Austrian ski jumper Lukas Mueller underwent surgery on his lower spine Wednesday, hours after crashing during test ski flying on the world championship hill in Austria.

An equipment problem caused Mueller to fall while landing a 120-metre jump in heavy snowfall. Initially he couldn't move his legs but his condition improved while receiving first aid, according to emergency doctor Ulf Karner.

"It's a very serious spine injury with yet unforeseeable consequences," Karner told the Austria Press Agency.

Mueller was taken to the university clinic in Graz by ambulance as an emergency helicopter couldn't fly because of fog.

The 23-year-old Mueller, who was the 2009 junior world champion on the normal hill, competed in 16 World Cup events from 2009-13, and finished sixth at the first stop of the 2009-10 Four Hills Tour in Oberstdorf, Germany, for his best result.

Race director Harald Haim said Mueller's jump seemed normal until he lost balance as his left ski suddenly went up, causing him to turn over and fall.

"TV footage seems to show that his shoe buckle opened because of the pressure," Haim said.

The hill has been tested by jumpers since Monday. The worlds start with training and qualifying on Thursday, followed by the two-day individual competition, and will be concluded by the team event on Sunday.

It's the second serious crash on the Kulm, one of the world's largest ski flying hills, in two years.

In 2014, three-time Olympic champion Thomas Morgenstern suffered skull and lung bruises during a training crash. The Austrian returned to competition at the Winter Olympics in Sochi just three weeks later before ending his career.

The hill has been reconstructed since, allowing athletes to jump even further. Severin Freund of Germany set a new Kulm record of 237.5 metres last year.