Which body part?
The spine, just above the shoulders. Every nerve runs through it. The current sent through it can be altered at will, so the limbs will correspond with the command given.
"Run for an hour and don't feel tired, while the bloodflow...etcetcetc."
Althrough aesthetically unappealing (A metal piece jutting out from behind your neck) its efficiency is unparralelled!
In my own fictional universe, anyway.