A Device to De-Stress Your Workout

AFTER a night that included several beers and not enough sleep, Keith Gillis, a 31-year-old cyclist in Truro, Nova Scotia, set out on a 74-mile road ride with the caveat that he was feeling fatigued. Yet two-thirds of the way through the ride, Mr. Gillis said, he was setting the pace, riding ahead of his cycling partner. “Even though I’d felt tired at the start, I had the energy to lead, and I wasn’t out of breath,” he said.

To what did he attribute his stamina? A flexible mouth guard by a Canadian company called Makkar that he has been biting on while riding since April. When fellow cyclists ask him why he isn’t winded when they are, Mr. Gillis tells them, “because I have my Superman guard in.”

Mr. Gillis is among a small but growing number of athletes wearing what manufacturers like to call “performance mouthpieces” while cycling, running or weight training. One of the newest tools in a performance-enhancement arsenal, these mouthpieces are light, flexible pieces of molded plastic that fit over the teeth — and are only vaguely reminiscent of that retainer from junior high school or the bulky mouth guards worn by football players.

Dentists say these high-end mouth guards can open up the airways, prevent teeth-clenching and align the jaw. Being able to take in more air while exercising has obvious benefits — more oxygen for working muscles — while a relaxed jaw can decrease stress and help an athlete’s body function more efficiently.

“There is research to support improved breathing mechanics and reduced jaw fatigue,” said Fabio Comana, an exercise physiologist with the American Council on Exercise. “Depending on how you look at it, there is some truth to the claims.”

Unlike regular mouth guards, which are....