Hi, my first post here in this great community! 
Id like to build such a device on my own and actually it seems quite simple: a few capacitor and electrodes connected to them..
But which capacitors for 5v source (like arduino) and what current?
And how to adjust the output? by changing frequency or current/voltage?
In general this should work like a real EMS but i havent found much information on how they work in the www.
Perhaps you could help me out? 
Thanks in advance,
Flok3r
I would seriously reconsider what you're trying to do here; while what your wanting to do could most certainly be "home brewed" - you could also end up with a potentially lethal device. I would also advise anyone else here who may want to give advice on how to build such a circuit to pause and think upon what it could mean, should fl0k3r accidentally harm himself in such a fashion, and his family decides to pursue legal action...
Hm, i expected that it is probably too dangerous, its a pity, because good ems's are very expensive.
But - for other purposes - im still interested, what capacitor to use and how.
Anyone knows these electrical fly paddles/rackets?
Electro shockers in general, how much current and voltage?
To stay safer with electro activities, search ebay for "slimming massager" and similar for items like below. There is techinal info on the net for EMS but it could be dangerous out side of a lab.
Wow, thanks, didnt know that these things are sold at this name too.. seems like i didnt search that good, sorry 
But i think ill buy one of these for such a low price to see, whats going on inside.
Thank you for taking the time picking up these links
I got some accidental muscle stimulation last week, reclaiming components from a single use camera 
400V - ouch.