What is HIIT?
High Intensity Interval Training (HIIT) is a cardiovascular and strength training workout format that alternates short bursts of exercise with rest intervals that let your heart rate come down. It's one of the most researched ways to get fitter in less time.
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Work hard. Recover. Repeat.
Every HIIT workout pushes your heart rate up during short bursts of work, then lets it come down during rest. Those heart rate spikes are the marker of a HIIT workout. Running sprints and resting after each one is HIIT. Most commonly, HIIT workouts include bodyweight, weighted and banded movements that also increase strength and muscle tone.
Example: 30 seconds of Jumping Jacks followed by 30 seconds of rest, repeated 5 times, makes a 5-minute HIIT workout.
What are HIIT intervals?
There are two types of intervals in a HIIT workout: exercise and rest. During an exercise interval, you perform a movement at the highest intensity you can while still keeping correct form. The rest interval that follows lets your heart rate come down so you can go hard again.
Intervals can be as short as a few seconds or as long as a few minutes, depending on who the program is for. In Group HIIT workouts, some movements are paired closely together, either to increase the intensity or to give you an "active rest": a lower-intensity movement that still lets your heart rate come down.
How hard should it feel?
How HIIT timers work
Our classes use many different interval timers. They all come down to two things: how much you work compared with how much you rest, and how the moves are arranged.
Group HIIT classes use simple timer beeps to tell you when to switch, so you can focus on the work, not the clock.
Our interval timers aren't only for HIIT. Some classes, like low-impact and strength-focused workouts, follow the same work-and-rest format at a lower intensity, so the format works at any level.
HIIT vs. steady cardio
| Steady cardio | HIIT | |
|---|---|---|
| What it looks like | Jogging, cycling or brisk walking at an even pace | Short, hard bursts with recovery in between |
| How hard | Moderate. You can talk in full sentences | High. Only a few words at a time |
| VO2 max & endurance | Improves | Improves more |
| Fat loss | Effective | Just as effective in about 40% less time |
| Variety | Usually the same movement throughout | Changes every interval |
Where HIIT came from
Athletes
Coaches have used interval training with runners and other endurance athletes for generations.
Tabata
Japanese researcher Izumi Tabata's 4-minute protocol of 20 seconds on and 10 off improved both aerobic and anaerobic fitness in 6 weeks.
McMaster & Norway
Martin Gibala's lab in Canada showed how little time HIIT can take, while Norway's "4x4" research brought HIIT to heart patients and older adults.
What HIIT does for you
HIIT is one of the most studied workout formats in exercise science. The strongest evidence is for fitness and time: you get more done in less time. Here's what peer-reviewed research shows.
Almost double the VO2 max gains
VO2 max is the standard measure of cardio fitness and endurance. In adults with heart disease, high blood pressure, metabolic syndrome or obesity, HIIT improved it almost twice as much as moderate continuous exercise.
Same fat loss in 40% less time
Across 13 studies of overweight and obese adults, HIIT matched steady cardio for fat loss and body composition while needing about 40% less training time.
One minute of intensity, 45 minutes of results
After 12 weeks, sedentary men doing a 10-minute workout with just one minute of all-out effort improved fitness and blood sugar control as much as men doing 45 minutes of steady cycling.
More energy in your muscle cells as you age
After 12 weeks of HIIT, mitochondrial capacity, your muscles' energy factories, rose 69% in adults aged 65 to 80. That was a bigger gain than younger adults saw (49%).
Better blood sugar control
Across 50 studies, HIIT lowered insulin resistance more than both no exercise and steady cardio. People with or at risk of type 2 diabetes saw lower fasting blood sugar.
Less depression, better well-being
Across 58 randomized trials, HIIT improved mental well-being and reduced symptoms of depression and stress compared with not exercising. The evidence was rated moderate-to-high quality.
The research by health goal
Tap a topic to see what the studies found, and how strong the evidence is.
