Health Benefits of High Intensity Interval Training (HIIT)

We’re all wanting to do more in less time. And exercise is no different. We often use a lack of time as a reason not to exercise. Or try to figure out how to squeeze a full workout in between other obligations while still getting the benefits. But what if you could exercise less and get more from it? It may sound too good to be true, but the answer may be in high intensity interval training (HIIT).

What is high intensity interval training?

HIIT consists of short bursts of high intensity exercise interspersed with  active rest. This is in contrast to doing an exercise session of continuous activity at a moderate, or easier, level. The intense intervals can range from 30 seconds to four minutes at 80%-90% of your maximal intensity. So it’s a pretty high intensity. The active rest is usually done at 50%-60% intensity for one to three minutes. A popular form of HIIT is the 4×4 where you do four intervals of four minutes each.

The popularity of HIIT has exploded in recent years. You can find it in fitness centres, YMCAs and community programs. A 2020 survey of more than 3000 health professionals listed it as the second most popular fitness trend. And it’s been in the top five since 2014. It’s also one of the few fitness trends backed by science.

However, HIIT is not new. Athletes have been practicing HIIT for decades as it allows them to train at, or above, ‘race pace’ in short bursts. These short, intense intervals build up their fitness and speed for when they race or compete.

The Health Benefits of High Intensity Interval Training

Over the years, researchers have compared HIIT to continuous exercise training. These studies have generally used a small number of people (20 to 50) for a short period of time (up to twelve weeks). The result is HIIT may be superior to continuous exercise for improving fitness. However, some studies have found no difference between the different formats.

A key consideration when comparing HIIT with continuous exercise is whether the same amount of calories is burned. The higher intensity the exercise, the more calories you expend per given time. So exercising at 80% or more, as in HIIT, will burn more calories per minute than exercising at a lower intensity. The caveat is we cannot sustain exercise at very high intensities for very long,

Where there seems to be an advantage for HIIT is when taking into account calories burned after the exercise has stopped. Referred to as excess post-exercise oxygen consumption, HIIT raises the body’s metabolic rate to a greater extent than less intense exercise. And this higher rate continues following the HIIT session resulting in more calories burned.

Whether this translates into differences in weight loss has been of great interest to many. A summary of studies found no difference between HIIT and continuous exercise across a range of body weight and fat measures. However, a more recent study reported HIIT to lead to greater reductions in fat mass (with no change in weight). So while it’s unclear if a definitive advantage for HIIT exists, it’s clear that similar benefits can occur in less time.

However, if weight loss is your goal, exercise isn’t all that effective compared to changes in diet. That being said, there’s more to exercise than burning calories. HIIT seems to be as good, or better than continuous exercise in reducing your chances for type 2 diabetes as it may be better at controlling blood sugar. And it can lead to similar improvements in heart function to continuous exercise.

Tips and Considerations Before Starting HIIT

Before starting HIIT consider these few tips:

1. If exercise is new to you, begin with a few weeks to months of continuous exercise before starting HIIT. HIIT puts more stress on the body and having a foundation of continuous exercise will help prevent injury.
2. Make sure your to do an active warm-up of at least 10 minutes before you start your HIIT session. And after, an active cool down.
3. With any exercise, there is a greater (albeit small) risk for a heart event than at rest. In addition, the higher exercise intensity the greater the risk. Although there is some indication that HIIT can be beneficial for patients with heart disease. If you have heart disease or over 40 years old, then see a doctor before starting HIIT.
4. As HIIT is at a higher intensity, there is a greater risk for injury and people may experience more ‘pain’ and muscle soreness. It’s important to listen to your body for these signals and take rest days (or continuous exercise days) in between HIIT workouts.
5. If you participate in endurance events such as  running, cycling or triathlons, then it’s essential to continue to do lower intensity continuous exercise (even if you add HIIT) to maintain your endurance capacity.
6. Because of the higher intensity, HIIT may not be comfortable for all. A summary of 33 studies found HIIT was less pleasant than continuous exercise during the session. But people reported higher enjoyment from HIIT after the session.

While many questions remain, including HIIT into your exercise regimen alongside continuous exercise gives you the best of both worlds, while adding variety to your routine.

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This post was originally published on August 30, 2017 and updated on September 2, 2020.