Measuring Power is a tool most useful in analyzing training or racing data after a ride. Power is a direct manifestation of the exercise intensity, whereas heart rate responds to changes in Power. Power will therefore change rapidly with each pedal stroke. Heart rate on the other hand is relatively stable and will not fluctuate because it takes heart rate longer to adapt to the intensity change. The combination of the two provides the full analysis. Polar Power Kit (S) system is a valuable tool to help develop your pedal stroke. It measures the Power put out by each leg independently, and can then tell how evenly you pedal with both legs. If you can distribute the power evenly between each leg, you will generate a more consistent power output and delay fatigue.  The Polar Power Output sensor kit is compatible with Polar Heart Rate monitors S725, S720i, S710, and S625X.