is available for cardiovascular disease (CVD). Studies show that HIIT can achieve greater reduction in CVD risk factors than MICT.  The rest intervals and shorter durations of higher intensity intervals of HIIT workouts may help cardiac patients to complete the workout and achieve greater stimulation of the heart. Use of HIIT programs has shown improvements in cardiovascular health in patients who have had a heart attack, coronary artery bypass surgery, or congestive heart failure. HIIT workouts can be modified depending on an individual’s fitness level and have been successfully prescribed for cardiac rehabilitation programs. 
A meta-analysis looking at the effect of HIIT and MICT on cardiovascular risk factors in adults with overweight or obesity found that both significantly decreased weight, body fat percentage, and total cholesterol, and improved the body’s use of oxygen. However, the HIIT sessions were almost 10 minutes shorter than the MICT sessions. 
A meta-analysis of randomized controlled trials comparing the effect of HIIT versus MICT on blood lipids did not find a significant difference, as both formats improved lipid profiles. One exception was HDL cholesterol, which appeared to improve more with HIIT than MICT.  Overall the authors concluded that a higher total amount of exercise appeared to have a stronger effect on lowering lipids than the intensity of exercise.
Regular physical exercise, particularly vigorous aerobic exercise, has been studied to control hypertension, or high blood pressure.  Meta-analyses of randomized trials found that HIIT was as effective as MICT, when compared with a control group, in lowering resting blood pressure and systolic blood pressure in people with borderline or established high blood pressure. [13,12] Some of these studies found that HIIT showed a greater reduction in diastolic blood pressure than MICT, and HIIT more effectively improved total oxygen efficiency (VO2max) than MICT.
HIIT has been found as equally effective as MICT in improving aerobic endurance and reducing shortness of breath in people with chronic obstructive pulmonary disease (COPD). [5,14] Those with COPD cannot usually engage in higher intensity exercises for long durations due to increased shortness of breath.  The HIIT rest periods allow participants temporary relief from the high breathing demands required with higher intensity exercise, which then promotes better engagement in the bouts of high intensity movements that are needed to improve lung functioning. Both HIIT and MICT workouts produced functional improvements of less breathing difficulties during activities of daily living, greater functional capacity, less fatigue, and improved quality of life.
Research on HIIT with other lung conditions such as asthma, cystic fibrosis, and lung cancer is limited or nonexistent. Although available studies in these populations suggest a functional benefit, they did not include control groups and participants were not randomized.
The Centers for Disease Control and Prevention and the American College of Sports Medicine recommend 300 minutes weekly of moderate-intensity aerobic exercise for modest weight loss, and even longer durations for additional weight loss.  The majority of adults do not meet these guidelines, with lack of time cited as a major barrier. Therefore, shorter HIIT workouts have been considered a possible format to help individuals meet recommended fitness goals. Research has found compliance rates to be as high and enjoyable with HIIT formats as with MICT. 
A meta-analysis of controlled trials following participants with overweight and obesity for an average of 10 weeks who exercised three times a week found that MICT and HIIT programs showed similar effectiveness in producing modest body composition improvements (i.e., decreased fat mass and waist circumference) despite no changes in body weight.  An attractive feature was that the time spent on HIIT programs was 40% less than with MICT. The authors found that HIIT programs incorporating running were more likely to show fat mass losses than with cycling. [18,17] Studies also suggest that HIIT may be more effective at reducing abdominal (visceral) fat than MICT. [19,17] This may be due to the release of hormones like epinephrine and growth hormone that promote the breakdown of fat, in response to the high exercise intensities achieved. 
However, another meta-analysis of studies comparing HIIT with MICT did not find significant differences in the participants’ body composition and fat reduction (both formats produced small reductions in total body fat and fat mass) when controlling for energy expenditure.  The authors cautioned that the quality of studies was low, as most were small with 20 participants or less, and short-term. Most also did not monitor changes in appetite or dietary habits, which could have affected the results. It has been shown that HIIT may regulate appetite better than MICT, but more studies are needed in this area.
Type 2 diabetes
HIIT should only be performed in people with diabetes who have well-controlled blood glucose levels, and should be avoided if one has diabetic retinopathy, which increases the risk of detachment of the retina.  If one has diabetic neuropathy, foot care should be performed to monitor for foot blisters or ulcers. In short to medium-term interventions (up to 16 weeks), HIIT has been found more effective than MICT at reducing fasting blood glucose, hemoglobin A1c, fat mass, and insulin resistance in people with type 2 diabetes. [10,5,20] However in a year-long randomized controlled trial, HIIT did not produce greater improvements in hemoglobin A1c, body composition, or cardiorespiratory fitness compared with MICT.  The authors noted that appetite and dietary changes were not monitored, which could have affected the results. Both the MICT and HIIT programs were found to be safe and scored high among participants for enjoyability.
HIIT is an effective exercise option to increase endurance and strength in those who have limited time to exercise. Because of the higher intensity format, it is advised to consult with a physician if you have any medical conditions before starting a HIIT program. All participants new to HIIT should choose a program that is facilitated by an exercise professional.
- Thompson WR. Worldwide Survey of Fitness Trends for 2021, ACSM’s Health & Fitness Journal. 1/2 2021. Volume 25; Issue 1: 10-19.
- Taylor JL, Holland DJ, Spathis JG, Beetham KS, Wisløff U, Keating SE, Coombes JS. Guidelines for the delivery and monitoring of high intensity interval training in clinical populations. Progress in cardiovascular diseases. 2019 Mar 1;62(2):140-6.
