Last Updated on December 5, 2024
High Intensity Interval Training (HIIT) and Sprint Interval Training (SIT) are often promoted as a superior way to do cardio, with greater improvements in fitness, along with better weight loss and body composition when compared to moderate intensity steady state conditioning.
However, most of the research on this topic is limited to untrained individuals and short-term studies lasting 6- to 8 weeks, making it difficult to know whether or not HIIT really lives up to the hype.
In this article we’ll discuss what HIIT and SIT are, its potential for improving fitness relative to other forms of cardio, and how-to put it into your program if you want to experiment with it. Let’s start out with some definitions.
Podcast Discussion on HIIT
Drs. Baraki and Feigenbaum took a deep dive into High Intensity Interval Training on Episode 318 of the Barbell Medicine Podcast. Listen below to learn more:
What is HIIT?
High-intensity interval training (HIIT) refers to repeated bouts of relatively hard exercise, alternating with rest periods. It is typically contrasted from Moderate-intensity Continuous Training (MICT), where a person exercises at a longer, moderate intensity without breaks or rest periods.
High intensity interval training generally involves heart rate targets above 80% of maximum heart rate. This is sometimes referred to as exercise in zones 3, 4, and 5 in the “5-zone” model of exercise, where each zone reflects an increasing intensity (see Fig. 1).
Sprint Interval Training (SIT) is a specific type of HIIT referring to exercise intensity higher than the person’s VO2max, which typically corresponds to a heart rate greater than 90% of maximum, and zone 5 or greater. VO2max describes a person’s maximum ability to take in, transport, and use oxygen during physical exertion. This is sometimes known as a person’s maximum “aerobic capacity”.
Much like in strength training, where lifting a weight at 50% of your 1-repetition maximum strength leads to different adaptations compared with 70% or 95% of 1-repetition maximum strength, each of these intensity zones tends to produce different types of adaptations, regardless if the exercise is performed continuously or using intervals.
There is no one “best” zone for exercise, particularly for health or general fitness. Instead, each zone offers a particular type of benefit, at a particular “cost” in terms of fatigue. If you would like to determine your maximum heart rate and these intensity zones for yourself, see our online heart rate zone calculator.
What Are The Health Benefits of Interval Training?
Both high-intensity interval training and moderate intensity continuous training can improve VO2max similarly, depending on both how it is done and how much of it is performed. When using interval training, intervals longer than 2 minutes appear more effective than shorter intervals at increasing VO2max. We also know that for every incremental improvement in cardiorespiratory fitness, a person will reduce their risk of premature death from any cause, so either method of exercise (o can be an effective strategy to improve health and longevity.
As it relates to body composition, exercise alone — including HIIT — has very little direct impact on weight loss. For example, there is no significant difference between high-intensity interval training and moderate-intensity continuous training for fat loss.
However, there are numerous indirect benefits of exercise on body composition and body fat levels. For example, exercise improves the body’s ability to signal and sense feelings of fullness and satisfaction from meals. It also promotes building muscle mass, prevents the loss of muscle mass during weight loss, and helps fight against a slowing metabolism during weight loss too.
How to Program HIIT Workout
As mentioned above, different intensity zones deliver different types of adaptations, with different costs. It is not feasible or realistic to perform maximum intensity interval workouts in every conditioning workout over the long term, much like how it is not feasible or realistic to lift absolute maximum weights in every strength training workout over the long-term. Training that is most productive and sustainable involves a variety of intensities.
In general, for individuals dedicating a significant amount of weekly exercise time to aerobic activity, it is reasonable for about 80% of training to be of moderate intensity (zones 1-2), and about 20% reserved for higher intensity work (HIIT or SIT).
These proportions are not a strict rule, however; for individuals who have much less time available for exercise, using a greater proportion of higher intensity exercise may be useful. Alternatively, high-level endurance athletes may perform an even greater proportion of lower and moderate-intensity work compared with the amount of true high-intensity exercise, depending on their specific performance goals.
Interval training is most effective using different levels of effort and intensity, depending on the duration of effort. Here are a few sample interval training strategies written based on running paces or heart rate targets, but these can be used across different modalities like running, cycling, rowing, or other activities, using similar effort targets.
Sample HIIT Workouts:
Zones 3 and 4 HIIT
Workout #1
- 4 to 6 rounds of: 30 seconds on at 600-800 m running pace (or a speed sustainable in the range of ~90-150 seconds), 4 min off / easy effort
Workout #2
- 8 to 10 rounds of: 1-minute on at 1 mile-5 km running pace (or a speed sustainable in the range of 6-25 minutes), 1 minute off
Workout #3
- 3 to 5 rounds of: 5 minutes on at zone 4 heart rate (85-95% max), 3 min rest
Zone 5+ SIT
Workout #1
- 4 to 6 rounds of: 30-seconds on at 400-600 m running pace (or a speed sustainable in the range of ~60-90 seconds), 4 min off / easy effort
Workout #2
- 4 to 8 rounds of: 15- to 30-seconds hill sprint, 3+ -minute rest periods (be conservative on pace and hill grade at first)
For a person just getting started who does not know these paces, starting out “easier” to get a feel for a manageable level of effort is wise, before pushing to harder efforts later on.
Safety Considerations
A common misconception is that high-intensity interval training necessarily requires you to go “all out” or “as hard as you can”. This is not accurate, and actually likely has downsides. The goal of training is to provide a stimulus to improve fitness, while also avoiding excessive fatigue and injury risk. Training at very high intensities under conditions of fatigue likely increases the risk of injury, and decreases the adaptations we get from exercise. Additionally, it is not actually possible to perform repeated “all out” efforts, truly at maximal effort. Attempting this will reveal that by the second interval, performance begins to decline dramatically.
Sprint interval training should be used sparingly, especially for beginners who are not yet adapted to this level of intense effort. Gradually building up is essential in these situations, starting with low intensity exercise before advancing to sprints. For example, the following progression can get from beginner stage to running sprints:
- Low- to moderate-intensity jogging
- Repeated moderate-intensity runs up a hill
- Higher-intensity hill sprints
- Moderate-intensity running intervals on flat ground
- High-intensity sprint intervals on flat ground
While most forms of exercise – including HIIT and SIT- are safe, doing too much, too soon can increase risk of injury quite a bit.
Conclusion
In summary, HIIT is safe and effective for improving aerobic capacity, VO2max, and improving health, but should not be the only form of cardio training a person uses. The intensity and volume should be programmed specifically for individual goals and current fitness levels rather than always pursuing maximum effort.
For example, short-duration HIIT (15 minutes including warm-up) performed 3 times weekly for 6 weeks can meaningfully improve cardiorespiratory fitness. In contrast, elite endurance athletes typically perform very high weekly training volumes, and distribute a high proportion of moderate-intensity exercise, with a much lower proportion of high-intensity work.
Adequate recovery and gradual progression are important for preventing burnout and injury while maximizing adaptations to be gained from this form of exercise.
If you liked this article and want to learn more about High Intensity Interval Training, check out Episode 318 of our podcast where we take a deep dive into the subject. We also publish a more in-depth summary, complete with academic citations, expert commentary from Drs. Baraki and Feigenbaum, ad-free listening, early access to episodes, and more exclusive content for our Barbell Medicine Plus members. Sign up today to support what we do and take your results to the next level!
