A method of differentiating between a strain and a contraction of the pelvic floor muscles (PFM) of a subject includes receiving data generated by an orientation sensor provided within a vaginal probe device that is located within the vaginal canal of the subject and utilizing a processor to process data generated by the orientation sensor to determine a direction of rotation of the vaginal probe device during a measurement period. When the processor determines that the vaginal probe device has rotated in the cranial-ventral direction relative to the subject, an output is generated indicating that there has been a contraction of the PFM during the measurement period; and when the processor determines that the vaginal probe device has rotated in the caudal-dorsal direction relative to the subject, an output is generated indicating that there has been a strain of the PFM during the measurement period.