A method for monitoring periodic motions of one or more subjects uses signal reflections from the subjects. The method includes emitting a transmitted signal from a transmitting antenna and receiving a received signal at one or more receiving antennas. The received signal includes a combination of a number of reflections of the transmitted signal, at least some of which are associated with the subjects. The received signal, including the reflections, is processed to determine an estimate of a fundamental frequency of the periodic motions.