Hi everyone, i have a project: controling an arm with an arduino.
Like, i push a button and my arm move, or hand.
This will be done with some electrodes, one arduino and some relays.
So my question is :
Do you know how much volt i need to put in my electrodes because i do not want to die while trying it 
?
It is the current that kills you, not the voltage.
The current is determined by the voltage, combined with electrode contact, skin and body resistance, which are highly variable and can't be predicted.
This so reminds me of a rather obscure film, Shanks (1974) featuring Marcel Marceau as a puppeteer who learned how to animate dead bodies with electrical probes.
Luigi Galvani discovered "animal electricity" by making the leg of a dead frog jump, in 1780.
First of all, don't do such things, you can harm yourself in various ways.
Next question, do you want precise activation of distinct muscles, or only activation for training muscles or burning fat?
As Galvani demonstrated many years ago, very low voltages are sufficient to make the leg of a dead frog jump. Of course the voltage must be very low, because animals and humans are not connected to a power plant, instead must create their own electricity in other ways 
The major problem is bringing the voltage to a distinct muscle. If you don't want to (let!) implant electrodes, any voltage applied to skin will activate any number of muscles or nerves, which happen to exist in the neighbourhood of the electrodes. It's quite impossible to address distinct muscles this way. Also a high voltage may be required, that penetrates skin and fat before it can reach the muscle - see the difference between a heart pacemaker (implanted) and a heart defibrillator (external).
Next comes feedback from the achieved motion, like stretching an arm may not only fatigue or overload muscles and fibers, it also may hurt if you hit a solid obstacle, or break your nose when bending the arm.
Nobody can hinder you from torturing or killing yourself, but you won't get assistance for such a project here.
Of course, your heart is a muscle too and if you can make your arm twitch you can make your heart twitch. 
A good start would be to discover how muscle toning tens machines work .
It takes only a few mA across the heart to stop it.
You neeed to be very sure you get the current path correct.
Well, that is why i think about using some electrodes, but with a fat-burning device, so it will not be deadly.I want to do it slowly, but i wanted your advice first, or to tell me if you tried it before?
Why does the image of some city boy peeing on the electric fence come to mind? :o
There are commercial devices designed to do just what you want (activate/move certain muscles to burn fat), but they are NOT simple (well the waveform they use for stimulation is very controlled and current limited). Do NOT start down this path yourself without doing a LOT of serious research into muscle stimulation or you run the risk of killing yourself or someone else.
gpsmikey:
Why does the image of some city boy peeing on the electric fence come to mind? :o
As a kid we had a party line , sometimes it was necessary to pee on it to get a connection.
And pray no one rang up while you were doing it.
Boardburner2:
As a kid we had a party line , sometimes it was necessary to pee on it to get a connection.And pray no one rang up while you were doing it.
I had heard this one before http://msgboard.snopes.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=74;t=001085;p=0 - it still isn't clear if this was real or just one of those urban rumors, but it could work that way. If I remember correctly, the ringer is 90vac superimposed on the -48v of the phone system (not sure why they use negative relative to ground -suspect it was an electrolysis thing from the past). You can definitely get a "charge" out of it though if you are touching the wires when a call comes in (want to drive somebody nuts - put a neon light (without a limit resistor) directly across the tip and ring wires - normal phone works fine, but any incoming call ionizes the neon light which "completes" the circuit for the phone company so it connects (except nobody has picked up the receiver). Effectively blocks all incoming calls. 
I forget how but the earth rod was required on a party line so one of the parties could sieze the line to make a call.
Or was it so that an incoming call could be directed to the right phone ?
Boardburner2:
I forget how but the earth rod was required on a party line so one of the parties could sieze the line to make a call.
Or was it so that an incoming call could be directed to the right phone ?
Not sure on that - I was pretty young here in the states when we had a party line for a short time. Seems to me the ring was different - double ring for one party and single ring for the other or something like that (that would have been about 60 years ago ... shortly after the old phones with the magneto and crank on them went out of style !! ) I do know if you picked your phone up when the other party was using it, you could hear the conversation.
UPDATE, i was thinking of that:
Marsat:
Do you know how much volt i need to put in my electrodes because i do not want to die while trying it
There are all sorts of wierd people in Internet forums, some of them may give answers to the most rediculous of questions.
What you need to think about, is if some of these wierd people give you the wrong advice, will you ever know ?