I would recommend you do work in mechanical engineering and/or controls, with as many electives as possible in physics, biology, and anatomy. While you do need a good understanding of the human body, you have to be able to replicate the muscles, joints, and nerves with mechanisms - and you need a deep understanding of mechanical design to do that elegantly.
If you want to see some really interesting stuff, check out
iWalk (who recently got $20M in venture funding for some seriously amazing robotic prosthetics), or
Hugh Herr's website from the MIT Media Lab. Look at the sort of work that's been done there, the background of the people who've worked with him, the background of the people at iWalk, and the job descriptions for companies that do the sort of thing you're interested in. That may give you a better idea of what you need for this field.