Like many people who own an iPad or smartphone, I sometimes wonder if I'm just a surgical procedure away from becoming a cyborg.

My first exposure to the idea of cybernetic enhancements in medieval fantasy was probably when I read The Drow of the Underdark, a Forgotten Realms supplement written by Ed Greenwood and published back in 1991. I was still relatively new to gaming back in the early 90's, not yet in secondary school and still with huge gaps in my geek knowledge. Drow of the Underdark didn't give rules for grafting metal limbs onto characters, but it did describe powerful mechanical devices which Drow warriors sometimes used to replace lost body parts. For a long time, this was the closest thing to RoboCop that D&D players could dream of, until 3rd edition came around with its half-golems, steampunk-inspired artificial hands, and uncountable third-party creations. One thing none of these books have ever really explored in-depth is the actual practicality of augmentic a human (or humanoid) body with shiny replacement parts... and who can blame them, when the last thing we usually want from our games is practicality? There's actually been some wonderful thought put into this question over the years by some of the greats of science fiction and fantasy, my favourite being a short diatribe against the whole thing from Warren Ellis' Global Frequency (I won't quote it here; if you want to read what he has to say, you should go buy the book, which is worth every penny). In brief: giving people metal body parts, especially ones with superhuman capabilities, is bloody complicated.

Here's the one problem which I always feel people forget about: metal is heavy. Suppose you have a character who's had one arm replaced with a snazzy artificial one, which is a common enough trope in fiction. Your average human arm isn't terribly heavy, but the same volume of most metals, even light-weight metals, is much, much heavier. In modern times, artificial limbs are usually made of plastics, which weigh a good deal less, but these plastics by and large don't have the strength to, say, punch through walls. The stronger the material you use, the denser it has to be, and while it's reasonable to assume that in a science-fiction setting someone has invented lighter-weight polymers or something, the resulting arm is still probably going to be a good deal heavier than an organic one, even before you start putting in, say, retractable steel claws. If all you do is attach one arm, the recipient is probably going to walk lopsided for the rest of their life, or develop very severe back problems. You'd presumably have to balance the creature with a similar sort of weight on the other side, which gets into additional problems... have you ever tried to walk uphill carrying an extra forty pounds? Pretty soon, you'll need artificial legs, too, or at least new knees.

The other big thing about artificial limbs which always gets me is the premise that they would grant some sort of superhuman strength. Granted, an artificial limb probably should convey some strength bonus in some respects; the fact that the limb doesn't feel pain or exhaustion probably contributes to that. The problem is, no matter how powerful the arm, it's usually still plugged into meat. The poor creature who tries to use that arm to do something excessively amazing could stand a very real risk of the arm simply ripping itself out of them. This, of course, is just one of the problems that comes from matching super-strength with human strength, and there are hundreds of essays critiquing the physics of Superman as further reading if you'd like to see more of them.

The big advantage characters have in a fantasy setting is that their cybernetic parts are less bound by physical laws; they can be created with magic to eliminate these problems, to say nothing of magic presumably making it much easier to attach a limb to a living creature's nervous system. This begs the question, though: what happens to the cyborg in an antimagic field? What happens to it when its magic is dispelled? The answers to these questions inevitably has to come down to players having some understanding of how such things might plausibly work in the real world, although in the end, we're probably forced to fall back on those two scariest words in the whole of the gaming lexicon: "storyteller's discretion." 

A little more than four years ago, Dr. Eris Lis, M.D., began writing a series of brilliant and informative posts on RPGs through the eyes of a medical professional, and this is the one that appeared here on March 2, 2013. Lis is a physician, gamer, and author of the Skirmisher Publishing LLC OGL sourcebook Insults & Injuries, which is also available for the Pathfinder RPG system.