A device for monitoring heart rate or blood oxygen level during exercise is disclosed having sensors (106) and emitters (104), wherein the sensors (106) are used to monitor transmission from each of the emitters (104) in turn. This provides a possibility that a smaller device can be provided since less sensors and emitters can be used while obtaining more number of observations. In some embodiments the repeated use of sensors with different emitters introduces redundancy into the device so that the device is more robust and may function even in the event that one or two of the emitters and sensors have broken down.
