A Method for Identification of Mechanical Response of Motor Units in Skeletal Muscle Voluntary Contractions Using Ultrafast Ultrasound Imaging—Simulations and Experimental Tests

Robin Rohlen,Christer Gronlund,Jun Yu,Karen-Helene Stoverud,Erik Stalberg

doi:10.1109/access.2020.2980053

Robin Rohlen, Christer Gronlund + Show 3 more

Open Access

https://doi.org/10.1109/access.2020.2980053

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Abstract

The central nervous system coordinates movement through forces generated by motor units (MUs) in skeletal muscles. To analyze MUs function is essential in sports, rehabilitation medicine applications, and neuromuscular diagnostics. The MUs and their function are studied using electromyography. Typically, these methods study only a small muscle volume (1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) or only a superficial (<1 cm) volume of the muscle. Here we introduce a method to identify so-called mechanical units, i.e., the mechanical response of electrically active MUs, in the whole muscle ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 4$ </tex-math></inline-formula> cm, cross-sectional) under voluntary contractions by ultrafast ultrasound imaging and spatiotemporal decomposition. We evaluate the performance of the method by simulation of active MUs’ mechanical response under weak contractions. We further test the experimental feasibility on eight healthy subjects. We show the existence of mechanical units that contribute to the tissue dynamics in the biceps brachii at low force levels and that these units are similar to MUs described by electromyography with respect to the number of units, territory sizes, and firing rates. This study introduces a new potential neuromuscular functional imaging method, which could be used to study a variety of questions on muscle physiology that previously were difficult or not possible to address.

Highlights

The central nervous system controls force production of the skeletal muscles in a quantal manner by successive recruitment of motor units (MUs) [1]
We found that the characteristics of the identified mechanical units were similar to those of MUs described by EMG [1], [7]
Electrophysiological methods, including needle and surface EMG, are well-established techniques that have been used for over 60 years to help our understanding of muscle physiology and diseases

Summary

Introduction

The central nervous system controls force production of the skeletal muscles in a quantal manner by successive recruitment of motor units (MUs) [1]. The muscle fibers are electrically depolarized and give rise to a contraction [3] that results in force production through shortening and thickening of the fibers (mechanical twitch), which is a subtle (micrometer amplitude) and tran-. The analysis is based on recording and extracting activation characteristics of multiple individual MUs and allows access to the neural input from the spinal cord and the muscle fibers’ condition. Electromyography (EMG) is the gold standard to study MUs, and there are multiple techniques available. Invasive needle EMG typically samples electrical potentials at the tip of a needle with a sampling volume of about one mm3 [6] but provides no quantitative depth or spatial information.

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