According to the wiki article on scabbards, the few exceptions might include Mongol horsemen, Chinese soldiers, and perhaps some groups of Celts. The rest is, indeed, modern invention:
I don’t know much about swords, but it does seem pretty impractical to have a heavy, 2-handed blade in a sheath on your back. Seems like pulling it out would be difficult.
Assuming you’re correct that swords are rarely worn like this in real life, I think it’s safe to assume that somebody thought that it looked cool, and now it’s so pervasive in movies and TV shows that people just expect to see it.
But equally, it would seem impractical for a five or six foot sword to be strapped to your hip where it drags on the ground.
Seems very unlikely that the whole “quick draw” of a sword for battle is a real historical concern, especially for large weapons. Not like you’re prancing around like a modern fencer with a six foot claymore, running various ‘named’ defenses and offenses.
I haven’t come across any depictions of knights or even mercenary groups from the middle ages, in Europe at least, that did this either.
Two handed swords (longswords) were worn at the hip. We don’t know much about the zweihanders of the renaissance, but I don’t think they wore those on their back either (they were too big for that).
I have a broadsword with a three-foot blade. A coworker asked to borrow it in the '80s, and he tried to draw it from its leather scabbard from his back. His arms were too short. (Never mind drawing a razor-sharp blade from an awkward position!) He used his experiment to disallow swords being drawn from a back scabbard in his D&D games.
Those Landsknecht Zweihänder swords were just about as big as they get, and they were carried like a WWI-WWII rifle, on the shoulder, the ricasso often having a dull or even leather covered area for this purpose.
I suppose the back might not be a bad place to keep a back-up throwing knife.
Suppose you were a Roman legionnaire and you were marching from one province to another. Were you really going to carry your sword on your hip like you were expecting battle at any minute?
Swords are awkward. They make it hard to do most everything (walking through doors, riding a horse, rescuing a leading lady). It is easier for an actor to wear a back-sword than to learn how to handle the fool thing.
You could argue that if yo were going to walk or ride long distances, carrying it onyour back could be a better option. nobody, however would go ino battle like that.
Two-handers can not be comfortably walked/rode with on your back or side. You can pick them up and carry them parallel to the ground.
Do you realize Roman swords were about one foot long? People tend to think “a sword” is the size of the one in the posters for Conan the Barbarian, but that’s like thinking that Mr Schwarzenegger himself is “of average size and build”.
A gladius was short but not that short. Most were somewhere between two and two and a half feet long.
And I think you’d be surprised how awkward even a small object can be when it’s worn on the hip. I used to carry a radio on a hip holster. It was constantly banging into corners and doorways. And I’ve carried nightsticks (which are about the same size as a gladius) and they’re a lot worse.
Scabbards would often be tied at the bottom, though (at least for short swords), which makes them less likely to… flounce about than nightsticks (which aren’t scabbarded) or even walkies.
A Roman soldier on the march would be carrying a lot of kit over his shoulder, spears, rations, cooking utensils, camping gear and his shield. I would think it would be preferable to keep your sword on your hip rather than add more weight to your back.
Actually, adding the sword to the backpack may make it easier to carry for a day’s march, but it wouldn’t be any more “battle ready” than if it was on muleback. Modern soldiers carry their weapons very differently when they’re travelling, when they’re on duty in a friendly area and when they’re on patrol in an unfriendly zone, too.
It’d be possible, though, to draw even a three-foot sword from a back scabbard so long as it actually was a “back scabbard”; that is, a scabbard modified to allow a functional sword-draw from the back. With a longsword, the easiest mod I can imagine is removing a percentage of the upper portion of the scabbard, but only the strip that would cover the upper cutting edge. The two flat sides and lower cutting edge-covering portions would remain intact for stability. Removing, say, one-third of that upper-cutting-edge bit of the scabbard would permit drawing a three-foot sword with a two-foot draw; you draw the sword up two feet, at which point the tip of the blade can swing clear through the cut-out side of the scabbard. Seems like the cut-out would make it easier to re-sheathe one’s sword, as well.
That’s just what I can picture, though; I’ve never seen or heard of such a thing in fact being created or used.
The other advantage is that you can see the sword in a head shot. This way, the director can you remind you of the character’s manly warriorness even when he’s showing his tender side.