Of all the metrics at a serious cyclist's disposal, none is more valuable than power. Rather than record variables such as heart rate (subject to rest) or speed (subject to weather conditions), your power output – measured in watts – remains constant. In other words, it's the only objective way to measure your performance – how many watts you're capable of producing for five, 10 or 20 minutes at a time. Which is why pro cyclists have been training and racing with them for years.
And, unlike less legal means, they're available to us mere mortals, too. There are plenty of models on the market, but you'll get the most bang for your buck with the SRAM RED Quarq. This crank-based, fully integrated unit is as accurate as it is sleek (not to mention durable), measuring output with an accuracy of +/- 1.5 percent, while calibration couldn't be easier: Simply start pedaling. Pick up an ANT+ compatible cycling computer, book an appointment with a cycling coach to test your functional threshold – a baseline measurement that determines the watts you can produce for 60 minutes – then get ready for some leg-searing workouts. Sure, training with power is harder, and you could blow the Quarq's two grand price tag on a lighter set of wheels or a fancy trip to France, but nothing will make you fitter or faster. [$1,900; quarq.com]