Investigating the Volume Conduction Effect in MMG and EMG during Action Potential Recording

Abstract

The study and measurement of the magnetic field from the skeletal muscle is called Magnetomyography (MMG). These magnetic fields are produced by the same ion currents which give rise to the electrical signals that are recorded with electromyography (EMG). Layers between the muscle and skin surface, known as volume conduction, play a critical role during the measurement. This paper presents the volume conduction effect on the electrical and magnetic signals with the finitedifference time-domain simulations using Sim4Life. The effects of 1 mm fat on the recorded electrical and magnetic signals from the skin surface have been evaluated in both EMG and MMG. The results indicate that due to 1 mm fat, the electrical signals decrease over 60% through traveling across layers between the muscle and skin surface, while these layers are transparent to the magnetic field. In a similar simulation procedure, when the new fibers are recruited, the interference among electrical signals makes the strength of recorded signals behave non-linearly proportional to the increasing number of active muscle fibers. Sim4Life simulations show that the recorded magnetic signals do not have the same trajectory as electrical signals. Hence, the changes in EMG signals caused by the volume conduction effect can result in signal misinterpretations.

Magnetomyography: A novel modality for non-invasive muscle sensing

“…We have demonstrated the similarities of MMG to sEMG and highlighted some differences which may allow for better localization using MMG compared to sEMG. Though advantages in localization have been shown in models (Arekhloo et al, 2022;Klotz et al, 2022Klotz et al, , 2023, there has yet to be empirical evidence. Magnetic sensing of the brain (magnetoencephalography (MEG)) has been demonstrated to provide higher localization accuracy compared to electrical sensing (electroencephalography (EEG)) in both a phantom as well as an implanted dipole in humans (Cohen et al, 1990;Cohen & Cuffin, 1991;Leahy et al, 1998), but the results cannot be extended to muscle sensing.…”

“…Though the origin of both modalities is the currents traveling through muscle fibers, there are notable differences between how they propagate. sEMG signals are warped by the conductive properties of the signal path -tissue, skin, and the impedance at the sensor interface -whereas the body is permeable to magnetic fields leaving MMG signals unaffected (Arekhloo et al, 2022;Hansen et al, 2010). This distortion introduced to sEMG is highly nonlinear and depends heavily on the subject and day-to-day environmental conditions, introducing difficulties in generalization and scaling with sEMG.…”

“…Several studies have demonstrated robust recordings of MMG and have applied analyses including stimulus triggered averages of muscle activity (Broser et al, 2018), decoding of individual motor units (Broser et al, 2023), and even discrimination between movements that rival results obtained with sEMG (Greco et al, 2023). Models and simulations have also shown MMG has higher specificity and better localization compared to sEMG, furthering excitement for the field (Arekhloo et al, 2022;Klotz et al, 2023).…”

Alignment of magnetic sensing and clinical magnetomyography

“…However, there are two key drivers in employing magnetic sensors rather than electrical sensors for detecting muscle activities. First, unlike electrical signals, magnetic signals have the advantage that their signal strength is minimally affected by the surrounding tissues like skin and subcutaneous fat, as the body’s tissues are effectively transparent to magnetic fields ( Arekhloo et al, 2022 ). Consequently, magnetic signals can be coupled to the electrical current flowing in the muscle fiber and offer significantly higher spatial resolution (~mm).…”