Effects of sample geometry and electrode configuration on measured electrical resistivity of skeletal muscle

Over the past 40 years, researchers from a variety of scientific background s have been using Rush's equations to analyze results of their electrophysi ological studies. A lack of understanding of the constraints and the domain in which these equations are valid, often results in situations in which i t is challenging to evaluate and compare results obtained by different inve stigators. In this paper, we reanalyzed the conditions for which Rush's equations were derived, and using mathematical modeling, computer simulation and in vitro measurements, we delineated areas of their appropriate application. Our st udies showed that both sample geometry and test electrode configuration aff ect the measured tissue electrical resistivities: 1) The sample can be cons idered semi-infinite only if its dimensions are >50 inter-electrode separat ion distances (IESD), and thickness >2.5 IESD, 2) smaller sample sizes incr ease the transversally measured resistivity, 3) semi-infinite samples thinn er than 2.5 IESD, and samples tested with needle electrodes demonstrate red uced anisotropy, and 4) when surface-spot electrodes are longitudinally ali gned, as the IESD/tissue thickness ratio decreases, the measured resistivit y increases. Our conclusion is that in most experimental situations, it is necessary to use modeling techniques to decouple the electrode configuration/sample geom etry influence from the measured tissue resistivity.