System identification of the mechanomyogram of the tibialis anterior muscle in twitch-like contraction

The purpose of this study is to identify a mechanomyogram system of the tibialis anterior muscle without electrical stimulation. A subject sat on a chair, and his thigh and leg were immobilized with nylon belts. Ag- AgCl electrodes were attached to the skin over the common peroneal nerve. A capacitor microphone was attached on the skin of the tibialis anterior muscle to measure a mechanomyogram. The subject was instructed to generate an instantaneous dorsiflexion torque like twitch contraction. The neural action potential and mechanomyogram were measured, and the transfer function between them was identified using the state-space subspace system identification (4SID) method. The system was approximated with the third-order model. The natural frequency of the system was arroximately 2 to 3 Hz and agreed with those estimated using electrical stimulation. The proposed technique provided a novel method for identifying the transfer function without electrical stimulation.