Michael Schumacher remains in a "critical but stable" condition, his spokeswoman said on Saturday, as French investigators hoped a helmet-mounted camera he was wearing at the moment of his ski accident could provide answers.

It was not known whether the GoPro camera filmed the moment when the retired seven-time world champion slammed his head against a rock, or whether images have been damaged by the impact, which was so hard it split the helmet he was wearing.

But investigators may yet find another clue in a film made by a German witness, who claimed to have captured Schumacher's accident by chance, German news magazine Der Spiegel reported on Saturday.

Caps, flags and shirts with get-well wishes and birthday greetings for former F1 driver Michael Schumacher hang on the fence of the karting circuit owned by Schumacher on Friday, his 45th birthday, in Kerpen near Cologne, western Germany. [Patrik Stollarz / Agence France-Presse]

The magazine quoted the witness describing Schumacher descending the slope at a "leisurely" pace and "at a maximum speed of 20 kilometers an hour".

In the background, a skier could be seen descending on an unmarked run between two groomed pistes before falling, the magazine reported.

Key among the elements investigators are hoping to establish is the speed at which Schumacher was skiing, as it could determine responsibility in the Dec 29 accident in the French Alps near the Meribel ski resort.

French prosecutors and the ski resort say Schumacher was skiing at great speed. His spokeswoman, Sabine Kehm, has challenged that, saying he could not have been going fast "because it appears he helped a friend who had just fallen".

Edouard Bourgin, a specialist on accident claims said: "There could have been a catapult effect that explains the violence of the shock, even in the absence of excessive speed."

Prosecutors are also looking at whether the limits of the ski runs next to the accident site were correctly marked and whether the rock in question was lying close enough to the piste to require some kind of protection or signage.

In addition, they are examining whether the safety releases on Schumacher's skis operated properly.

"I don't think it's normal that between two marked slopes there would be this passage with rocks showing that is not fenced off," said Philippe Streiff, a former French racing driver, in the French sports daily L'Equipe.

A paraplegic since an accident in Rio de Janeiro, Streiff visited the Grenoble hospital on Friday to hand a message to Schumacher's wife, Corinna. Other family members at his bedside included his two teenage children, his father, Rolf, and brother, Ralph, who is also a racing driver.

Schumacher turned 45 on Friday, and fans marked the birthday with a silent vigil outside the facility, part of which was organized by Ferrari, Schumacher's former team.

Kehm said Schumacher remains in a "critical but stable" condition and no more media conferences were scheduled before Monday.

As all eyes turned to Schumacher's helmet-mounted camera for clues, Kehm denied some reports his family had been reluctant to hand over the camera - taken by authorities on Friday - because of privacy reasons.

"Michael's helmet camera was voluntarily given to the investigating authorities by the family," said Kehm in a statement. "That this should have been done against the wishes of the family is untrue."

The GoPro miniature camera, robust and popular among extreme sports enthusiasts, has a 170-degree wide angle lens that takes in much of the view and can show parts of the user's body including the face depending on how it is mounted.

The latest models can take up to 30 images a second for six hours.

Any usable images should shed light on the circumstan-ces of the accident on a small, seemingly innocuous off-piste section of Meribel located between two ski slopes - one classed as easy and the other as intermediate.

Police have also obtained eyewitness testimony from Schumacher's 14-year-old son, Mick, who was skiing with his father at the time, as well as a friend, at the Grenoble hospital on Friday.

Although Schumacher was conscious when airlifted from the unmarked run, he was agitated and soon fell into a coma, prompting his transfer to the Grenoble hospital.

He has undergone two operations to remove blood and pressure from his brain.