Here's a keyword that'll get you on your way: myoelectric.
It's the basis of the technology that a lot of sensors in the field act on. Google it up.
Six months or so ago I talked to some folks from Hanger Orthopedic Group who use this technology for exactly what you're looking into. Here's the hand they're working on http://www.hanger.com/sections/po/po_ilimb_hand.html
The sensors can only detect larger muscle groups if you're applying them on the surface. The only current way to get really fine muscle control (small muscle bundles actuating the sensors) is to have a trans-dermal implants. I work on the medical device industry and I'll tell you right now trans-dermal is limited to 3 days max by the FDA. So while it's the best resolution, you have to replace the entire sensor network every three days (and relearn how to use it!) so it's been not exactly... useful.
I did try out the large-muscle detectors and just by having the sensors touching my skin I was able to actuate a fully articulated hand. It's pretty amazing stuff.