An activity monitor determines whether the current value of a users heart rate from a heart rate sensor is correct based on a motion-based heart rate and an associated spread which are determined by a model. The model provides a probability density function (PDF) of the heart rate based on confidence levels of previous heart rate values. The spread of the PDF is inversely proportional to the confidence levels. The confidence level may be based on an amplitude of a spectral peak relative to a noise floor. Multiple spectral peaks within the spread can be processed based on a shape of the PDF to determine which peak to use as the current value from the heart rate sensor. Either the value from the heart rate sensor or the motion-based heart rate is provided as the current heart rate.
