If you read the link given, you'll see that EMG signals are only in the 20-30 mV
maximum range, so you need an amplifier to boost the signals into the range
that Arduino can use = 0-5V. So, figure an amp gain of 200 or so.http://en.wikipedia.org/wiki/Electromyography
Also, you have to be careful when hooking up anything to the human body that
uses voltages, so professional amps have isolation circuitry and/or run off batteries,
etc, to preclude electrocuting the humans. Also, since the signals are so small, you
have to design the ckt to reject extraneous noise sources, like 60-hz mains, etc.
Also, the electrode contact potentials with the skin drift over time, so you need to
take that into account too in the amp design. On and on. You can probably
google for EMG amp design.