Tracking Electromechanical Muscle Dynamics using Ultrafast Ultrasound and High-density EMG

Abstract

Muscles generate force and enable movement. After excitation of a muscle the muscle fibers contract. Methods to assess muscle contraction in vivo are scarce. Electromechanical delay (EMD), defined as the time lag between muscle excitation and contraction onset, has been proposed as a measure for contraction efficiency, but provides limited insight in electromechanical muscle dynamics. The current paper proposes and evaluates a novel non-invasive method to simultaneously track the propagation of both electrical and mechanical waves in muscles using high density electromyography and ultrafast ultrasound imaging (5 kHz). The method successfully tracked the propagation of the excitation-contraction (E-C) coupling in electrically evoked twitch contractions of the Biceps Brachii in three healthy participants. The excitation wave (i.e. action potential) had a velocity of 3.90 ± 0.65 m/s and the subsequent mechanical (i.e. contraction) wave had a velocity of 3.52 ± 0.89 m/s. Both waves propagated from distal to proximal and had similar spatiotemporal characteristics, indicating that our method can track the propagation of the E-C coupling. The experimental results were compared to simulated contractions of a newly developed multisegmental muscle fiber model, consisting of 500 sarcomeres in series. Both the experiment and simulation showed evidence that excited muscle sarcomeres pull on sarcomeres that were not yet reached by the action potential. In conclusion, our method can track the electromechanical muscle dynamics with high spatio-temporal resolution. Ultimately, the method can be used to characterize E-C coupling in patients with neuromuscular disease to assess contraction efficiency, monitor the progression of the disease and determine the efficacy of new treatment options.