Differentiating between strain and contraction of the pelvic floor muscle, PFM, of a subject comprises using an orientation sensor 112, e.g. accelerometer, geomagnetic field sensor, magnetometer or gyroscope sensor, provided within a vaginal probe 110. In one embodiment data is received from the sensor located within the vaginal canal. A processor 123 is utilised to determine from the data a direction of rotation of the probe during a measurement period. When the probe rotates in the cranial-ventral direction an output is generated indicating there has been a contraction of the PFM. When the probe has rotated in the caudal-dorsal direction an output is generated indicating there has been a strain of the PFM. In another embodiment the probe comprises a transmitter 113. An internal and external calibration period may be used to calculate noise within the subject and within the device respectively. The probe may comprise a force sensor 114 to detect compressive force acting on the probe.
