Lance Armstrong doesn’t track his speed when he trains for next month’s Tour de France. The number he craves is measured in watts, not miles per hour. Power output dominates the training of top cyclists like Armstrong because it’s a much more accurate gauge of performance than speed (at the mercy of the wind) or even heart rate (too fickle), say experts.

Power never lies because it is a direct measure of the force applied to the bike (torque) that is converted into a measure of power output (watts). By measuring how many watts he expends on a mountain climb, Armstrong can develop a training program that duplicates those race efforts down to the watt. For the past seven years, Armstrong has used a $2,600 device called the SRM Powermeter, developed by German medical engineer Ulrich Schoberer in the late 1980s. The Powermeter measures deflection of the pedal crankarm using tiny strain gauges and converts the measures into watts on a handlebar computer that can store and download 70 hours worth of wattage (and heart rate) for analysis—numbers that Armstrong pores over obsessively.

Armstrong sure can crank. During the final hour of a seven-hour stage of the Tour, he can pedal at an average of 400 watts. (Track racers have pushed it over 2,000 watts for a few seconds. The average cyclist can barely light a 100-watt lamp.) "Pro cyclists used to train according to how they felt," says David Cathcart, marketing director for CycleOps Performance Training Products, which makes a watt measurement device called Powertap. "Now it’s all about metrics."