Abstract
Quadriceps femoris muscle torque was measured in 40 subjects during transcutaneous neuromuscular electrical stimulation (NMES). Three different electrode types (carbonized rubber, sponge, and adhesive) were used on each subject, permitting determination of the factors that influenced the magnitude of quadriceps femoris muscle torque induced by NMES. This goal was accomplished by entering the various factors into a multiple-regression model. The electrodes differed significantly in their characteristics. The carbonized-rubber electrode delivered the greatest current with the lowest impedance, resulting in the highest knee extension torque. We found that the most important factor in determining torque generation level was the quadriceps femoris muscle’s intrinsic ability to be activated (as opposed to electrode size, current, current density, or skin impedance). These data suggest that NMES efficacy is primarily determined by the intrinsic tissue properties of the individual (defined in this study as “efficiency”) and is not dramatically changeable by using high stimulation currents or large electrode sizes. The precise physiological basis for interindividual differences in efficiency is not known. [emphasis mine]
My comments:
This was one of those things I figured out the hard way during my year of electric stimulation, only to find research from 1991 had already been there. These researchers compared carbonized rubber electrodes to sponge electrodes and the sticky self-adhesive gel electrodes that generally come free with an EMS or TENS machine.
The results of the sponge electrodes were about as good as the sticky (that’s what I call them) electrodes, but the size of the sponges were about half the size, so I don’t think they got a fair comparison. Since sponges always look dirty to me, I don’t use them anyway, so no great loss. The big difference (which I definitely agree with and think I can feel myself) was that the rubber electrodes, for which the size was comparable (109 cm square rubber, vs 104 cm square sticky), were almost twice as good as the sticky ones.
Under otherwise identical conditions skin impedance with rubber was less 53 vs. 98 k-ohms (84.9% better), percent of maximal voluntary contraction was 21.2% rubber vs. 12% sticky (76.7% better), and subsequent quadriceps torque produced was 33.5 vs. 17.9 N-m (87.5% better). The idea being that skin impedance is about half as much with the carbonized rubber electrodes, making the electric stimulation about twice as comfortable, so you can activate almost twice as much muscle, producing almost twice as much torque, thus generating a better training effect. Their findings were something I largely figured out on my own, but it was good to see numbers and an explanation put to it.
If using the electric stimulation for pain control (TENS rather than EMS), the rubber carbon electrodes should still let you more comfortably handle higher levels of electric stimulation, and I would expect greater reductions in pain per a higher dose of gate control theory. This would in turn go along with other researchers’ findings where EMS reduced pain better than TENS. Where EMS is not designed for pain control, but it’s still a larger dose of mostly the same thing.
Other reasons I like the carbonized rubber electrodes are that they are easy to clean between applications, while the sticky electrodes just look like bacteria traps. The carbonized rubber electrodes work fine, even if with greater amounts of body hair, and the carbonized rubber electrodes seem to last indefinitely. Longer than a year with repeated daily use thus far. Whereas the sticky pads don’t last near as long. However, a periodic spray with a water bottle helps extend the life of the sticky pads.
The downside of the carbonized rubber electrodes is that they don’t stick to you so they have to be held in place with straps, which depending on the location can be lesser (legs, arms, core, feet) or greater of a challenge (chest and shoulders). Still, for clinical use or home for me and pretty much every patient I get an EMS machine for, carbonized rubber is THE way to go. My preferred size for most uses is ~4 inch or 10 cm diameter circular pads. For conductivity, I just spray them dripping wet with tap water; no gel or special spray necessary.
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