Image 1 of 3 Chris Froome (Sky) is protected by his teammates (Image credit: AFP) Image 2 of 3 Christopher Froome (Sky (Image credit: Bettini Photo) Image 3 of 3 Chris Froome (Sky) won stage 8 and took over the Tour de France race lead (Image credit: Bettini Photo)

Chris Froome's performances in the Tour de France's first mountaintop finish and first time trial have left him with a sizeable buffer of 3:25 at the head of the overall standings but they have also triggered a litany of estimates of his power output from a variety of external analysts.

In an article for Le Monde earlier this week, for instance, former Festina trainer Antoine Vayer estimated that Froome produced an average power output of 446 watts on the final climb to Ax 3 Domaines on Saturday, but the yellow jersey dismissed the figures when they were put to him during his press conference after stage 12 to Tours.

"I don't think it's humanly possible to average 440 watts for a whole stage so that's out of the question. That's far, far from what I think is possible," said Froome.

Froome did not, however, divulge what his own power-meter had actually read after his stage victory at Ax 3 Domaines on Saturday, in keeping with Team Sky's policy on the matter. Manager Dave Brailsford has dismissed estimates such as Vayer's as pseudo-science and says that he will not publish his riders' power data because "interpreting it is not as simple as it seems."

Interpreting Froome's improvement against the watch in recent years is no simple matter either. Prior to the 2011 Tour de Suisse, Froome had never finished in the top 10 in a time trial in a WorldTour event, yet a little over two years later, the 28-year-old put two minutes into his overall rivals in a 33-kilometre time trial at the Tour de France.

Twenty-four hours on from his efforts at Mont-Saint-Michel, Froome was keen to put his display in perspective and said that he does not expect to make similar gains on Alejandro Valverde, Alberto Contador et al in the second, hillier time trial to Chorges next week.

"If I look at the two time trials in the Tour, I think the one yesterday was the one where I could take the biggest advantage as it was flat and quite windy," Froome said. "The second time trial is made up of two mountains and two descents and that will favour climbers more, so I think the results will be much more balanced in terms of GC riders."

Not that mountains of any shape or size have posed any problems for Froome thus far. After winning emphatically on Ax 3 Domaines and coolly fending off Movistar's collective offensive the following day, Froome is not concerned by the race's next major rendezvous, the climb of Mont Ventoux on Sunday. "I haven't spent too much time thinking about it but I'm looking forward to going uphill again," Froome said.

Thursday's stage to Tours was not without its problems for Team Sky, however, as Edvald Boasson Hagen crashed in the frantic closing kilometres. The Norwegian was diagnosed with a fractured scapula after the stage and will not start on Friday, reducing Sky to seven riders.

Froome was not aware of the extent of Boasson Hagen's injury when he faced the press after the stage but he said that he was glad not to have been caught up in an accident himself. "I heard the crash but I didn't actually see it, although I heard on the radio that Edvald was caught up in it," Froome said. "The team did a fantastic job keeping me up there, it was a hard day. Every time I come across finish line, I breathe little sigh of relief."