Any workout fiends feeling puffed up should stay out of one of those "how-many-calories-did-you-burn-yesterday" conversations with Grete Eliassen.

Eliassen is a 27-year-old freestyle skier. She has the sort of sculpted, 5-foot-10, 140-pound physique that looks like it burns 7,000 calories a day. Some days, according to her Fitbit Flex, that's exactly what happens.

On Nov. 20, the bronze medalist at last year's world championships in slopestyle, an event that requires skiers to perform highflying acrobatics as they zip down a hill, burned 7,601 calories during her training. Like plenty of people these days—everyone from hard-core triathletes to new mothers looking to drop baby weight—Eliassen quickly got hooked on the little gadget on her wrist that was tracking her movement.

"I kept telling my teammates, 'Girls, I need to eat more. I just burned 5,000-plus calories,' " Eliassen said.

With the Olympic cauldron set to burn in Sochi, Russia, in February, and the wearable technology craze steaming toward a boil, The Wall Street Journal decided to hold a little experiment among a few Olympic hopefuls.