1. You’re not supposed to feel anything actually the lower frequencies are what you feel the most. Anything having your nerves react to the frequencies doesn’t make them any more effective, just uncomfortable and usually ineffective.
2. And as stated above, you will only usually feel frequencies below around 5 kHz. Higher frequencies will not be felt when doing contact sessions through the TENS Pads because they are moving at too high and oscillation rate. Increasing the power output so you can feel it probably wouldn't help and could potentially cause harm. More is not always better.
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