IN THE STORE Do you want a GPS-based system or one that uses wireless foot and bike sensors? Advantages of the former: Just turn on the unit and go. Cons: It's not reliable in the concrete canyons of Manhattan, and it won't work indoors. Meanwhile, sensor-based units work anywhere but can take hours to accurately calibrate. The running pods are especially problematic; change your pace, or even switch shoes, and you'll need to reset them.

IN THE FIELD At high speeds on a bike, your heart-rate monitor might confuse a flapping jersey zipper for your thumping ticker. To foil this, try rotating the strap to position the sensors across your back, not your front. >> When calibrating a shoe-mounted running sensor, run farther (at least 2,000 yards) to more accurately measure your pace and distance.

IN THE FUTURE Swimmers will soon reap the benefits of heart-rate training. Currently, water's conductivity screws with the wireless transmission, but a customized chest strap may hold the key. Caveat: It will be for pool and freshwater swimming only.