Power measurement (wattage) is the most precise gauge of fitness and exertion. Polar's Power Output Sensor W.I.N.D. adds actual, average and maximum power feedback to compatible heart-rate monitors (sold separately). Start training with this all-in-one power meter and you'll see great improvement in your riding. It also provides cadence and a measurement of right-versus-left-leg efficiency that allows you to smooth out your pedal stroke for a flawlessy efficient spin.