[Wumpspawner]

The composite longbow was a 6' monstrosity, stiffer than hell, not the modern compound bow you can buy in any sporting goods store. But while it did require some strength to pull, one didn't have to have superhuman strength. If that was the case, I'm sure it would have never hit on so good. So yes, that rule is pretty unrealistic. But, in a world where men can throw flame from their hands, and survive an axe cleave from a 20' tall humanoid, or better yet, where a 5 ton reptile can fly and spew flame or acid or other such abrasive stuff from it's mouth, who needs realism, you know?

Archaelogists can recognize the skeleton of a professional longbowman by the unnatural twist to the spinal column caused by the strain of pulling back the bowstring a few thousand times. So maybe you'd need an 18 strength to prevent back injuries.