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