The MMG signal properties were analyzed during electrical stimulation of the tibialis anterior muscle. A group of 4 healthy volunteers were stimulated with train of pulses of increasing intensity. Two different pulse waveforms were tested: a rectangular monopolar and rectangular bipolar with inter-pulse interval (IPI). The EMG, force and the MMG signals were recorded. The MMG signal was recorded transcutaneously by using 3D accelerometer located over the muscle belly. The muscle response recorded for particular stimulus intensity and waveform were averaged. In the analysis four time intervals of MMG has been analyzed: P1 in range 2–12 ms, P2 in range 12–27 ms, P3 in range 27–65 and relaxation phase (RP). As the motor units (MUs) respond in all-or-none manner the recruitment process can be analyzed on the base of the MMG signal changes generated by increasing stimulation intensity. The obtained results have revealed significant difference in P3 phase of MMG courses dependent on the stimulus shape. The computer model-based analysis showed that the response recorded in P1 phase is generated by easy excitable fast MUs, while in P3 phase is determined by number of activated slow MUs. Concluding, the MMG signal profiles can provide the information concerning an influence of electrical stimulation pulses waveforms on recruitment of different types of motor units.
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