The current requirements and the pain response for various sizes of surface stimulation electrodes

Two electrode types (gel and garment) in five sizes from 20 to 60 cm² were studied on ten normal male subjects to determine the effect of electrode size and type on the stimulation current (constant current pulses of 300 μs at 40 Hz) required to obtain 25% of the maximal voluntary contraction (MVC) strength of the quadriceps muscle and the pain response. The distal electrode was placed 7.5 cm above the patella and the proximal electrode at two-thirds the length of the femur. The total current required to produce 25% MVC (approximately 55 mA) was not significantly different for the different size or type electrodes, but the two smaller electrodes had a significantly higher pain rating. There were large individual variations in both the current and pain rating for the same relative force, but near-constant values were obtained for each subject over the different size electrodes. The larger electrodes result in less pain and require the same current as small electrodes, but are not commonly available for muscle stimulation.