The paradoxical arms of T. rex

The puny size of Tyrannosaurus arms is so much a part of it's iconic image there is a Tumblr dedicated to things that T. rex would have been unable to do, because of it`s short arms (T-Rex Trying). Paradoxically the arms of a T. rex are actually quite robust and would have been rather muscular in life. 
So were those puny arms merely vestiges inherited from a longer armed ancestor, or were they actively used.. and if so, what for?

T. rex head & arms (original image from Wikipedia, by user FunkMonk)

More than just a Bone-crunching Bite

Tyrannosaurus rex had the most powerful bite of any land animal that ever lived, but recent research shows that their was more to it's bite than brute force alone.

Image via Paleoblog

That T. rex had a powerful bite had long been known, a mechanical test of it's biting strength done by Paleontologist G.M. Erickson and engineers from Stanford University built a mechanical rig to replicate the teeth marks found on fossilized T. rex victims. They found a minimum bite strength of 3.300 pounds (~16.000 Newtons).
A recent computer analysis put the bite force even higher, with a minimum bite force of 20.000N and a maximum estimated  at an astonishing 57 thousand Newtons. And it should be noted that because of the differently sized teeth of Tyrannosaurs the bite force would initially be applied only to the tallest teeth, which would punch their way into the flesh (and any bones they happened to meet) ahead of the rest.

There is however evidence that T. rex wasn't optimized for brute force alone, and it comes from yet another computer analysis of the T. rex skull.