There is provided a wrist-worn apparatus for measuring a user's heart activity, the apparatus comprising: an optical heart activity sensor configured to measure the user's heart activity by optical heart rate measurement； and a wrist strap adapted to physically couple the optical heart rate activity sensor against the user's skin by attaching the wrist-worn apparatus around the user's wrist, wherein a first curved connection member is adapted to slide inside a second curved connection member, and wherein the wrist strap comprises at least one groove and a fixing claw acting as a counterpart for the at least one groove, wherein the fixing claw is arranged to engage the at least one groove when the first connection member is slid inside the second connection member.
