Chris Mosier on Saturday competed in an Olympic trial, making him the first openly transgender athlete to participate in the gender with which they identify.

Mosier participated in the Team USA 50 kilometer racewalk trial held outside San Diego, California.

In an Instagram post, Mosier explained that he decided to participate despite a knee injury to make history. Mosier withdrew mid-race.

“I wanted [to] come here today and have this experience and make history and celebrate,” Mosier wrote. “[T]he truth is my leg has been hurting for a long time and this morning I had two options: go all out today and hurt myself even more, or exit early and live to race another day.”

“I made the tough but wise choice.”

“Since making it to the Olympic Trials was my main goal last year, I feel extremely accomplished,” he added.

Mosier, an All-American duathlete, accomplished triathlete, and transgender rights advocate, last year began racewalking to compete in the Olympics. With only two races, Mosier qualified for the Olympic trials.

Mosier, 40, promised he was going to “fix this knee and really let it rock!”

In 2012, Keelin Godsey became the first transgender athlete to qualify for the Olympic trails. However, Godsey, who identifies as a man, competed in the women's division.