Its been a while since I tool biomedical electronics, however suspect the current research is still fairly much the same.
Muscles are basically ionic pumps. when electricity moves through the skin this transfer from electric to ionic energy happens. originally sodium chloride (salt water) was used to help with this transfer with some of the first ecg machines.
The frequency that current can be highly felt (and and also interact highly with the heart) is between 50 and 60 hz.
I remember a discussion in class that happened during a lab where the students were wondering why 1000hz wasn't used instead since the chance of biological interaction would have been much less dangerous. At the time I believe the ad-hoc conclusion that was arrived at was that until switching power supplies were invented the transformers for that frequency would have been too large and expensive comparatively and motors and lighting of the day would have required these kind of transformers to work properly.