Chris Mosier, the first openly transgender athlete to compete on a U.S. men's national team, stars in Nike's latest ad from its "Unlimited" campaign.

The light-hearted ad follows Mosier as he goes through his grueling training regimen, running, cycling and lifting weights. A voiceover questions him on how he knew he would get past certain obstacles-- like being able to compete against other men, being accepted by his teammates. Throughout, he bluntly explains, "I didn't," but pushes on, undaunted and seemingly without a care.

35-year-old Mosier, Nike's first trans athlete, first realized at the age of four that he didn't identify as female and continued to feel so as he went on to compete in women's athletics. But he then went on to achieve athletic success, as a man, in 2015 when he came in seventh at the sprint duathlon national championships in the male 35-39 group, landing a spot on the U.S. national team. As an advocate for LGBT athletes and founder of transgender resource site transathlete.com, Mosier has also helped to achieve change on the international level. He played a part in convincing the International Olympics Committee to amend its rules so that transgender athletes would not need to have reassignment surgery in order to compete as the gender they identify with.

"Everything that I've done in the last five, six years since I started to transition, has been with [a] 'Just Do It' mindset," he explained in a statement. "I didn't know if I would be competitive against men; I just did it. Every success that I've had since then has shown me that anything is really possible. By not stopping myself, not limiting myself and just really going for it I've learned a lot about myself and also had the opportunity to further the conversation on trans inclusion in sports."

Nike also created an accompanying behind-the-scenes film. The new ad, created out of Wieden & Kennedy, Portland, follows the "Unlimited Future" and "Unlimited You" spots, the latter of which debuted during the Olympics' opening ceremony. The campaign celebrates how everyday and professional athletes strive beyond their own limits to reach new heights.