Changes in the electrical properties of the electrode–skin–underlying tissue composite during a week-long programme of neuromuscular electrical stimulation

Particular neuromuscular electrical stimulation (NMES) applications require the use of the same electrodes over a long duration (>1 day) without having access to them. Under such circumstance the quality of the electrode–skin contact cannot be assessed. We used the NMES signal itself to assess the quality of the electrode–skin contact and the electrical properties of the underlying tissues over a week. A 14% decrease in the skin's stratum corneum resistance (from 20 to 17 kΩ) and a 15% decrease in the resistance of the electrodes and underlying tissues (from 550 to 460 Ω) were observed in the 14 healthy subjects investigated. A follow-on investigation of the effect of exercise-induced sweating on the electrical properties of the electrode–skin–underlying tissue composite during NMES indicated a correlation between the decrease in the resistance values observed over the course of the week and the accumulation of sweat at the electrode–skin interface. The value of the capacitance representing the dielectric properties of the skin's stratum corneum increased after exercise-induced sweating but did not change significantly over the course of the week. We conclude that valuable information about the electrode–skin–underlying tissue composite can be gathered using the NMES signal itself, and suggest that this is a practical, safe and relatively simple method for monitoring these electrical properties during long-term stimulation. (paper)