Australian Olympic BMX rider Sam Willoughby has no movement from his chest down after a training accident in the United States of America, his family says.

The South Australian, who won a silver medal at the 2012 Olympics and recently competed at the Rio Olympics, underwent surgery in California after sustaining fractures to his C6 and C7 vertebrae which severely compressed his spinal cord.

Australian BMX rider Sam Willoughby will move to a rehab centre within days. ( Supplied: Road2recovery )

His parents, Colin and Sharon Willoughby, said the 25-year-old had the accident on September 10.

They said surgery was "successful enough at decompressing his spinal cord and aligning his vertebrae that a second operation for further stabilisation was not necessary".

"Sam still had no movement from his chest down but has regained use of his arms and is slowly regaining some sensation in his legs," the statement said.

The surgery involved removing the C6 vertebra and replacing it with a titanium cage.

His C5 and C7 vertebrae were also fused with a plate and four screws.

Willoughby is expected to be moved into a USA-based rehabilitation centre within days.

He crashed at his local track in Chula Vista, near San Diego, and was airlifted to hospital.

His parents said they had received a "huge outpouring of support" from friends, family and fans.

The family have sent up a Road2Recovery webpage to update fans about Willoughby's progress and collect donations.

Willoughby is also a two-time world champion.

His American fiancee Alise Post won BMX silver in Rio - the pair are due to be married on April 15 next year.

"He's a fierce competitor, we know that, we've seen that, he's a World Champion, he's an Olympic silver medallist and he's a stoic character, you know and those characteristics will come out in his recovery phase and he'll need every strength of character for him and his family to get through the next phase," said Cycling Australia chief Nick Green.

"I don't think he's thinking of getting back on the bike. What's been reported to me is that one of his first challenges is, he wants to walk down the aisle when he marries his fiancée and that's his immediate goal."

ABC/AAP