Effect of subcutaneous fat thickness and surface electrode configuration during neuromuscular electrical stimulation

The effect of subcutaneous fat thickness, electrode size and inter-electrode distance on the minimum stimulus current necessary for fiber excitation was examined in an attempt to improve the efficacy of neuromuscular electrical stimulation (NMES) in obese populations. A three-dimensional finite element model of the human thigh was developed and used to calculate the potential along a myelinated nerve fiber due to NMES. The activating function was used to examine alterations in the excitation of the fiber due to fat thickness, electrode size and inter-electrode distance. The finite element model was coupled to a neural model to examine the stimulus current required for action potential propagation. The stimulus current required to evoke 10% of the maximum M-wave amplitude was measured experimentally. Both experimental and modeling studies indicated that the stimulus current required to reach the threshold for muscle activation increased with fat thickness, electrode size, and inter-electrode distance. However, as fat thickness increased, the threshold for muscle activation became less sensitive to inter-electrode distance and electrode size. These results suggest that by using larger electrodes above regions of high subcutaneous fat thickness, the efficacy of NMES could be maintained while reducing the current density at the skin and the associated subject discomfort.