A wrist device comprising: a wristband being substantially elastic and forming one integral entity, the wristband comprising first, second and third portions , the third portion being situated between the first and second portions , wherein the first and second portions are configured to mechanically connect to each other in order to enable detachable and adjustable attachment of the wrist device to a wrist of a user; and an electronics module comprising an optical heart activity sensor configured to measure heart activity of the user and a processing circuitry configured to obtain heart activity measurement data from the optical heart activity sensor and to process said data into a heart activity metric characterizing a heart activity of the user, the third portion of the wristband comprising a holder configured to enable detachable mounting of the electronics module to the wristband , the holder and the electronics module being adapted and dimensioned so that the holder produces a spring force to the electronics module in order to keep the electronics module in the holder , the electronics module , when mounted to the wristband, stiffening a structure of the third portion in order to enable the wrist device to firmly attach against the wrist of the user and to enable the optical heart activity measurement by the electronics module, characterized in that the electronics module comprises a multifunction button, and the holder comprises a hollow for receiving the multifunction button and a protrusion arranged together with the hollow, the protrusion and the hollow configured to enable mechanical energy transfer from the protrusion to the multifunction button of the electronics module , and in that the holder further comprises first and second mounting elements being less elastic compared with the first, second, and third portions , the first and second mounting elements enabling firm attachment of the electronics module to the wristband . a wristband being substantially elastic and forming one integral entity, the wristband comprising first, second and third portions , the third portion being situated between the first and second portions , wherein the first and second portions are configured to mechanically connect to each other in order to enable detachable and adjustable attachment of the wrist device to a wrist of a user; and an electronics module comprising an optical heart activity sensor configured to measure heart activity of the user and a processing circuitry configured to obtain heart activity measurement data from the optical heart activity sensor and to process said data into a heart activity metric characterizing a heart activity of the user, the third portion of the wristband comprising a holder configured to enable detachable mounting of the electronics module to the wristband , the holder and the electronics module being adapted and dimensioned so that the holder produces a spring force to the electronics module in order to keep the electronics module in the holder , the electronics module , when mounted to the wristband, stiffening a structure of the third portion in order to enable the wrist device to firmly attach against the wrist of the user and to enable the optical heart activity measurement by the electronics module, characterized in that the electronics module comprises a multifunction button, and the holder comprises a hollow for receiving the multifunction button and a protrusion arranged together with the hollow, the protrusion and the hollow configured to enable mechanical energy transfer from the protrusion to the multifunction button of the electronics module , and in that the holder further comprises first and second mounting elements being less elastic compared with the first, second, and third portions , the first and second mounting elements enabling firm attachment of the electronics module to the wristband . <EOS>