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.
Bibliographical noteFunding Information:
Alon  studied three sizes of electrode from 9 to 81 cm' using a high voltage galvanic stimulator on the quadriceps femoris muscles of normals. He determined that the larger electrodes were more tolerable and produced less pain. Using a high voltage galvanic stimulator with different size electrodes will result in undetermined changes in the current and effective pulse width because the output capacitor is typically discharged into the nonlinear impedance of the electrodes and body tissue. Since the peak current and the shape of the pulse affect the response of excitable tissue, it is difficult to extend these results beyond the exact experimental stimulator-electrode combination used. In a later report , Alon concludes Manuscript received February 25, 1993; revised April 2, 1993. This work was supported in part by a Grant from the University of Minnesota Hospital and Clinics. The authors are with the Department of Physical Medicine and Rehabilitation, University of Minnesota, Minneapolis, MN 55455. IEEE Log Number 9209416.
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- electric stimulation