See
Electrical stimulation of excitable tissue: design of efficacious and safe protocols
By Daniel R. Merrill, Marom Bikson, John G.R. Jefferys
PDF full text available from link here: https://scholar.google.com/scholar?hl=nl&as_sdt=2005&sciodt=0%2C5&cites=10867594465599094979&scipsc=&q=Electrical+stimulation+of+excitable+tissue%3A+design+of+efficacious+and+safe+protocols&btnG=
For a discussion of empirical evidence on minimum carrier wave frequency to avoid stimulation induced tissue damage based on forumlae in the the above, see:
https://github.com/diglet48/restim/wiki/estim-safety
See especially this quote:
"For a sheet of omega rubber ( 9 cm 2 ) placed on the shaft, k = 1.7 gives a damage threshold of 21.2 μ C / phase . The highest signal intensity I saw for this electrode position is Q 0 = 7.4 μ C . This suggests the carrier frequency should be at least 755 hz to fall in the "safe" region.
For the sheet of 9 cm 2 omega rubber, I've found that signals k = 1.4 result in 1-2 days of skin irritation, repeated exposure of such signal can cause what looks like thermal burns slightly under the skin, which will take a few days to surface. signals k = 1.10 result in a few hours of skin irritation. Lower values generally do not result in skin irritation."
Higher frequencies mean lower k values. I do not know how to calculate the frequency based on the formulae he gives though? Anyone know?