The Science Behind Hiit Training: Why It’s So Effective

When you want a workout that’s relatively short, challenging and effective, what’s your best option?  HiiT training, of course. As you know, HiiT stands for high-intensity interval training. HiiT is the antithesis of monotonous, boring moderate-intensity exercise. It takes the intensity up a few notches to give you a workout that’s challenging and effective.

HiiT training lets you perform at an intensity you wouldn’t normally be able to sustain for very long and then gives you time to recover so you can do it again. Try a HiiT session and you’ll quickly discover you enjoy it. It’s because it takes you out of your comfort zone in a good way.

Is HiiT training more effective than moderate-intensity cardio? If so, what’s the science behind it?

What Research Shows about High-Intensity Interval Training

Most people do moderate-intensity cardio to improve their aerobic capacity and to burn fat. One study, involving 36 healthy, untrained men, compared steady-state running for 150 minutes a week to interval running for 40 minutes weekly. Another group did strength training. At the end of 3 months, the group that worked out interval style enjoyed greater improvements in aerobic capacity compared to the other two groups. They experienced an average 14% increase in V02 max (a measure of aerobic capacity) versus 7% in the steady-state group and 2-3% in the strength-training group. In addition, the interval training group experienced comparable reductions in blood glucose in response to training. This study suggests that exercise intensity is more important than training volume for improving aerobic capacity and for cardiovascular health.

Although this study didn’t show HiiT training was more beneficial for fat loss, a number of studies do, especially belly fat loss. A study published in the International Journal of Obesity showed HiiT training three times weekly for 15 weeks led to greater reductions in belly fat and total body fat compared to steady-state exercise. High-intensity exercise also improved insulin sensitivity more. In another study, overweight young men who performed HiiT training for 20 minutes three days a week experienced a 17% decrease in belly fat. Pretty impressive for an hour of HiiT training weekly.

Improvements in cardiovascular health and aerobic capacity, fat loss and improved insulin sensitivity – those are some of the things you can expect when you do a high-intensity interval workout. Plus, HiiT training is more time efficient. You can get fitness benefits in half the time compared to steady-state training. That’s all good but why is high-intensity exercise so effective?

 Hormonal Response to HiiT Training

Hormones are the driving force behind changes in body composition. HiiT interval training, due to its intensity, causes a significant increase in catecholamines, hormones activated during periods of stress. These hormones are important because they also mobilize fat stores to be used as fuel. In addition, high-intensity interval training maximizes release of the fat-burning hormone growth hormone.

One study found higher levels of growth hormone in the bloodstream after high-intensity compared to moderate intensity. In fact, research shows there’s a minimal exercise intensity needed to elicit the significant release of growth hormone. In one study, ten minutes of high-intensity exercise consistently increased growth hormone levels in healthy men. The release of these hormones maximizes the benefits of HiiT training. Once you’ve finished your workout, these hormones boost fat burning for hours after you’ve recovered. One of the benefits of HiiT training is the afterburn it creates, the ability to burn more fat during the post-exercise period. In contrast, when you finish a moderate-intensity workout, the afterburn effect is minimal.

When you do long periods of moderate-intensity exercise, an hour or more, your cortisol level rises. High-intensity exercise also boosts cortisol due to the stress of training but the rise is short term. Exercise of longer duration causes a more sustained increase in cortisol. Studies have found endurance athletes that run for long periods of time have more prolonged increases in cortisol. You don’t want that. Cortisol breaks down muscle tissue and increases appetite and cravings for high-carb foods. People who have sustained elevations in cortisol also put on more belly fat.

Improvements in Aerobic and Anaerobic Capacity

One of the benefits of HiiT training is it improves aerobic fitness without causing a sustained rise in your cortisol level and it does it in the shortest time possible. HiiT also improves anaerobic capacity, the point at which your body has to turn to anaerobic metabolism to generate energy. When you exercise at a high intensity, your body is forced to use anaerobic energy pathways to make ATP, your body’s energy currency, because your aerobic system can’t keep up with the demand. As a result, lactic acid builds up in your bloodstream because you can’t remove it quickly enough. That’s why HiiT exercise is so fatiguing and why a rest interval follows each high-intensity interval. The rest intervals help remove some of the lactic acid and restore your body’s pH to normal.

When you do regular high-intensity workouts, your body becomes more efficient at removing lactic acid and you can exercise at a high intensity longer without having to stop. Most experts believe your anaerobic threshold, the intensity with which you can exercise without building up lactic acid, is a better indicator of fitness than aerobic capacity. It’s even a better predictor of how well you’ll perform during aerobic exercise than V02 max. Steady-state exercise improves aerobic capacity but not anaerobic capacity. HiiT training is designed to improve both.

Enjoy the Benefits of High-Intensity Interval Training in Moderation

As you can see, HiiT training offers a variety of health and fitness benefits. With growing evidence that exercise intensity is more important than duration, high-intensity interval training is a no-brainer. Plus, it’s time expedient, perfect for those days when you have minimal time to work out. Don’t overdo HiiT training. You need more time to recover between workouts when you’re exercising at a high intensity. Two to three times a week is enough to get the benefits.

Is HiiT a replacement for moderate-intensity exercise? It doesn’t have to be. If you’re a beginner, start by training at a moderate intensity until you’ve developed a certain level of endurance. Then gradually add some HiiT sessions to your routine. You can even periodize your training by focusing on HiiT training for a week or two and then doing a week of moderate-intensity cardio. This will challenge your body in different ways, avoid plateaus and prevent staleness and boredom.

Don’t forget about the importance of strength training. HiiT training has benefits but building lean body mass isn’t one of them. Keep your workouts balanced.

References:

Med Sci Sports Exerc. 2010;42(10):1951-1958.

J Obes. 2011; 2011: 868305.

Poliquin Group. “Is Aerobic or Anaerobic Training Best for Getting Rid of Belly Fat?”

Int J Obes (Lond). 2008 Apr;32(4):684-91. doi: 10.1038/sj.ijo.0803781. Epub 2008 Jan 15.

Psychoneuroendocrinology. “Elevated Hair Cortisol Concentrations in Endurance Athletes” (2011)

Am J Physiol Endocrinol Metab. 2014 Oct 1;307(7): E539-52. doi: 10.1152/ajpendo.00276.2014. Epub 2014 Aug 5.

J Clin Endocrinol Metab. 1992 Jul;75(1):157-62.

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