All waveforms except a sinus waveform.
Generally a square wave.
There seems to be positive experiences with inverted sawtooth.
A (optimal) sine wave with a frequency contain only this one frequency and nothing else.
All other waveforms produces harmonics. They contain multiplies of the main frequency (but with lower amplitude/power as the main frequency.)
If you want that the electric current went through the cell itself you need a high frequency (> 100 kHz)
Lower frequencies went only around the human body cells.
Although you use a low frequency square wave it contain these high frequency (within the harmonics at a lower amplitude/power).
This may be the difference that a square wave has a higher impact if you use it.
If you want to see what i mean, you can use this Audio Spectrum Analyser: http://techmind.org/audio/specanaly.html (needs no installation, only one file)
This works over your soundcard, mainly over the mic input (very sensitive input so use an output amplitude of 0.01 V if you want to experiment with this. Higher Amplitudes will maybe generate distortions and a false measurement. Depends on your hardware. Also use a shielded cable if possible.)
For more details, please check the link from: