Ryan Kremsater hears a lot of trash talk for someone brave enough to fly down a mountain bike trail while balancing on one wheel. “It’s hard to get acceptance from other sports because unicycling looks weird,” he says. “I’ve gotten used to the ‘You’re missing a wheel’ jokes.”

Unicycling is more commonly associated with circus or street performers. But, Mr. Kremsater, 26, is an athlete pushing the boundaries of the niche sport of mountain unicycling.