The Auvon AS8016 TENS/EMS unit produces bipolar pulses with no net DC offset, so the UI controls the negative and positive amplitudes equally. The range has 20 steps, with the screen shots here set to 10 units. The actual output voltage depends on the mode, with some modes producing a peak voltage well above the others at the same UI setting.
It’s worth noting the effect comes from current passed through skin and muscle, rather than voltage applied to it. The test setup uses a 500 Ω resistance to make the current vary linearly with the voltage (which is definitely not the case with human bodies): a 20 V pulse passes 40 mA through the resistor:
The simplest bipolar pulses always start with the negative phase. The shortest pulse width is 100 µs:
And 150 µs:
And 200 µs:
Up to 250 µs:
Some modes have a short zero-voltage pause between the negative and positive phases:
The pause can be the same duration as the negative and positive phases:
Some modes have pulses starting with the positive phase, others switch the leading phase during the course of the output modulation.