VO2 max, endurance & heart health Strong
VO2 max is the most oxygen your body can use during exercise. The higher it is, the longer and harder you can go, and it's one of the strongest predictors of long-term health. It's also where HIIT stands out most: across studies, HIIT raises VO2 max more than steady, moderate cardio. In a Norwegian study of heart failure patients, 4x4 HIIT raised fitness 46% compared with 14% for moderate training. A larger trial of 261 patients later found HIIT and moderate training worked about equally well, and both beat exercise advice alone.
Weight & body fat Strong
A review of 39 studies found HIIT reduced total, abdominal and visceral (deep belly) fat, with no difference between men and women. Compared with steady cardio, fat loss is similar, but HIIT gets there in about 40% less time. For fat loss, consistency and nutrition matter most.
Blood sugar & type 2 diabetes Strong
Across 50 studies, HIIT reduced insulin resistance more than both no exercise and continuous training. People with or at risk of type 2 diabetes saw lower fasting blood sugar. The effects are meaningful but modest, so HIIT works best alongside healthy eating. If you take insulin or medication that lowers blood sugar, talk to your doctor before starting.
Blood pressure Moderate
In adults with elevated or high blood pressure, HIIT and moderate cardio lowered resting blood pressure by similar amounts, and HIIT improved fitness more. Both are good options. HIIT just takes less time.
Healthy aging Moderate
HIIT boosted muscle energy capacity more in adults aged 65 to 80 than in younger adults. In Norway's Generation 100 trial, 1,567 adults aged 70 to 77 exercised for five years. The group doing HIIT twice a week had the best fitness and a trend toward lower mortality, though that difference wasn't statistically significant.
Mood & stress Moderate–high
A review of 58 randomized trials found HIIT improved mental well-being and reduced depression and stress compared with not exercising, with small extra gains in well-being compared with other exercise.
Liver health Moderate
HIIT reduced liver fat as much as moderate cardio, often with less time and energy. Any regular aerobic exercise above a moderate intensity helps.
Kids & teens Moderate
In a review of 9 studies of overweight and obese kids and teens aged 6 to 17, 4 to 12 weeks of HIIT lowered blood pressure and improved aerobic fitness more than other types of exercise, in less time. Looking for workouts made for kids? Try Rocket Fit Kids.
Cellular aging (telomeres) Emerging
Telomeres are protective caps on your chromosomes that shorten as you age. In a 6-month trial of previously inactive, healthy adults, HIIT and endurance running each raised telomerase activity (the enzyme that maintains telomeres) two- to threefold and lengthened telomeres in blood cells. Resistance training alone didn't have this effect. It's one well-designed trial, so the finding still needs to be confirmed.
Cancer Emerging
For people during or after cancer treatment, reviews show HIIT improves cardio fitness, with high adherence and few side effects. In a small trial of 52 men with early-stage prostate cancer under active surveillance, 12 weeks of HIIT lowered PSA levels, and their blood slowed the growth of prostate cancer cells in the lab. A similar lab study found blood taken right after a single HIIT session slowed the growth of colon cancer cells in colorectal cancer survivors. These are early results. HIIT is not a cancer treatment, so talk to your care team before starting.
Arthritis, lung health & brain health Emerging
HIIT is being studied for arthritis, COPD and brain health. Early results are promising, but the research isn't mature enough yet to call these proven benefits.
Many HIIT studies are small and run 6 to 16 weeks, and most used bikes or treadmills. Results vary from person to person.
HIIT myths, busted
Knowing what HIIT can and can't do helps you set the right goals, and stick with them.
"HIIT is just cardio, like burpees and sprints."
Fact: HIIT is a format, not a type of move. Squats, lunges, push-ups and weighted or banded strength moves get your heart rate up too when you do them in timed intervals, so you build strength and fitness in the same workout.
"The harder you go, the more fat you burn."
Fact: HIIT and steady cardio lead to similar fat loss. Intensity mainly drives fitness. For fat loss, showing up consistently and eating well matter most.
"HIIT is only for young, fit people."
