There are a number of reasons one would experience shocks and a number of ways to avoid it. Please review to see if this applies to your situation.
1. Low Frequencies. Anything under 10000 Hz is very likely to cause a reaction due to this frequency range having a high probability to react with your nervous system, causing involuntary muscle contractions.
Remedy: Use Reduce Amplitude < 10 kHz setting as depicted in the User Guide.
2. High Voltage. This couples well with point #1. There has to be enough current to also engage the nervous system; however, there also has to be enough voltage to penetrate the resistance of the skin. When the frequencies are low, there is a way to find the happy medium.
Remedy: Use Amplitude Ramp 5 Seconds (UP/Down) to gently start and end each frequency. Recommend one uses this with Reduce Amplitude < 10 kHz settings as also depicted in the User Guide.
3. I still feel too much when applying both #1 and #2.
Remedy: Plug into the CS port of the Boost. This has a resistor that will limit current, yet still, provide a proper contact treatment.
4. Wired to more than one generator. Current from multiple generators finding their pathway back into one generator.
Remedy: Do not use more than one generator in contact mode at a time.
5. The possibility that the TENS pads are no longer good. A small area of contact to the skin will result in an increased sensitivity to the electrical current flow.
Remedy: Replace TENS pads.
In the unlikely event that your generator has somehow started to short circuit, which can be verified by contacting customer support, they would be able to further address the issue should none of the above apply.
Authored by: Jeff Kaczor