BARCELONA, Spain - Nico Rosberg turns around with a grin during an uphill section, shouting "Hey, what's the pulse?"To which his personal coach Daniel Schloesser responds: "104."It's training unplugged on a bicycle as the Mercedes Formula 1 driver listens to his own body (and the trainer) without wearing a pulse watch or other monitoring equipment.EVERY KILO COUNTSRosberg says his bike training route is "some two to two-and-half hours" as his route leads away from the Barcelona race track where teams were testing ahead of the 2013 F1 season.Rosberg, who normally trains at his Monaco home, is not the only F1 driver who enjoys cycling as part of his fitness routine. McLaren's Jenson Button is a dedicated tri-athlete who has already qualified for the world championship over the half-Ironman distance. Ferrari's Fernando Alonso also excels on two wheels riding as much as 12 000km per year.Rosberg, unable to run much due to issues with a knee, manages 5000km on his bike which forms part of his all-around fitness regime.Schloesser, who spends 200 days a year with Rosberg, says: "you can't train specifically for your sport as a Formula 1 driver" but adds that the routine gives drivers diverse training."How is my pulse now?" asks Rosberg 20 minutes into the session. "107" is the answer from Ironman competitor Schloesser.F1 drivers' dilemma is that they have to keep very fit but at the same time can't be musclemen as every kilogram counts in a sport where 642kg is the low limit for a car plus driver.Red Bull's Sebastian Vettel is another example as the 25-year-old looks more like a bright high-school graduate than someone who can control the immense centrifugal forces of a 560kW F1 car and keep cool in the most extreme situations in the sport.Doctor Johannes Peil, head of a sports clinic in Bad Nauheim, Germany, said: "The demand on F1 drivers has risen constantly since the 1990's. Basic things such as power, coordination, speed, endurance and reaction time are trained in a way not imaginable in the past."A split second can be the different between success or defeat.Rosberg's new team mate Lewis Hamilton crashed into a tyre wall, aiming straight at the obstacle rather than sideways because that would have caused more damage to the car. A solid preparation is needed to be ready for these moments and the drivers are all engaged in their own fitness regime long before the start of a season.‘IT IS HELL’Planning a driver’s training regime during the 19 or 20-race season is difficult; there are sponsors' events, travel and other obligations. Rosberg normally trains for three days followed by a one-day break. He readily admits that he sometimes wants too much, "because I enjoy it and because I want to be a good cyclist".Crazy or not, the training helps in extreme conditions.Rosberg: "It is hell. You could put your bike into a sauna and then pedal for 20 minutes."The idea alone is enough to break a sweat and Rosberg says he knows of nothing that compares with the conditions of the race set again for March 24 in Sepang. The Malaysian Grand Prix comes a week after the season-opener in Australia and travelling can take its toll, even though drivers enjoy first class or travel on private jets.Still, the lower pressure in the cabin at an altitude of 10 000m, the dry air and less oxygen are far from ideal for a sportsperson. Schloesser said: "The flights are a big problem. The immune system is challenged, the whole system affected. Anyone who trains too much afterwards falls ill. It is not easy to make the right decision and skip training despite feeling bad about it." The long-haul flights also take the drivers to different time zones throughout the season: Australia, Malaysia, Bahrain, then Europe, a stint to Canada, back to Europe, then Asia, the United States and Brazil.All these factors make it hard to maintain the physical fitness level, which makes Schloesser keep a close eye on Rosberg - and his pulse watch.