What is High-intensity Interval Training?
High-intensity Interval Training (HIIT or HIT) is training at the upper end of the range where your body can provide energy from the oxygen you breathe. This effort is hard to sustain, so HIIT adds low-intensity or rest periods between the high-intensity periods. The high-intensity periods should be hard enough to leave you breathless — difficult to say more than a few words — but the pace should be one you can sustain for a minute or more. In general, the target heartbeat range is 85-100% of maximum heart rate, although, for shorter intervals, it’s challenging to reach that rate during the initial repetitions.
Some famous examples of HIIT workouts are:
- The Tabata Protocol: Published by Izumi Tabata in 1996, this protocol popularized HIIT training in the eight repetitions of 20 seconds maximal effort interspersed with 10 seconds of rest.
- The 4×4 Workout: This workout is recommended by the Norwegian University of Science and Technology. It starts with 10 minutes warm-up of moderate jogging, followed by four repetitions of 1-2 minutes of high-intensity running and then 3 minutes of jogging. Here’s a video explaining it, complete with a Norwegian accent.
Different exercises can be used for HIIT, including running, cycling, jumping jacks, or burpees. Generally, any activity that allows for intense effort can be used for HIIT.
Four Ways HIIT Improves Fitness
To allow increased performance at near the maximum sustainable rate, HIIT improves each step of how the body converts oxygen to energy:
1. Improved Lung Capacity
The first step to increasing oxygen use is to increase lung capacity, and indeed, HIIT can improve maximum inspiratory pressure, or the ability of the lungs to pull in air, by over 40%.1
2. Improved Heart Capacity
To get more energy from the oxygen you breathe, HIIT increases the maximal cardiac capacity, which is the ability of your heart to pump blood.2
3. Increased Capillary Density
HIIT also increases the capillary density to get more oxygen to the muscles that use it. The capillaries are the delicate blood vessels that deliver oxygen to your cells as the blood transfers from arteries from the heart to veins going back to the heart.3
4. Increased Mitochondria
Six Benefits of HIIT
Some benefits of HIIT follow from its fitness effects listed above, but some come from other aspects of HIIT:
1. Improved Cardio-respiratory Fitness
Having the energy to do the activities you want is vital to health, and HIIT can significantly boost cardio-respiratory fitness, a critical factor in how much energy you have. Cardio-respiratory fitness is often measured by your VO₂max, which indicates the maximum amount of oxygen you can process for energy. Training in HIIT for six weeks can increase VO₂max (a measure of cardio-respiratory fitness, which gives the maximum rate your body can use oxygen during exercise) by 10%.5
2. Fat loss
By raising your resting metabolic rate more than resistance exercise or AE, HIIT helps metabolize fat.6 Some studies have shown fat reductions from HIIT without changing dietary habits.7
3. Time Efficient
Aerobic Exercise (AE) also improves cardio-respiratory fitness. Still, the gains from HIIT are available with less total time exercising, especially if you only count the high-intensity effort time and not the lower-intensity rest periods. In general, a HIIT workout, including the rest intervals, takes about half as long as AE that would provide a similar cardio-respiratory boost.8
4. Lower Blood Pressure
Similar to AE, HIIT can lower blood pressure by up to 10 points after eight weeks of practice,9 which is as much as the effect of common blood pressure treatment medications.10
5. Reduced Insulin Resistance
The primary cause of Type 2 Diabetes is insulin resistance, and HIIT has been shown to have a significant impact in reducing insulin resistance.11
Signs of insulin resistance are fasting glucose level or hemoglobin A1C (HbA1C),12 which are standard blood tests. HIIT was found to reduce blood sugar by nearly one mmol/l and HbA1C by about .2%.
6. Longer life
Even for people already doing AE, the health benefits from adding HIIT to your weekly exercise routine can add years to your life. A 2020 Norwegian study showed that adding just two sessions of 4×4 HIIT reduced all-cause mortality by almost 50% compared with people only doing AE.13 Even starting from 50 years old, that could add seven years of life.
What are the best exercises for HIIT?
The best exercise is the one you can do at high intensity, and you like. Brisk walking is probably not difficult enough to reach high-intensity, but good options are running, cycling, or swimming. Fitness exercises like jumping rope, body squats, or burpees can also be good choices.
How much HIIT should I do?
A good target is about two 4×4 workouts per week. This is enough to provide the longevity benefits above, and when done consistently, will provide a steady improvement in cardio-respiratory fitness.
With this base of 2 HIIT workouts per week, the cardiorespiratory benefits of HIIT improve linearly by the product of the total amount of time exercised by the percentage of your maximum effort in watts.14 While the percent of your watt max is a little lower than the percentage of your heart rate maximum (HRmax), it’s a good guideline for how many weeks of HIIT are required to reach a particular V̇O2 max improvement. Fourteen months of two 4×4 workouts/week could be enough to increase V̇O2 max by about 10%. If you doubled the repetitions from 4 to 8 (an 8×4 workout), the same gain of 10% could be achieved in 7 months.
Frequently Asked Questions
1. How much does skipping some HIIT reduce fitness?
Detraining or stopping HIIT for two weeks or more has a significant impact on cardiorespiratory fitness. A two-week break could reduce V̇O2 max by 4%,15 which would take over 5 hours of high-intensity effort (not including rest intervals) to recover or half a year of two 4×4 sessions per week. Keeping up training at one session per week seems enough to maintain cardiorespiratory benefits. However, endurance may drop.16
2. Does playing sports count as HIIT?
Mostly, yes. While some sports, like golf, don’t have enough intensity, sports that often require speed dashes, like racquet sports, soccer, or basketball, give similar benefits to HIIT. Studies on sports participants find that these kinds of sports have the highest reductions in all-cause mortality.17
3. How much HIIT is too much?
So HIIT seems great, right? But, you can get too much of a good thing. As with other physical activities, there is a U-shaped curve of the benefits, where benefits reverse at high levels due to overtraining. After 160 minutes of HIIT effort time a week, performance flatlines, glucose intolerance starts to increase, and mitochondrial energy production decreases. Direct analysis of mitochondria extracted from thigh muscle showed that while the total amount of mitochondria was continuing to grow during the overtraining, but the amount of energy production per mitochondria dropped by 40%.18
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