What Is HIIT? Benefits and How to Start

HIIT, short for high-intensity interval training, is a way of exercising in which short bursts of hard effort alternate with short periods of easier recovery. An umbrella review for the 2018 US Physical Activity Guidelines Advisory Committee, published as an official American College of Sports Medicine (ACSM) pronouncement, described it as short bouts of high-intensity exercise separated by short periods of recovery at a lower intensity (Campbell et al., 2019). It found moderate evidence that HIIT can improve insulin sensitivity, blood pressure and body composition about as much as steady, moderate-intensity exercise (same source). Sessions can also be much shorter, as the trial described below shows.

How hard is “high intensity”?

There’s no single agreed definition. The same review notes that there’s no universally accepted length for the hard or easy periods, no set number of rounds, and no agreed intensity for the hard part. It groups HIIT protocols into three broad bands (Campbell et al., 2019):

Type	Intensity of the hard intervals
Sprint interval training	Above the level of maximal oxygen uptake
Near-maximal interval training	90% to 100% of maximum heart rate or oxygen uptake
Vigorous interval training	64% to 90% of maximal oxygen uptake

Physiologists MacInnis and Gibala draw a similar line between HIIT, which uses “near maximal” efforts, and sprint interval training, which uses “supramaximal” ones (MacInnis & Gibala, 2017).

Without lab equipment you can use the CDC’s talk test. At vigorous intensity you can’t say more than a few words without pausing for a breath (CDC). The lowest of the three bands above is vigorous intensity, so the hard intervals should feel at least that hard. The NHS calls this “very vigorous” activity, meaning short bursts of maximum effort broken up with rest. Its examples include interval running, sprinting up hills, running up stairs, circuit training and spinning classes (NHS).

What a HIIT session can look like

One 12-week trial in sedentary men compared two cycling programs, each done three times a week (Gillen et al., 2016):

• Sprint intervals: a 2-minute easy warm-up, three 20-second all-out sprints separated by 2 minutes of easy pedaling, and a 3-minute cool-down. That’s about 10 minutes in total, with 1 minute of hard work.
• Steady cardio: 45 minutes of continuous cycling at about 70% of maximum heart rate, about 50 minutes with warm-up and cool-down.

Peak oxygen uptake, a standard measure of aerobic fitness, rose by 19% in both groups. Insulin sensitivity and muscle mitochondrial content also improved similarly (same source). It was a small study of men averaging 27 years old, on stationary bikes, so other programs and other people may get different results.

What the research shows

Aerobic fitness

The reviews that the 2018 advisory committee examined consistently found that HIIT raises VO2max in adults across a range of body weights and health conditions. In one meta-analysis of 50 studies, HIIT raised VO2max more than not exercising and, by a smaller margin, more than continuous training (Campbell et al., 2019). MacInnis and Gibala also report that HIIT tends to raise VO2max more than moderate continuous training when the total amount of work is matched (MacInnis & Gibala, 2017).

Heart and metabolic health

HIIT’s improvements in insulin sensitivity, blood pressure and body composition were comparable to those from moderate continuous training. They were more likely in adults with overweight or obesity, or at higher risk of heart disease and diabetes, especially with 12 weeks or more of training. Healthy adults at a normal weight typically didn’t see those particular changes, and blood lipids didn’t appear to change with HIIT (Campbell et al., 2019).

Fat loss

A meta-analysis of 31 studies that compared interval training directly with moderate continuous exercise found it comparable, but not superior, for reducing body fat (Campbell et al., 2019).

What’s still unknown

The committee found too little evidence to say how much HIIT produces how much benefit, or whether results differ by age, sex or race/ethnicity. It also said people’s willingness and ability to stick with HIIT over the long term aren’t well known (Campbell et al., 2019).

Is HIIT safe?

Harder exercise carries more risk. Writing for ACSM, Kenneth Powell said injury and other adverse event rates are likely higher at high intensity, and that injury risk rises with the size of the overload. Reported injury and dropout rates in HIIT studies are low, he added, but those studies have been small and selective with short follow-up (ACSM). In one meta-analysis, only about a third of studies reported adverse events. Those that did recorded 18 musculoskeletal injuries, 14 of them during HIIT, and none were serious (Campbell et al., 2019).

If you have a heart condition, talk to your doctor first. One large safety study comes from supervised cardiac rehabilitation. Among 4,846 patients with coronary heart disease at three Norwegian cardiac rehabilitation centers, there was one fatal cardiac arrest in 129,456 hours of moderate exercise and two nonfatal cardiac arrests in 46,364 hours of high-intensity intervals. The authors judged the risk low in that setting (Rognmo et al., 2012). That was supervised training, which is different from doing intervals alone at home.

More generally, the US guidelines say people with chronic conditions or symptoms should be under the care of a health care provider (HHS). The CDC says to talk to your doctor before starting vigorous activity if you have been inactive, have a disability, or have overweight (CDC). ACSM’s screening approach for deciding who should get medical clearance looks at three things: how active you are now, whether you have signs or symptoms of, or known, heart, metabolic or kidney disease, and how hard you plan to exercise (Riebe et al., 2015).

How to start

If you’re new to exercise, build a base first. The US guidelines advise inactive people to “start low and go slow”, beginning with lower-intensity activity and gradually increasing how often and how long they’re active. They also advise choosing activities that suit your current fitness level (HHS). Our guide to how much exercise you need explains the weekly targets HIIT can count toward.

Once regular moderate activity feels comfortable:

1. Pick a movement you already know, such as brisk walking, cycling or a simple bodyweight exercise.
2. Warm up at an easy pace first, as the trial above did.
3. Add a few short efforts hard enough that talking becomes difficult, each followed by easy recovery until your breathing settles.
4. Finish with an easy cool-down, and see how your body responds before you push harder next time.
5. Add rounds or shorten the recovery gradually, not all at once.

HIIT also doesn’t replace strength training. The US guidelines still ask adults to do muscle-strengthening activities on 2 or more days a week (HHS). A bodyweight workout for beginners and a home workout plan can help you fit both into your week. As Powell put it for ACSM, the best physical activity regimens are the ones people can and will keep doing for years (ACSM).