The embodiments relate to a sensor strip (13) and to a flexible support strip (12) for receiving the sensor strip and for fixing the strip to a surface e.g. to the back of a test person. According to the embodiments, the sensor strip (13) has a reference point (18), (for example a snap fastener connection), with which it can be fixed to a reference surface (16) of the support strip. This allows the position of the sensor strip (13) to be unambiguously defined on the support strip (12). The further extension of the sensor strip (13) lies in a pocket (23) so that it can slide, allowing the relative movement between the support strip and the sensor strip if the support strip is lengthened (represented by a dot-dash line). The entire sensor unit is thus advantageously more comfortable to wear, as the elastic support strip (12), for example, can freely follow the back movements of the test person.