How HIIT Training Improves Exercise Endurance

At one time, the recommendation for improving exercise endurance was to engage in sustained periods of moderate-intensity exercise. Based on this advice, jogging and cycling became popular years ago. When you do prolonged periods of moderate-intensity exercise, your muscles adapt in a way that makes them more efficient energy producers. With more energy available, your muscles can sustain exercise for a longer period of time before fatiguing to the point that you have to stop.

Adaptations to Endurance Exercise

How do these positive adaptations take place? One adaptation your body makes in response to endurance exercise is to increase the number of mitochondria inside your muscle cells. The formation of new mitochondria is referred to as mitochondrial biogenesis. Mitochondria produce the energy currency that all cells use to carry out the functions needed to sustain life. ATP fuels muscle contraction and is the energy source that keeps your muscles contracting when you do any form of exercise. More mitochondria are synonymous with greater exercise endurance because you have more “energy factories” to make ATP.

So, now we know one of the secrets of developing greater endurance – increase the number of mitochondria inside your muscle cells. Doing this could have benefits that go beyond increasing exercise endurance. For example, research shows that increasing the number of mitochondria in brain cells may enhance memory and learning. Plus, studies now suggest that damage to mitochondria, from exposure to free radicals, may play a role in aging.

HIIT Training vs. Endurance Exercise

Interestingly, HIIT (high-intensity interval training) boosts mitochondrial biogenesis similarly to endurance exercise, even though HiiT training workouts are shorter. Until recently, researchers weren’t sure how HIIT workouts, being brief, could offer this benefit. Researchers at the Karolinska Institute in Stockholm believe they have the answer.

The researchers discovered that as little as three minutes of high-intensity exercise damages a muscle cell’s calcium channels. When calcium moves through these channels, it tells muscle fibers to contract. When calcium channels are damaged by vigorous exercise, the muscle cell is forced to adapt. In this case, they make new mitochondria. When muscle cells gain new mitochondria, they can make more ATP to fuel exercise and you’re rewarded with greater endurance.

The question is why does HIIT training damage a muscle cell’s calcium channels? High-intensity exercise places stress on the body and on cells. In response, cells, including muscle cells, get exposed to more damaging free radicals. The free radicals break down the calcium channels and force muscle cells to build more ATP-producing mitochondria.

As you can see, free radicals, formed through the stress of intense exercise, is damaging to muscle cells, but, ultimately, they benefit from them. Without these free radicals, muscle cells would have no stimulus to make more energy-producing mitochondria. It’s an example of how a controlled amount of stress can actually have benefits.

The Problem with Flooding Your Body with Antioxidants

You may have heard that consuming large amounts of antioxidants prior to a workout interferes with adaptations to exercise. This explains why. If you take an antioxidant supplement before a HiiT Training workout, it reduces free radical formation so that the muscle cell’s calcium channels aren’t damaged. In turn, there’s no stimulus for the cells to make new mitochondria and you don’t get the benefits of increased endurance.

Once you become a “seasoned athlete,” you don’t produce as many free radicals in response to exercise. The researchers in this study know this because they measured the number of free radicals produced by untrained and elite endurance athletes. When they biopsied the thighs of each, they found the elite endurance athletes didn’t produce as many free radicals. That’s because they had already adapted to endurance exercise and built up a stronger antioxidant defense system. They also found that vitamin C and E, both antioxidant vitamins, blocked the free radical effect on muscle calcium channels.

What Does This Mean?

It looks like high-intensity interval training, even when short in duration, leads to adaptations similar to what you get from long periods of endurance exercise. It also looks like you can block these adaptations if you flood your body with antioxidants prior to exercise. This means you don’t want to take antioxidant supplements or antioxidant vitamins, including vitamins C, and E, prior to a workout. Other supplements that might block this effect include green tea extract, alpha-lipoic acid, coenzyme Q10, beta-carotene, selenium, and acetyl-L-carnitine. However, you should still get antioxidants from food sources, like fruits and vegetables.

The Role of Mitochondria in Health and Aging

Mitochondria may be important for healthy aging as well. Every cell in your body has mitochondria, with the exception of red blood cells. People who are sedentary or who have type 2-diabetes tend to have fewer functional mitochondria. As you might expect, having fewer mitochondria to make ATP gives you less exercise endurance and less stamina in daily life. For example, you might fatigue easily due to your lack of conditioning. Plus, research shows loss of functional mitochondria in the heart muscle may contribute to heart disease. Not surprisingly, your heart is highly dependent on having enough ATP.

Although aging and chronic health problems like type 2 diabetes can reduce the number of functional mitochondria you have, exercise is a stimulus for making new ones. Yes, you need resistance training but it’s not the type of exercise that forces your body to make new mitochondria. Endurance exercise and high-intensity interval training are the forms of exercise that boost mitochondrial biogenesis and helps you retain more functional mitochondria.

The Bottom Line

HIIT training provides similar benefits to endurance exercise by stimulating the formation of new energy-making mitochondria. The advantage is you can get the benefits without doing hours of moderate-intensity cardio each week. As the research shows, it’s best not to load up on antioxidants before a workout, by taking supplements, since this interferes with calcium channel damage, a signal that tells your muscle cells to build more mitochondria. Plus, in terms of antioxidants, it’s best to get them naturally from food sources and not supplements.

References:

IFL Science. “Scientists Figure Out Why High-Intensity Training Can Be As Effective As Longer Exercise”

PNAS. December 15, 2015. Vol. 112, No. 50.

HIIT vs Continuous Endurance Training: Battle of the Aerobic Titans. Micah Zuhl, Ph.D. and Len Kravitz, Ph.D.

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