Inertial sensors measure the acceleration of the S1 foot pod more than 1000 times per second and use this data with advanced algorithms to calculate foot angles and gait velocity. Polar S1 foot pod is a highly advanced running speed and distance sensor it is not merely a pedometer. Onetime calibration. Superior water resistant design. Requires only one battery. Longer battery life up to 40 hours. Low battery indicator. Compatible with Polar RS200, RS200sd, RS400, RS400sd,
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