A prototype of hybrid probe for surface EMG and MMG joint recordings

Cescon, Corrado and Nannucci, Lorenzo and Orizio, Claudio (2002) A prototype of hybrid probe for surface EMG and MMG joint recordings. In: Proc. of the XIV Congress of the International Society of Electrophysiology and Kinesiology, 22-25 June 2002, Vienna, Austria.

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The mechanomyographic signal (MMG) is generated by the transversal oscillations of the muscle due to the dimensional changes of the fibers during the contraction. The joint study of the MMG and surface EMG signals can give additional information in the study of the electro-mechanical properties of the muscle fibers. Within the NEW project (Neuromuscular assessment in the Elderly Worker) a prototype of a hybrid probe for the simultaneous detection of the surface EMG and MMG signals has been developed and realized. The acceIerometric sensor used for the detection of the MMG signal (Analog Devices model ADXL202JE) was selected due to the best performance/price ratio, identified by the characterization of 4 different sensors made at the Dynamometric Laboratory of the National Research Center (CNR) in Torino. The accelerometer has the following characteristics: 1 gram weight, 5x5x1.5 mm dimensions, ± 2 g dynamics and 0-100 Hz bandwidth. The first phase of the prototype design was aimed to establish the maximum mass that can be applied on a muscle without altering its mechanical response. In order to do that, the MMG signal was picked up on the biceps brachii muscle placing on the surface of the muscle different masses from 2 to 62 grams (the weight is comprehensive of the accelerometer). The signals were acquired from 7 subjects (24-29 years old), during 10 s contractions at the 30% of the Maximum Voluntary Contraction (MVC). For each subject two repetitions of the same experiment were made to assess the repeatability. The parameters of the MMG signal investigated were the mean frequency (MNF), the RMS amplitude and the asymmetry of the spectrum (ASI). The results indicated 16 grams as the maximum mass that can be applied on a muscle with acceptable spectral modifications (MNF<10%). Having this result in mind, a prototype of a hybrid probe was realized, that includes on the same printed circuit board 4 silver bar electrodes with interelectrode distance of 10 mm for the EMG detection, the voltage followers for the EMG signal buffering and the accelerometer for the MMG detection. The total mass of the probe after the incorporation with epoxy resin is 5 grams. The probe has been used to study the mechanical response of single motor units with spike triggered averaging technique.

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