Kristin Borda, who is the program lead for a Little Bellas chapter in New Jersey, witnessed how concerns about what their peers might think can melt on the bike trail on her group’s first outing. “There’s a super steep dirt hill on the trails we ride,” she explained, and many of the girls were wary. But then a couple of them asked if they could try it.

The first girl didn’t make it to the top of the hill. That spurred another rider to try. “Before you know it, everyone is saying ‘I want to try next.’ I mean, they’re not making it up this hill. We spent 30 minutes, and they were not making it, but they would not give up. They wanted it so badly,” she said. And no one seemed to care how they looked when, stalling out mid-climb, they tumbled over sideways.

Concerns about beauty standards can likewise lift the moment a mud puddle turns a girl riding a bike into a Jackson Pollock painting. “On cold, rainy days we have mud competitions, so it takes the girls’ minds off of ‘Oh, it’s 35 degrees and raining.’ It’s kind of the opposite of gender norms for girls,” said Ms. Davison.

Dr. Whittington, who takes young women on canoe trips as part of her research, adds: “I love to watch them transition from caring what they look like to not caring. I love how it challenges their femininity and the social constructs they live in.” By day five, when the girls need to portage over land, they happily embrace the challenge. “When someone asks if they want help they’re like, nope, we’re all set, we’ve been carrying these for several days.”

Of course, there’s mud on soccer fields, and you can learn stick-to-itiveness from basketball drills or batting practice. But mountain biking doesn’t come laden with the politics and expectations of team sports. “No one sits on a bench,” said Ms. Davison, “and no one gets stuck playing just a single position.”

Even better: Parents are often left sitting in cars at the trailhead. “Parents are parents,” she says, and every now and then one gets aggressive about his or her child’s performance. But as soon as the kids ride off and into the woods, they’re free to recreate on their own terms.

What they do, once they get into those woods, can be pretty empowering. “There’s a certain work ethic you have to have to be a mountain biker,” said Annika Peacock, who is now 15. “If there’s a section of the trail that’s really hard for me, I’ll go try it five more times. I say to myself, yes, yes, yes, I can do this.” And then the next day? “I go back and do it again.” She now competes against other teens as part of the National Interscholastic Cycling Association, an organization working to bring mountain biking programs to high schools throughout the United States.

Annika’s mother says of her daughter, “She’s this petite little bundle of smiles, but she has this self-talk inside her that says ‘I can do this’, and she will ride and re-ride something until she gets it. There’s this resilience she’s trained in herself. She’s fierce.”