Champion distance runner Lauren Fleshman still remembers the first time she lost a race to a boy. She was in middle school, and had developed a reputation as the fastest overall runner, the one who consistently won the mile. Until one day, she wasn't fastest.

"When I first got beat by one of my male peers ... it was because he hit puberty and kind of skyrocketed his performance in the mile in a very short period of time," she says. "It was very disorienting to find out that puberty was going to create two different paths for my male peers and my female peers, and that I was on the one that I wasn't so sure I wanted to be on."

Growing up in what she calls the "girl power revolution of the '90s," Fleshman had been led to believe that she could do anything that her male peers could do. But as an athlete, puberty hit hard. She describes getting her period as an impediment, an "added burden that my male peers didn't have to deal with." The development of breasts and hips, she says, felt "scary, like they threatened the future that I wanted in sport."

Nevertheless, Fleshman went on to have a very successful running career, breaking the American junior record in the 5,000 meters race the first time she ran it, which qualified her for the Olympic trials. She was a five-time NCAA champion at Stanford University, and later, as a professional athlete, she won two national championships.

But, along the way, she noticed a surprising number of her female teammates leaving the sport. Many who did stick with it developed eating disorders or other physical or mental health problems. Fleshman says too many coaches seemed to assume — falsely — that what worked for male bodies would also benefit female bodies.

"The male body, between 18 and 22, is getting more juice out of every squeeze when it comes to training. Their hormonal profile is such that their recovery time is quicker," she says. Meanwhile, Fleshman notes, improvement times for female runners tend to slow down between those same ages.

"That's where a lot of tension is created for female athletes around their body," Fleshman says. "There's a basic level of education that coaches need to get, not just in physiology, but also in understanding socially what they're doing, culturally, the environment they're creating, that is working against their goals of having a healthy, consistent team that performs at their best."

Fleshman became a coach and is now an activist working to promote equity in sports by recognizing the differences in male and female bodies. Her new book, Good for a Girl: A Woman Running in a Man's World, is a memoir and a critique of how the sports world treats female athletes.

Interview highlights

On why she loves running

I loved running the way a lot of little children do, when they would just burst into run naturally. It felt like flying. It felt like freedom. ... I joined [the team] in high school, and by then what it meant to me was belonging, exploring. We would take off on these runs as a group into the foothills around our town and get to see vistas of my town from a new perspective, get to explore different neighborhoods. My world got a lot bigger through the sport of running. And I also loved that when you run alongside somebody, you can have a more vulnerable, honest conversation than you can when you're sitting across the table from one another. There's something that just opens you up with the movement. And so I just developed these deep bonds and also these deeper understandings of myself. It just felt like a natural fit.

On how menstruation is often invisible and thought of negatively

When I got my period, it was later than most of my peers, it was around age 17 and I didn't want it. ... It felt like something I had to navigate alone, and the effects that it would have on my mood or my body composition, bloating, all those things felt like this roller coaster that I had to navigate ... and my male peers didn't. And I felt resentful of that, especially since it was invisible to my coaches and to the health professionals around me. It was kind of like, "Oh, just figure out how to deal with it." So it's understandable why so many girls don't have a positive view of their period, which is really unfortunate, because our menstrual cycle is so critical to the healthy functioning of our bodies.

On how breast development in girls affects their participation in sports

All of the research currently shows that this is extremely common, it extends well beyond runners. Seventy-three percent of girls reported at least one breast-related concern related to sports in middle-school age, and half of them felt that breasts affected their participation. But the way that we talk about breasts is very sexualized or we don't talk about them. They're a little bit of a tough subject when they really should just be a factual, basic lived experience of half the population. Eighty-seven percent of girls wanted to know more about breasts and sports bras specifically. So we know that the lack of sports bras is one of the reasons why we're losing girls in sport.

On coaching strategies related to weight that are damaging

One of the most common ones is just having an "ideal" athlete body in mind that you expect your team to work their way toward. When you consider all of the diversity in our genetics and our individuality, that's an absurd idea that everyone should mold themselves into some particular model. There are public weigh-ins that happen regularly in programs, or body-fat tests consistent enough to have athletes become fixated on it. Athletes are given very small ranges of acceptable body fat for an elite athlete that are based on 28-year-old Olympian bodies and not 20-year-old adolescent bodies that are in the thick of developing.

