There are mixed opinions about HIIT (High Intensity Interval Training) and LISS (Low Intensity Steady State) cardio - how do you know when to incorporate HIIT or LISS into your program? Initially, it's important to understand the concept of exercise intensity. Both high and low intensity are defined by your heart rate and how many times your heart beats per minute (bpm); the faster your heart beats, the higher the intensity.
Exercise intensity is divided into 5 zones that represent a percentage of your maximum heart rate and links to a specific training benefit. You can estimate your maximum by subtracting your age from 220. For example, if you’re 30 years old your max heart rate is around 190 beats per minute (220 – 30) and each zone is a percentage will be based around 190 bpm. As seen in Figure 1, your max heart rate naturally decreases with age.
HIIT vs. LISS
HIIT consists of short bursts of effort that can lead you to your max heart rate, which is followed by slower recovery periods to get your heart rate back down. LISS (Low Intensity Steady State) on the other hand, is a constant pace at 50 - 70 percent of your maximum heart rate. Generally, HIIT workouts utilize maximum effort, hence sustained for shorter durations. However, LISS workouts can be sustained consistantly for long durations because they don’t take a big toll on the body.
The advantage of HIIT is that your average heart rate is higher compared to LISS, which means you burn more calories. HIIT burns 40 - 60 percent more calories per minute than LISS. Figure 2 shows an example of fluctuations in the heart rate during a 20 minute HIIT workout compared to a 30 minute LISS workout. The HITT workout includes 5 intervals of 1 minute on and 2 minutes off, compared to 30 minutes of low intensity steady state cardio. In this case, both workouts burn 300 calories (20 minutes equals to 15 calories per minute. 300 calories and 30 minutes multiplied by 10 calories per minute equals to 300 calories).
Burning carbs vs. burning fat
While it’s true that HIIT burns more calories than LISS, it’s important to know that HIIT exercises burn more calories from carbs, while LISS exercises burn more calories from fat. HIIT only burns 90 calories from fat (30 percent of 300 calories), however LISS burns over 135 calories from fat (45 percent of 300 calories). At around 70 percent of your max heart rate, LISS cardio hits the sweet spot for burning the highest amount of fat per minute. Since LISS exercises typically last over 30 minutes, it has more fat-burning potential than HIIT. As a result, LISS exercises would be the best option if you're measuring calories burned from fat.
What About EPOC?
Proponents of HIIT argue that you burn more calories after high intensity training. This is due to EPOC (Excess Post-Exercise Oxygen Consumption). Also called the after-burn effect. Basically, you can’t get enough oxygen to replace what you use during intense exercise. And that creates an oxygen deficit. With an oxygen deficit, your respiratory rate increases for a while after exercise, which burns calories. Since EPOC is greater after HIIT than LISS, the thought is that you burn more calories while recovering.
However, LISS consumes more oxygen during exercise compared to HIIT. So the 24-hour oxygen consumption (total green area) is pretty much the same for HIIT vs LISS. Indeed multiple studies show that the difference in 24-hour energy expenditure between HIIT and LISS is not statistically significant1,2,3,4. That means greater EPOC doesn’t necessarily mean more fat burning.
Which One Should You Do?
At the end of the day, the HIIT vs. LISS depends on your goal well as your personal preference. Often, the best solution is a combination of both HIIT and LISS. It’s important to find the form of exercise you actually enjoy so you’ll do it consistently. For maximum fat loss, you should focus on LISS. While you don’t really need HIIT to maximize fat loss, you can include it if you like more challenging workouts or to mix things up.
Aim for at least 150 minutes per week
Split it up into 3 to 6 LISS sessions, 30 to 60 minutes each
Target around 70% of your max HR
Examples: Fast walking, slow jogging, cycling
Limit HIIT to 0 to 2 sessions per week
For optimal fitness, a mix of HIIT and LISS works best. Before doing HIIT workouts, make sure you’re healthy enough for max effort bouts. And include a 5-10 minute warm-up and cool down at 70% max HR.
Aim for at least 150 minutes per week
Split it up into 2 to 3 HIIT sessions, 10 to 20 minutes each
Target 90-100% max HR during intervals, with 30 sec to 3 min recovery periods
Examples: sprint/walk, row/rest, circuit training
Do LISS on days you don’t do HIIT