The present invention relates to a portable device (10) for determining a heart rate of a person (20), comprising a heart rate measurement unit, a motion measurement unit for measuring the motion of a body part (12), and a processing unit. The processing unit is adapted to measure a signal quality of the heart rate signal and accordingly switch between two calculation modes: If the signal quality is above a predefined threshold, the heart rate is calculated based on the heart rate signal. If the signal quality is so poor that a reliable calculation of the heart rate is technically not possible anymore based on the heart rate signal, the processing unit switches to its second calculation mode, in which the heart rate is estimated based on the motion signal by estimating a heart rate constant, which depends on the frequency of the motion signal, and defining an exponential development of the heart rate, starting at the last reliably measured heart rate and finishing at the estimated heart rate constant.
