Estimation of Tissue Thickness Changes Due to Electrical Muscle Stimulation Using Wearable Ultrasonic Sensor in Pulse Echo Mode

Skeletal muscle monitoring provides valuable information about the muscle contractile properties which could be useful in various clinical applications. A wearable ultrasonic sensor (WUS) was constructed using a 110-µm thick polyvinylidene fluoride polymer piezoelectric film to perform a continuous and hands-free muscle monitoring. The lightweight and flexible properties of WUS enables the stable attachment to the skin surface without affecting the tissue motion of interest which results in less motion artifacts. In order to perform reliable estimation of tissue thicknesses by the WUS in an ultrasonic pulse echo mode, the accuracy of selected signal analysis techniques was evaluated using a laser displacement measurement result as a reference. In addition, the effects of moving averaging on the signal-to-noise ratio of the ultrasonic signals and the estimation accuracy of the tissue thickness were investigated. In order to demonstrate the feasibility of the proposed ultrasound technique for in-vivo measurements, continuous monitoring of the lateral head of triceps muscle of a healthy male subject was performed using the WUS. The tissue thickness changes due to the evoked muscle contraction by an electrical muscle stimulation (EMS) were monitored. It was observed that the muscle was unable to fully relax at the EMS frequency at 16 Hz, indicating the tetanic contraction.