HIIT Science Revealed: Unpacking the Research Behind High-Intensity Interval Training’s Powerful Health Outcomes

Every year since 2007, the American College of Sports Medicine (ACSM) has published its “Worldwide Survey of Fitness Trends” report, a comprehensive review based on responses from thousands of fitness and wellness professionals from across the globe. 2023 marked the 10^th year that High-Intensity Interval Training (HIIT), a type of exercise that involves short bursts of intense exercise followed by periods of rest or low-intensity exercise, made it in the top 10 trends.

However, the history of HIIT dates back much further, to the early 20^th Century, when Swedish coach Gösta Holmér developed a training method called “fartlek,” which translates to “speed play” in English. This method combined fast and slow running intervals, serving as the foundation for modern HIIT.

In the 1930s, German physician Hans Reindell and coach Woldemar Gerschler formulated a systematic interval training approach involving alternating high-intensity and recovery periods to improve cardiovascular endurance. However, it wasn’t until the 1970s that HIIT truly gained momentum, with the work of athletic coach Bill Bowerman and physiologist Per-Olof Åstrand. Bowerman, co-founder of Nike, played a significant role in popularising HIIT in the United States, while Åstrand’s research on the physiological effects of interval training deepened our scientific understanding of the approach. Over the years, HIIT has evolved into a versatile and highly effective training method, embraced by athletes, fitness enthusiasts, and health-conscious individuals alike.

What makes HIIT so effective?

There are several mechanisms which explain why High-Intensity Interval Training (HIIT) is effective. These include:

1. Increased aerobic and anaerobic capacity: HIIT pushes the body’s energy systems to their limits, stimulating adaptations in both aerobic and anaerobic metabolism.
2. Enhanced oxygen utilisation: HIIT improves the efficiency of the body’s ability to take up and use oxygen, increasing VO2 max and overall cardiorespiratory fitness.
3. Elevated fat oxidation: HIIT has been shown to increase the body’s ability to burn fat during and after exercise.
4. Improved insulin sensitivity: Regular HIIT workouts can help enhance insulin sensitivity, reducing the risk of type 2 diabetes and other metabolic disorders.
5. Increased production of growth hormones and other beneficial metabolic responses: HIIT stimulates the release of various hormones, such as human growth hormone (HGH) and testosterone, which can aid in muscle growth, recovery, and overall well-being.

In summary, the effectiveness of HIIT lies in its ability to trigger a range of physiological adaptations that contribute to improved cardiovascular fitness, metabolic health, and overall performance.

HIIT, VO2 max. and longevity

A measure called VO2 max. represents the body’s ability to take up and use oxygen. VO2 max. also reflects overall physical health, as it represents the outcome of many critical physiological systems working together. Such is the importance of VO2 max., some clinicians argue that it should be considered a vital sign alongside measures such as blood pressure. The more you can increase your VO2 max.  while you are younger, and the better you can maintain it as you age, the longer and healthier your life is likely to be.

A recent study described how VO2 max. values can predict when someone will likely cease to be independent in later life. Someone in the 10th percentile for VO2 max. could expect to begin relying on other people due to reduced physical capacity in their early 70s. In contrast, someone in the 90th percentile could expect to remain independent for the rest of their life.

By being more physically active and by increasing our VO2 max, it’s likely that we can look forward to much higher levels of health and vitality for longer. While traditional moderate-intensity training can also improve VO2 max., HIIT appears to result in larger improvements in less time (1)

Could HIIT training benefit your brain, too?

A recent study (2) indicates that just six minutes of HIIT could substantially increase levels of a molecule which supports memory & brain health. A short but intense bout of cycling was shown to increase the production of a specialised protein: Brain-Derived Neurotrophic Factor (BDNF). Animal studies indicate that increasing BDNF can improve memory, learning and cognitive performance. BDNF may also have a neuroprotective effect and protect the brain from age-related cognitive decline. The researchers compared four approaches to increasing BDNF:

• Fasting for 20 hours,
• Light exercise (90-minute low-intensity cycling),
• HIIT exercise (a six-minute bout of vigorous cycling),
• Combined fasting and exercise.

The HIIT session, featuring six 40-second intervals at 100% VO2 max, increased BDNF 4- to 5-fold more than light exercise. Interestingly, fasting did not change BDNF concentration, despite previous research indicating this may be possible.

Randomised controlled trials are needed to conclusively demonstrate that HIIT can promote cognitive or brain changes more than other modalities. However, the results are promising, and it’s another good indication that HIIT training has benefits beyond physical fitness.

