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.