Two electrode types (gel and garment) in five sizes from 20 to 60 cm- were studied on 10 able-bodied male and three spinal cord injured (SCI) subjects to determine the effect of electrode size and type ori 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. The distal electrode was placed 7.5 cm above the superior border of the patella and the proximal electrode at two-thirds the length of the femur. Constant current pulses of 300ms at 40 Hz were used for stimulation. 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 higher pain rating. The average conductance of the 20 cm2 electrodes was 886 μS which 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 size electrodes. Electrode size does not influence electrical efficiency. The larger electrodes result in less pain but the largest 60 cm2 electrodes are not commonly available for muscle stimulation.
- electric stimulation