Ex-U.S. Olympian Paralyzed In Training Incident ... Lolo Jones Leads Fundraising Campaign

A former U.S. Olympic star was partially paralyzed in a training incident over the weekend ... and now his friend Lolo Jones is spearheading a fundraising campaign to help pay for his medical bills.

TMZ Sports has learned ... Jamie Nieto -- a high jumper who finished 4th at the 2004 Games and 6th in 2012 -- was injured during a backflip training exercise while working with Olympic hopefuls at Azusa Pacific University in California on April 23rd.

Nieto's manager, Paul Doyle, tells us the 39-year-old landed on his head -- and the injury was so bad, a helicopter was called to the scene to transport him to USC Medical Center.

"We were very close to losing him at one stage," Doyle says ... "but thankfully, the MRI came back with no broken neck and the spinal cord remained intact."

"He’s slowly getting better. He’s starting to get feeling in his hands and legs ... doctors are optimistic."

But Nieto has another huge problem -- Doyle says Nieto had financial issues before the incident and had cancelled his health insurance to save money. Bottom line ... he's in trouble.

Enter Lolo Jones ... who trained with Nieto for years and became close with the track star. Now she wants to help raise funds to cover some of the medical costs.

In fact, Jones launched a fundraising page and made a video saying Nieto "has always been a light for me even when I've had some of my toughest moments."

"I just beg of you to please help donate to him now so that we can help him get back on his feet and he can soar to great heights once again."

So far, Jones has raised about $9,000 and counting.