Fact: Studies have safely used HIIT with adults in their 70s and with heart patients in supervised rehab. "High intensity" means high for you.
"You have to go all-out every time."
Fact: Going all-out every session raises your risk of injury and burnout. Adjust your effort day to day, choose low-impact options when you need them, and take rest days so your body can recover.
HIIT works when you hit the intervals and keep showing up
Research points to two things that make or break HIIT: reaching the right intensity and staying consistent. In one large heart-failure trial, about half the patients trained below their HIIT target, even with supervision. Group HIIT is built to help with both.
Timed for you
Timer beeps tell you when to work and when to rest, and instructors show you every move.
How it works →A program for every goal
Each program targets a goal, like losing weight or building muscle. Spark Fit is for beginners, Scorch Fit is low impact, and Flash Fit and Blaze Fit are there when you're ready for more.
Compare programs →Strength built in
Over 400 bodyweight, weighted, banded, plyometric and stabilization movements, so every workout is different. That matters, because HIIT on a bike or treadmill builds less strength than resistance training.
Browse workouts →Better together
Work out with friends, family or coworkers. People who train with others are more likely to stick with it.
Why it works →How to start HIIT the right way
You don't need to be fit to start HIIT. You just need to start at your level and build from there.
Four simple rules
- Follow your program's calendarEvery program lays out your workouts day by day, so you never have to guess how often to train. Feeling tired or sore? Lower the intensity or take an extra rest day.
- Warm up and cool down every time5 to 10 minutes each. Try our warm-ups and cool-downs.
- Aim for about 7 out of 10 at first"High intensity" means high for you. Choose low-impact options, like stepping back instead of jumping.
- Progress one thing at a timeAdd rounds first, then longer work intervals, then shorter rest.
Start with a plan, not a guess.
Spark Fit is our 6-week beginner program, and Scorch Fit builds low-impact strength. Both take you day by day, so you always know what to do next.
Start with Spark Fit Prefer low impact? See Scorch Fit →Check with your doctor before starting HIIT if you:
- Have heart, lung, kidney or metabolic disease
- Take insulin or other medication that lowers blood sugar
- Have chest pain, fainting or unexplained shortness of breath
- Have high blood pressure that isn't controlled
- Are pregnant, especially if you're new to vigorous exercise
- Have had recent surgery or have joint problems
- Are older and haven't been active lately
In supervised cardiac rehab, HIIT has had a low rate of serious heart events (1 per 17,083 sessions in one review), but those patients were medically monitored. Source: Wewege et al., Journal of the American Heart Association, 2018.
HIIT questions, answered
What does HIIT stand for?
High Intensity Interval Training. It alternates short bursts of hard exercise with rest or easier "active rest" intervals that let your heart rate come down.
How long is a HIIT workout?
Group HIIT workouts range from about 5 to 30 minutes. Add a warm-up and cool-down of 5 to 10 minutes each.
How often should I do HIIT?
Follow your program's day-by-day calendar. You don't have to go all-out every day: on days you feel tired or sore, lower the intensity, choose low-impact options or take an extra rest day.
Is HIIT better than cardio for fat loss?
For fat loss, HIIT and steady cardio work about equally well. The difference is time: HIIT gets similar results in about 40% less training time, and it improves cardio fitness more. Pair it with healthy eating for the best results.
Is HIIT OK for beginners?
Yes. "High intensity" means high for you, so work at your own pace. Start with Spark Fit, our 6-week beginner program, or Scorch Fit for low-impact strength. If you have a health condition, check with your doctor first.
Does HIIT build muscle?
It can. HIIT has been shown to increase lean mass and strength, but dedicated strength training builds more. That's why Group HIIT workouts use bodyweight, weighted and banded movements.
Do I need equipment?
Most workouts need little to no equipment. We have bodyweight-only programs, plus options with dumbbells or resistance bands if you have them.
Research summaries are provided for general information and are not medical advice. Group HIIT is a fitness program, not a medical treatment. If you have a health condition, consult your doctor before starting a new exercise program.
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