- American College of Sports Medicine. ACSM’s guidelines for exercise testing and prescription. 11th Philadelphia, PA: Wolters Kluwer, 2021.
- Martland R, Mondelli V, Gaughran F, Stubbs B. Can high-intensity interval training improve physical and mental health outcomes? A meta-review of 33 systematic reviews across the lifespan. Journal of sports sciences. 2020 Feb 16;38(4):430-69.
- Ross LM, Porter RR, Durstine JL. High-intensity interval training (HIIT) for patients with chronic diseases. Journal of sport and health science. 2016 Jun 1;5(2):139-44.
- Costigan SA, Eather N, Plotnikoff RC, Taaffe DR, Lubans DR. High-intensity interval training for improving health-related fitness in adolescents: a systematic review and meta-analysis. British journal of sports medicine. 2015 Oct 1;49(19):1253-61.
- Beetham KS, Howden EJ, Fassett RG, Petersen A, Trewin AJ, Isbel NM, Coombes JS. High‐intensity interval training in chronic kidney disease: A randomized pilot study. Scandinavian journal of medicine & science in sports. 2019 Aug;29(8):1197-204.
- Korman N, Armour M, Chapman J, Rosenbaum S, Kisely S, Suetani S, Firth J, Siskind D. High intensity interval training (HIIT) for people with severe mental illness: a systematic review & meta-analysis of intervention studies–considering diverse approaches for mental and physical recovery. Psychiatry research. 2020 Feb 1;284:112601.
- Ribeiro PA, Boidin M, Juneau M, Nigam A, Gayda M. High-intensity interval training in patients with coronary heart disease: prescription models and perspectives. Annals of physical and rehabilitation medicine. 2017 Jan 1;60(1):50-7.
- Su L, Fu J, Sun S, Zhao G, Cheng W, Dou C, Quan M. Effects of HIIT and MICT on cardiovascular risk factors in adults with overweight and/or obesity: A meta-analysis. PLoS One. 2019 Jan 28;14(1):e0210644.
- Wood G, Murrell A, Van Der Touw T, Smart N. HIIT is not superior to MICT in altering blood lipids: a systematic review and meta-analysis. BMJ open sport & exercise medicine. 2019 Dec 1;5(1):e000647.
- Leal JM, Galliano LM, Del Vecchio FB. Effectiveness of high-intensity interval training versus moderate-intensity continuous training in hypertensive patients: a systematic review and meta-analysis. Current hypertension reports. 2020 Mar;22(3):1-3.
- Costa EC, Hay JL, Kehler DS, Boreskie KF, Arora RC, Umpierre D, Szwajcer A, Duhamel TA. Effects of high-intensity interval training versus moderate-intensity continuous training on blood pressure in adults with pre-to established hypertension: a systematic review and meta-analysis of randomized trials. Sports Medicine. 2018 Sep;48(9):2127-42.
- Sawyer A, Cavalheri V, Hill K. Effects of high intensity interval training on exercise capacity in people with chronic pulmonary conditions: a narrative review. BMC Sports Science, Medicine and Rehabilitation. 2020 Dec;12(1):1-0.
- Keating SE, Johnson NA, Mielke GI, Coombes JS. A systematic review and meta‐analysis of interval training versus moderate‐intensity continuous training on body adiposity. Obesity reviews. 2017 Aug;18(8):943-64.*Disclosure: JS Coombes has received an unrestricted research grant from Coca Cola and project specific funding from Renew Corp, Pfizer, Cyanotech, Terumo, Gatorade, Numico, Northfields and Baxter, and has received honorariums to present at meetings from Novartis, Amgen, and Roche
- Vella CA, Taylor K, Drummer D. High-intensity interval and moderate-intensity continuous training elicit similar enjoyment and adherence levels in overweight and obese adults. European journal of sport science. 2017 Oct 21;17(9):1203-11.
- Wewege M, Van Den Berg R, Ward RE, Keech A. The effects of high‐intensity interval training vs. moderate‐intensity continuous training on body composition in overweight and obese adults: a systematic review and meta‐analysis. Obesity Reviews. 2017 Jun;18(6):635-46.
- Maillard F, Pereira B, Boisseau N. Effect of high-intensity interval training on total, abdominal and visceral fat mass: a meta-analysis. Sports Medicine. 2018 Feb;48(2):269-88.
- Maillard F, Rousset S, Pereira B, Traore A, Del Amaze PD, Boirie Y, Duclos M, Boisseau N. High-intensity interval training reduces abdominal fat mass in postmenopausal women with type 2 diabetes. Diabetes & metabolism. 2016 Dec 1;42(6):433-41.
- Winding KM, Munch GW, Iepsen UW, Van Hall G, Pedersen BK, Mortensen SP. The effect on glycaemic control of low‐volume high‐intensity interval training versus endurance training in individuals with type 2 diabetes. Diabetes, Obesity and Metabolism. 2018 May;20(5):1131-9.
- Magalhães JP, Júdice PB, Ribeiro R, Andrade R, Raposo J, Dores H, Bicho M, Sardinha LB. Effectiveness of high‐intensity interval training combined with resistance training versus continuous moderate‐intensity training combined with resistance training in patients with type 2 diabetes: A one‐year randomized controlled trial. Diabetes, Obesity and Metabolism. 2019 Mar;21(3):550-9.
Last reviewed November 2021
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