World champion Simpson won't return to running after revealing rare heart condition that caused her collapse

World champion runner Jenny Simpson revealed it was a rare heart condition that threw her into cardiac arrest at a race in June and that she will not return to running.

In an Instagram post Thursday, the 39-year-old Simpson said she was diagnosed with Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC), a condition that triggered "dangerous heart rhythms that ultimately caused my heart to stop."

She said the diagnosis means it would be dangerous to return to running.

Simpson was pacing a group of girls in a mile run at an event in Raleigh, North Carolina, when she collapsed. She underwent CPR at the track and an automated external defibrillator was also used before medics rushed her to the hospital.

"You might be wondering what would be the right thing to say to me after an event like this," she wrote. "Please do not say that you're sorry or that you feel bad for me. If it is true in your heart, please tell me you're glad that I'm here. That's how I feel — so glad that I'm still here."

For about a decade beginning in 2007, Simpson was America's most successful women's 1,500-meter runner. She won worlds in 2011, finished second there twice and won Olympic bronze in Rio in 2016.