Ultrasound Strain Rate Imaging of Individual Muscle Motor Units

Abstract

Skeletal muscle is organized in motor units, each comprising a motor neuron and all its connected muscle fibres. The arrival of an action potential (a neural “impulse”) causes the fibers to contract, they exhibit a twitch. This study aimed for the spatiotemporal detection of individual twitches, which may allow for a more detailed real-time study of muscle physiology and higher fidelity in applications such as prosthesis control. We used a commercial ultrasound scanner and a linear probe in a clamped and fixed position over the biceps, with the image plane perpendicular to the muscle fibers. The strain rate scans were made with a frequency of 15MHz and a frame rate of approximately 215 FPS. Recordings of tiny voluntary isometric muscle contractions were made, with ElectroMyoGraphic (EMG) electrodes placed on each side of the probe to detect the Associated Motor Unit Action Potentials (MUAPs). The recordings were analyzed using the scanner’s Quantitative analysis (Q-analysis) tool for measuring strain rates within selected Regions Of Interest (ROIs). The results indicate that it is possible to image the mechanical response of a single motor unit by using ultrasonic strain rate imaging. This technique could thus be a future supplement to EMG in certain applications.