Optimizing measurement of the electrical anisotropy of muscle

Abstract

Skeletal muscle is electrically anisotropic, with applied high-frequency electrical current flowing more easily along than across muscle fibers. As an early step in harnessing this characteristic for clinical use, we studied approaches for maximizing the measured anisotropy by varying electrode size and applied current frequency in the tibialis anterior of 10 normal subjects. The results were compared to those from two patients with amyotrophic lateral sclerosis (ALS). Current was applied percutaneously, first parallel and then perpendicular to the major fiber direction of the muscle at frequencies ranging from 20 kHZ to 1 MHZ, using a fixed voltage-electrode length and varying the current-electrode length. The measured anisotropy was most pronounced using the longest length current electrodes and with a 125-kHZ applied frequency for the major outcome parameter phase. In addition, the two ALS patients showed very distinct anisotropic patterns. These results support the belief that, with the appropriate measurement technique, non-invasive assessment of electrical anisotropy of muscle may have useful clinical application.

FIGURE 1

Experimental arrangement for obtaining EIM measurements at 0° on tibialis anterior. For 90° measurements, the entire array is rotated 90°, with the electrodes being kept the same distance apart.

FIGURE 2

Mean (± standard deviation, represented by dotted lines) resistance (R), reactance (X), and phase (θ) measurements at 0° and 90° vs. log (frequency), for all 10 normal subjects.

FIGURE 3

Electrode sizes and effect on anisotropy, showing the average data for all 10 subjects. Resistance, reactance, and phase are plotted against log (frequency) for three different lengths of the current electrodes described. The greatest anisotropy is obtained with the longest current electrodes. Standard deviations are omitted for the sake of clarity.

FIGURE 4

Mean (± standard deviation, represented by dotted lines) anisotropy ratio using the longest length electrode data for the 10 normal subjects (filled circles) with comparison to the data from the two ALS patients (squares and triangles).