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.
Actually, required size of the transformer drops with the frequency. Mains switching power supplies still use regular transformer for isolation which run on comparatively high frequency and thus can be very small.
50/60Hz mains is motivated by compromise between visible flicker from lightbulbs and required frequency for practical synchronous motors with useful rotational speeds (some electrified rail systems use lower AC frequencies because they are better match for motors). Also, significantly higher frequencies will cause even relatively short mains runs to behave like a (RF) transmission line and also cause more significant skin effect.
Agree on that generally for electical equipments size falls off as square of frequency, the main motivation was the light bulb flickering 50/60 times was close to human persistence of vision which allowed the light bulb to appear constant.
Sorry, only eink kindles are being sold to Canada, and if I bring one in, software portion of device de-activates after 30+ days of being outside of the US, So thus no comparison unfortunately.
