The Influence of Electrode Size on Selectivity and Comfort in Transcutaneous Electrical Stimulation of the Forearm

Abstract

Transcutaneous electrical stimulation (TES) is a technique to artificially activate motor nerves and muscles. It can be used for rehabilitation or the restoration of lost motor functions, e.g., in subjects with brain or spinal cord lesions. Apart from selectively activating motor nerves and muscles, TES activates sensory fibers and pain receptors, producing discomfort and pain. Clinicians try to minimize discomfort by optimizing stimulation parameters, electrode location, and electrode size. There are some studies that found optimal electrode sizes for certain stimulation sites (e.g., gastrocnemius), however the underlying effects why certain electrode sizes are preferred by patients is not well understood. We used a TES model consisting of a finite element (FE) model and a nerve model to assess the influence of different electrode sizes on the selectivity and the perceived comfort for various anatomies. Motor thresholds calculated using the TES model were compared with motor thresholds that were obtained from measurements performed on the forearm of ten human volunteers. Results of the TES model indicate that small electrodes (0.8 x 0.8 cm(2)) are more comfortable for thin fat layers (0.25 cm) and superficial nerves (0.1 cm) and larger electrodes (4.1 x 4.1 cm(2)) are more comfortable for thicker fat layers (2 cm) and deeper nerves (1.1 cm) at a constant recruitment.

A review of the design and clinical evaluation of the ShefStim array-based functional electrical stimulation system

“…There are a number of limitations with the model, including the prismatic geometry and assumptions regarding the nerve depth, which undoubtedly varies significantly between subjects. Further, in contrast to Kuhn et al [29], we did not experimentally validate the model. However, the array geometry and hydrogel properties derived using the model proved to be similar to the array design successfully used in the final take-home study.…”

“…The activation area is similar in concept to the measure used by Kuhn et al [29], who based their measure of selectivity on an activation volume. As our model assumes the nerve depth to be known (at 10mm in this case), the cross-sectional area of the stimulation pool at 10mm is the measure of the selectivity of stimulation.…”

Influence of Different Geometric Representations of the Volume Conductor on Nerve Activation during Electrical Stimulation

“…In addition, this result supports studies that use cylindrical models for electrode optimization [14, 28]. Also, the electrical conductivities of the model tissues can be tuned to reduce the error [29, 30]; nevertheless, the model needs to be retuned when simulation conditions change, such as stimulation site, interelectrode distance, and electrode size.…”

“…As the CM and PM are usually constructed with concentric cylinders and cubes [2, 11–14, 28], the discrepancy of the fat thickness and bone location between the inspected models and the AM-R at each stimulation site could partly influence the activation error (Figures 4 and 5). Thus, fat thickness and bone location were adjusted to match the average fat thickness and bone-to-muscle distance for the anterior and medial sites to observe whether the error could be reduced or not.…”

Minimizing muscle fatigue through optimization of electrical stimulation parameters

“…Other researchers have used frequencies in similar ranges. [29][30][31] In each session, the PA was set to 30, 40, or 50 mA, and the PD was set to 150, 300, or 450 µsec, producing 9 (PA,PD) pairs (3×3) that were tested in separate sessions. Based on our previous observation, these ranges of PA and PD generate measureable ankle torque that is well-tolerated, while larger PAs or longer PDs either generate torque that saturate or that is not well-tolerated by participants due to discomfort from the stimulation.…”