TECHNICAL ASPECTS OF MECHNOMYOGRAPHY RECORDING WITH PIEZOELECTRIC CONTACT SENSOR

The piezoelectric contact sensor has been widely utilised in mechanomy ography (MMG). The authors aim to clarify the mechanical variables (i. e. acceleration, velocity or displacement) reflected by the MMG signal detected with a piezoelectric contact sensor (PEC), and compare the r esults with those obtained simultaneously by an accelerometer (ACC). T o measure the acceleration-frequency response, a mechanical sinusoidal excitation of 5 to 300 Hz at a constant magnitude of 0.01 G was appli ed to the two transducers. The acceleration-frequency response of the ACC transducer was confirmed to be almost flat. The PEC without any re striction of the transducer housing (including the combined seismic ma ss) demonstrated a similar response to the ACC transducer. The PEC tra nsducer output with restricted housing decreased with increasing sinus oidal frequency and an attenuation slope of -40 dB/decade and phase an gle of -180 degrees. The voluntary MMG signal during isometric knee ex tension was recorded simultaneously with the two transducers. The ampl itude spectral density distribution of the MMG from the PEC transducer was narrow and the mean frequency was approximately one-half that obt ained from the ACC transducer. The amplitude spectral density distribu tion with the PEC transducer resembled that of the double integral ove r time of the ACC transducer signal. The phase angle of the PEC transd ucer signal was different from that of the ACC transducer signal by ap proximately -180 degrees. These results suggest that the PEC transduce r acts as a displacement meter of muscle vibration. In addition, diffe rences in the MMG frequency components relating to the transducer type must be taken into consideration when investigating the mechanical ac tivity of muscle.