I'm a software guy so this isn't my area of expertise; I can't give you a numbers answer. The motors are quite powerful though, they can easily break bones, so user-safety was an important concern (especially since most users of the devices had no feeling and wouldn't know they'd been injured right away). However, in general terms - you command the motors to go to a particular position and stop; in zero-G, that would be the end of the story, but under gravity, there will be "sag" or "slack" in the system - it moves a little bit. It takes very little difference between "where I am" and "where I believe I am", to affect the machine's balance. Every linkage adds a bit more "looseness". As far as I know, wires were never even considered, because it's a pretty high-spec'ed wire that doesn't stretch at all. Hope that makes sense.