A device for monitoring the efficiency of physical training in a human or animal includes a millimeter wave generator that generates millimeter wave radiation and a millimeter wave antenna that emits millimeter waves generated by the millimeter wave generator in a predetermined direction. The device includes a coupling element that couples the millimeter wave antenna to the millimeter wave generator and that transmits the millimeter wave radiation to the millimeter wave antenna. The device also includes a reference element that intermittently receives millimeter wave radiation from the millimeter wave antenna and a detector that detects millimeter wave radiation and produces a first data stream. The device further includes a processing unit that receives the first data stream and generates a signal indicating development of a body lactate level over time and an output device indicating the development of the body lactate level over time, to a user.