HIIT: What's the HI in IT? - Breaking Muscle

In the fitness community there are always things that come and go. The key to success is learning the difference between those things that are trends or fads. Trends are usually longer lasting and usually have some validity within the research community.

Fads are those things that lose popularity just as fast as they gain popularity. One of the things that I believe is here to stay is high-intensity interval training (HIIT). I want to propose a few reasons why this is such a hot topic item and why it has made it on the top five fitness trend list for the last 10 years.

Say Hi to HIIT

The play on words in the heading is a suggestion that the “HI” in HIIT training is exactly that. This type of training modality uses a very high-intensity training regimen, which is what the acronym actually means.This can also mean that many of the processes that are involved in this type of cardio regimen are all increased, and moving at a higher expression of their purposes.

The body is always trying to reach a point of homeostasis and many hormones and metabolic processes are trying to return to their normal baseline levels. This causes an inefficiency for the body to use fuel for energy, therefore, causing an increase in metabolic functions.

Photography by Jeffrey Perez of Oahu, Hawaii

One study that was conducted using the Wingate protocol saw a rise in cortisol, growth hormone, and catecholamine. Many of these hormones by definition are catabolic or breakdown hormones, but in these instances, they are used to break down substrates for the body to use for energy.

Since the pure nature of high-intensity exercise uses the immediate and intermediate energy sources fairly quickly, the rise in these hormones increases in order to supply the body with energy it needs to fuel the grueling exercise intensity. Growth hormone increases as lactate levels go up.

These increases in growth hormone levels have been shown to increase fatty acid transport. Fatty acids can be used for energy by a process more in-depth than this article, but the take home is this is another way the body uses substrates the body breaks down to convert into energy for a high-intensity workout.

Catecholamines are hormones the body increases to increase metabolic processes to produce energy, drive lipolysis, and release fat from subcutaneous and intramuscular stores. These are made up of norepinephrine and epinephrine, released in response to “flight or fight,” which increases in both trained and untrained individuals up to 20 minutes after training has been concluded.

This isn’t to say the body has fully recovered because the research suggests that the carryover for the benefits of HIIT is up to 39 hours after one session of training. A few benefits of HIIT can include decreases in abdominal subcutaneous fat, increases insulin sensitivity, and increases in VO2 max or cardiorespiratory benefits.

This is not an exhaustive list as there can be some increases in lean body mass leading to better muscle retention and body composition.

HIIT Will Be Around for Awhile

High-intensity interval training is here to stay. There is a reason why it has gained so much popularity. Although all the mechanisms of how HIIT works so tremendously well isn’t fully understood on every level of biochemistry, what we do understand of basic physiology leads us to believe that HIIT will be around on the fitness trend list for many more years to come.