most back mounted weapons are fastened to the user through the usage of a Baldric. a peice of gear which has been used since ancient times to hold weapons to an individual.
these weapons where slung across the back for ease to carry on long marches and distant travel. however, because of the design of most baldrics, these weapons where often drawn and put away much like a hip mounted scabbard. When holstered, the belt was shifted or placed so that the blade was slung across ones back for ease of travel.
a Point to make.
most all ancient and Midevil scabbards, baldrics, and holsters where not designed to have the weapon slid into and out of them constantly. It was intended to hold the weapon during the times between battles, If any threat was present soldiers would already have their weapons drawn and would not put them away until all threat was gone.
the usage of a scabbard as you see in video games today represents a modern concept of combat, in that conflicts are resolved quickly. this is just not true, most ancient and midevil conflict would last Days, not hours. Sword fights between professional Plate clad soldiers could last up of 10-20 minutes before one is inevitably slain.
But this level of realism just simply isn't very fun.
better example
Look at fighting games like street fighter and mortal combat...
you have 3 rounds of 60-90 seconds.
look at Boxing or MMA fighting.
you usually have 3-5 rounds of 3-5 minutes.