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It seems too good to be true: Short, intense bouts of exercise are as good for your muscles and cardiovascular system as lengthy endurance regimens — and may in fact be even more effective. Now researchers at Karolinska Institutet in Sweden have discovered a cellular mechanism behind the surprising endurance and cardio benefits of high-intensity interval training (HIIT).

Their findings, published in the Proceedings of the National Academy of Sciences on November 3, 2015, may also offer clues as to why antioxidants undermine the effects of endurance training.

But how can just a few minutes of high-intensity exercise be so effective?

The researchers examined what happens in muscle cells after HIIT workouts by having 14 active men do three to six sets of 30-second-long all-out sprints on stationary bicycles. Each maximum-exertion session was followed by four minutes of rest. Researchers then took muscle-tissue biopsies from the thighs of the exercisers 24 hours later.

“Our study shows that three minutes of high-intensity exercise breaks down calcium channels in the muscle cells,” says Håkan Westerblad, PhD, professor and principal investigator at Karolinska Institutet’s Department of Physiology and Pharmacology, in a press release. Calcium is critical for muscle function, causing muscles to contract and relax.

During HIIT exercise, the body produces huge amounts of free radicals — highly active molecules that react within the body, helping fight against infections, among other things. The free radicals break down the channels that calcium ions flow through in the muscle cells, allowing calcium to leak out from the internal calcium stores in the cells. These calcium ions then trigger adaptions in the muscle cells, forcing the cells to become more fatigue resistant — and more efficient.

“This causes a lasting change in how the cells handle calcium, and is an excellent signal for adaptation, such as the formation of new mitochondria,” Westerblad explains.

Mitochondria are the tiny power plants within each of our cells that produce energy from the food we eat and the oxygen we breathe. So, they’re essential to our metabolism as well as our vitality and focus. (For more on mitochondria, see “The Care and Feeding of Your Mitochondria.”)

“During the short HIIT bursts, massive production of free radicals occurs and this triggers the changes,” he says. “The increase in free-radical production is much smaller during longer exercise, and therefore longer, low-intensity exercise does not induce the same changes on the calcium channels. Thus, free radicals are essential for triggering the positive adaptations.”

The production of too many free radicals, however, can wreak havoc, causing inflammation, muscle weakness, and eroding of our bodies. Our bodies’ cells have a protective antioxidative system for neutralizing excess free radicals. These free radicals can also be counteracted by antioxidants such as vitamins C and E.

As part of the study, the researchers also investigated what happens when muscles are treated with antioxidant supplements before and after exercise, examining the muscles of mice put on simulated HIIT exercises.

“Our study shows that antioxidants block the effect on the calcium channels,” reports Westerblad.

So exercisers who currently take antioxidant supplements like vitamin C after a workout to counter the buildup of the stress hormone cortisol may be better off not taking them if they want the most bang for their buck from HIIT workouts, according to the findings.

Researchers also found that elite endurance athletes were less affected by the three-minute HIIT workouts because they have already “built up more effective antioxidative systems.” But for recreational athletes, HIIT can be a big boost.

Interested in doing HIIT exercises? Go to “HIIT It” for more information.

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