The invention relates to wearable electronic devices for monitoring physical activities of a person. The invention also concerns related methods and computer program products. According to the invention, a first sensor for measuring acceleration data and a second sensor for measuring heart rate data is provided. A data processing unit is configured to operate in a first mode when data is read from said first and second sensors and in a second mode when data is not read from at least one of said first and second sensors. In the first operating mode the processing unit cumulatively computes and updates a heart rate offset value based on at least one of the following data: a predetermined pulse of the person a rest, a predetermined fitness level of the person, a current recovery pulse of the person, and/or the cumulated acceleration data during the current activity. It also computes a current activity gain value based on acceleration data and heart beat rate data. The data processing unit is further configured to compute an estimate of the heart beat rate based on said offset value and said gain value during said second operating mode.