There's also food policing — where coaches, will not allow certain types of food for their athletes. They will make body comments on athletes in front of their teammates. Another thing they'll do is point out when someone looks "fit" and give a lot of personal attention to athletes that attain this body ideal and then withhold positive interaction from athletes that don't. And those are subtle ways of consistently telling athletes that in order to be invested in and cared about, they must change who they are. And it's not even based on real science. That's the thing. It would be wrong even if it was, because it creates such an unhealthy environment for athletes.

On both acknowledging sex-based differences in sports while also being inclusive of transgender athletes

It's obviously a very contentious, complicated issue that we're wrestling with in culture right now. And I have evolved my perspective a lot on this subject from a place of defensiveness of what I viewed as women's sports from a sex-based perspective to being very pro-inclusion of trans athletes in every aspect of life, including sports. But that took a little bit of a journey because I am so familiar with sex-based differences in sport. I've lived it. I've watched it. They exist — to have some trans-rights activists in this space denying that those exist, or being afraid of looking at that science or looking to debunk it created a lot of resistance to me, and I see it in a lot of the athletes that I have raced against over time. It's a thing that we have to acknowledge — that sex-based difference exists and hold that in one hand and hold in the other hand that inclusion is extremely important and that our definition of fairness is so narrow.

If we're only looking at fairness as who's competing in the Olympic Games and who has experienced what kind of puberty and whatever, you can do that if you want to. You can spend all your time focused on that, but fairness is about a lot more than that. And we can hold the sex, these differences and still be for inclusion. And I think that's actually critical to the inclusion of trans people --not denying the science that we know, not denying the lived experiences of female-bodied people, but just deciding that even given some of those things, we still choose to compete together to be an inclusive space and experience all the benefits of having trans people on our teams in our lives and competing alongside us.

On women's running uniforms

Male runners generally wear looser fitting shorts and a jersey that covers the entire torso. In some events in running, especially the faster sprint events, the male outfit will be a tight fitting shirt that also covers the torso. Female athlete uniforms are like a little bathing suit bottom that your butt cheeks hang out of or a very, very short short that they call cheeky bottoms or something like that. And then a crop top that exposes your midriff, that's also form fitting and tight.

If there was a true sports advantage to wearing the outfit that female athletes are bound, by rules even, to wear in sport, male athletes would do it, too. The best athletes in the world will want to do what the biggest performance advantage is. The history of female uniforms being designed as they are now started in the wake of Title IX, when there was a lot of fear that sports was masculinizing girls, that it was making them gay — all of these homophobic fears around participating in activities that were traditionally viewed as men's spaces.

On refusing to be naked for a Nike ad campaign — and instead using the ad to comment on objectification of female athletes

My first big shot at an ad campaign with Nike, I was so excited. I just couldn't believe that I was going to get this chance to be used in a commercial and poster campaign, media notice around it. But then when I got the look and feel from the creative agency, it was a picture of Brandi Chastain, the soccer player, from an old ad where she was bent over naked with a soccer ball. It was very provocative. ... And I just felt crestfallen when I saw that. ... [Just like] being in Playboy magazine as a female athlete was kind of a sign you've made it, or being on the cover of another magazine depicted in a gown or lingerie or feminized in some way. And I just thought, why are we doing this? Why? That has nothing to do with the excellence that got you the opportunity in the first place. And so I got the courage to ask them to do it differently, to not be depicted in that way.

I came up with an ad where I was standing in my running clothes that I train in every day with my arms crossed, looking directly at the camera. And the ad was in the first person voice, so I was very much in control of how I was being viewed and which added a lot of power to the ad, and it made it a very successful campaign. ... [The caption was] "objectify me," ... and it was meant to kind of grab your attention. ... And then underneath it was the fine print of, "We study the female body so that we can make them the best running shoes."

Sam Briger and Seth Kelley produced and edited this interview for broadcast. Bridget Bentz, Molly Seavy-Nesper and Deborah Franklin adapted it for the web.