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Skeletal muscle is considered as a near-constant volume system, and the contractions of the muscle are related to the changes in tissue thickness. Assessment of the skeletal muscle contractile parameters such as maximum contraction thickness ( T h ), contraction time ( T c ), contraction velocity ( V c ), sustain time ( T s ), and half-relaxation ( T r ) provides valuable information for various medical applications. This paper presents a single-element wearable ultrasonic sensor (WUS) and a method to measure the skeletal muscle contractile parameters in A-mode ultrasonic data acquisition. The developed WUS was made of double-layer polyvinylidene fluoride (PVDF) piezoelectric polymer films with a simple and low-cost fabrication process. A flexible, lightweight, thin, and small size WUS would provide a secure attachment to the skin surface without affecting the muscle contraction dynamics of interest. The developed WUS was employed to monitor the contractions of gastrocnemius (GC) muscle of a human subject. The GC muscle contractions were evoked by the electrical muscle stimulation (EMS) at varying EMS frequencies from 2 Hz up to 30 Hz. The tissue thickness changes due to the muscle contractions were measured by utilizing a time-of-flight method in the ultrasonic through-transmission mode. The developed WUS demonstrated the capability to monitor the tissue thickness changes during the unfused and fused tetanic contractions. The tetanic progression level was quantitatively assessed using the parameter of the fusion index (FI) obtained. In addition, the contractile parameters ( T h , T c , V c , T s , and T r ) were successfully extracted from the measured tissue thickness changes. In addition, the unfused and fused tetanus frequencies were estimated from the obtained FI-EMS frequency curve. The WUS and ultrasonic method proposed in this study could be a valuable tool for inexpensive, non-invasive, and continuous monitoring of the skeletal muscle contractile properties.

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.

Skeletal muscle is considered as a near-constant volume system and the contractions of muscle are linked to the changes in tissue thickness. Assessment of muscle contractile properties provides valuable information for various medical applications. The Ultrasound method provides a high spatial and temporal resolution measurement of the architectural changes of skeletal muscle during contractions. However, one of the challenges in ultrasound methods is motion artifacts due to the weight, size, and attachment instability of the conventional ultrasonic probe employed. A flexible, lightweight, thin, and small size ultrasonic sensor would provide a secure attachment to the skin surface without affecting the muscle contractions dynamics. Thus, in this study, a wearable ultrasonic sensor (WUS) made of a polyvinylidene fluoride (PVDF) film was employed to measure skeletal muscle contractile properties as a quantitative assessment tool. The isometric contractions of gastrocnemius (GC) muscles at a lower leg were electrically stimulated. The tissue thickness changes were measured by means of an ultrasound time-of-flight method in the through-transmission mode. Maximum contraction thickness, contraction time, contraction velocity, sustain time, and half-relaxation time were successfully extracted from the monitored thickness changes. The results demonstrated the potential of the developed WUS in measuring skeletal muscle contractile parameters.