A data transmission method for use in the transmission of a performance transmitter (102, 104) of a portable user-specific performance monitor (100) by using wireless data transmission that is based on radio-frequency electromagnetic radiation, and the method comprises measuring with the performance monitor (100) the movement and/or organ system state of a user. In the solution, the availability of wireless data transmission in the performance transmitter (102, 104) is determined. If wireless data transmission is not available, measuring data is stored into a memory (110, 126) of the performance transmitter (102, 104), otherwise the measuring data is transmitted using wireless data transmission.
