The influence of electrode size and type on surface stimulation of the quadriceps

Two electrode types (gel and garment) in five sizes from 20 to 60 cm² were studied on ten able-bodied male and three spinal cord injured (SCI) subjects to determine the effect of electrode size and type on the current required to obtain 25% of the maximal voluntary contraction (MVC) strength of the quadriceps muscle (25% of max. stimulated strength for the SCI subjects) and the pain response. Constant current pulses of 300 ms at 40 Hz were used for stimulation. The total current required to produce 25% MVC (approximately 55 mA) did not differ significantly, but the two smaller electrodes had a higher pain rating. The average conductance of the 20 cm² electrodes was 886 μS and increased to 1286 μS for the 60 cm² electrodes. 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 electrode sizes. Electrode size did not influence electrical efficiency. The larger electrodes result in less pain, but the 60 cm² electrodes are not commonly available for muscle stimulation