World champion runner Jenny Simpson has revealed that a rare heart condition caused her cardiac arrest during a race in June, and confirmed she will not return to competitive running.

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

She stated that the diagnosis means returning to running would be dangerous.

"That chapter has ended," she said. "Thankfully, my story hasn't."

Simpson collapsed while pacing a group of girls during a mile run at an event in Raleigh, North Carolina. She received CPR on the track and an automated external defibrillator was used before medical personnel rushed her to the hospital.

The "Sir Walter Running Team" issued a statement on Instagram shortly after Simpson’s collapse, expressing gratitude for the swift response from medics at the event.

"We are incredibly grateful to the individuals who responded immediately, as well as EMS and the medical professionals who handled the situation with such care, urgency, and professionalism," the statement read.

It added that: "Jenny is receiving excellent medical care, and our thoughts are with her and her family during this time."

Reflecting on her feelings after the concerning incident, Simpson wrote: "You might be wondering what would be the right thing to say to me after an event like this. 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 starting in 2007, Simpson stood as America's most successful female 1,500-meter runner. She won the world title in 2011, finished second at worlds twice, and took Olympic bronze in Rio in 2016.