A heart rate monitor is disclosed comprising two main components: a user system in form of a wristwatch type device including a plurality of sensors, and a second device, such as a smart phone, that receives and processes the transmitted sensor signals. The user system includes an optical blood concentration sensing system (310) for detecting the heart rate, a motion sensor (320) to sense changes in the position of the user system with respect to the skin in order to compensate for noise, and an accelerometer (330) to provide information about the motion of the user system with respect to the users heart. A heart rate Kalman filter is configured to compute a heart rate on the basis of signals obtained from the plurality of sensors.