Could HIIT temporarily offset some of the metabolic disruptions from not sleeping enough?

Sleep loss could increase your risk of developing Type II diabetes as much as physical inactivity. But what can we do about it? Ideally, we would all sleep for at least 7 hours per night, every night, but a new study indicates that, for a short time, we may be able to offset some negative impacts of sleep loss with brief, high-intensity workouts.

The evidence is clear that sleep loss is associated with disrupted:

• Glucose tolerance
• Mitochondrial function
• Protein synthesis
• Biological rhythms

However, a recent study (3) demonstrated that exercise could temporarily offset the disruptions associated with sleeping 4 hours per night for five nights. Participants carried out the following HIIT (High-Intensity Interval Training) session during the sleep restriction period:

• 3-minute warm up at 60 Watts
• 10 x 60-second intervals @ 90% of their peak power output.
• 75-seconds of active recovery at 60 Watts between each interval

I encourage you to do what you can to ensure that you get adequate sleep, but if your sleep is shortened for some reason, this simple exercise protocol may offset some adverse health outcomes.

How can you get started?

Providing you don’t have any underlying health conditions to hold you back, here are three considerations for planning HIIT sessions.

Work-Recovery Ratio

Each ‘high-intensity interval’ may last from five seconds to eight minutes, performed at 80% to 95% of maximum heart rate. Recovery periods are usually at 40-50% of maximum heart rate. For example, you could begin with four to six 30-second sprints, each separated by 4-minutes of recovery. However, benefits are possible with various work-recovery ratios, so feel free to experiment.

Session Duration

A typical HIIT session lasts 20 to 60 minutes, but don’t let lack of time hold you back. In one study, 10-minute sessions featuring a series of 10-20 second all-out sprints improved insulin sensitivity by 28% and VO2 max. by 12-15% (4).

Frequency

Just three short HIIT sessions per week for two weeks can improve both VO2 max. and energy metabolism (5). You could consider doing a few HIIT sessions back-to-back during time-pressed weeks. However, aim for distribution of approximately eight low-intensity sessions for every two high-intensity sessions (6) if you were to average over a month.

You can use a wearable device with a heart rate monitor to record how much load and strain you’re accumulating to avoid overloading on intense training. Again, provided you don’t have any underlying health conditions, you could try integrating a few HIIT sessions into your week. The evidence indicates that HIIT could be an effective way to improve your health, longevity and performance.

If you’re interested in finding out more about whether you can do too much HIIT, you’ll enjoy this blog.

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Disclaimer: The information and exercise recommendations provided on this blog are for informational and educational purposes only and are not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare professional before beginning any exercise program, especially if you have any pre-existing medical conditions, are pregnant, or are unsure about your current fitness level. The author and publisher of this blog are not responsible for any injuries or adverse effects that may occur as a result of following the exercise recommendations provided herein. Proceed at your own risk and always listen to your body, modifying or discontinuing any exercise that causes pain or discomfort.

References

1) Milanović, Z., Sporiš, G., Weston, M. Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials. Sports Medicine 2015; 45(10): 1469-1481;

2) Gibbons TD, Cotter JD, Ainslie PN, Abraham WC, Mockett BG, Campbell HA, Jones EMW, Jenkins EJ, Thomas KN. Fasting for 20 h does not affect exercise-induced increases in circulating BDNF in humans. J Physiol. 2023 Jan 11.

3) Saner NJ, Lee MJC, Kuang J, Pitchford NW, Roach GD, Garnham A, et al. Exercise mitigates sleep-loss-induced changes in glucose tolerance, mitochondrial function, sarcoplasmic protein synthesis, and diurnal rhythms. Mol Metab [Internet]. 2021;43(October 2020):101110.

4) Metcalfe, R.S., Babraj, J.A., Fawkner, S.G. Vollaard, N.B.J. Towards the minimal amount of exercise for improving metabolic health: beneficial effects of reduced-exertion high-intensity interval training. European Journal of Applied Physiology 2012; 112(7): 2767-2775.

5) Gibala, M.J. & McGee, S.L. (2008) Metabolic adaptations to short-term high-intensity interval training: a little pain for a lot of gain? Exercise and Sport Sciences Reviews 36(2)p. 58-63

6) Seiler, K.S. & Kjerland, G.Ø. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an “optimal” distribution? 2006; 16(1):49-56.