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)

Arm musculature
Recent research on the musculature T. rex arms was done by Lipkin and Carpenter. Interestingly they start of by pointing out that no Theropod would have been able to bring it's manus to it's mouth. Perhaps part of our obsession with T. rex arms is that for us as primates hands are our most important means of interacting with the environment (and to bring food to our mouths).
This is also suggestive of a certain style of predation, the mouth was most likely the primary weapon for most Theropods (unlike for example felines which primarily attack using their paws).
Their actual research focused on creating a model of the musculature of the T. rex they found that the forearm of was both powerful and capable of resisting strong forces and moving quickly, which clearly suggests that they could have been used during predation.

Tyrannosaurus forelimb anatomy (image from Wikipedia, by user Conty)

Evidence of use
In their overview of Tyrannosaurus pathologies Rothschild and Molnar note that stress fractures affecting the manus are common in T. rex, since such stress fractures occur when large forces are acting on the hand this is evidence that they were actively used. Because such forces are unlikely in routine behavior this is highly suggestive of the arms being used against struggling prey.
Lipkin and Carpenter likewise note that injuries to the furcula (the bone that connects the shoulder blades in many Theropod Dinosaurs) are most likely to be the result from predator-prey interaction.


What use are small arms then?
As seen in the previous paragraph there is good evidence that the arms were actually used when catching prey, although Lipken and Carpenter note that "because of the small size of the forelimb relative to the body size it is unlikely that Tyrannosaurus would use the manus for striking prey".
More likely is that T. rex used it's claws like hooks, sinking them into the flesh of the prey to hold it in place, while the jaws did the grisly work of finishing off the victim.

T. rex rising (image from Wikipedia)

A completely different use for the arms has also been suggested. It has been speculated by Stevens, Larson, Wills and Anderson that T. rex might have used it's arms to rise from a resting position.
While they have convincingly modelled this using computer simulation, this remains highly speculative, very little is known about Dinosaur resting positions. Just because a Tyrannosaurus was physically capable of rising from a resting position by pushing itself upwards doesn't mean it actually slept on it's belly. For all we know T. rex may have only ever rested standing up (for some fascinating speculation on Dinosaur sleep and sleep posture see: Dinosaur Conundrums - How did Dinosaurs Sleep by Mark Wildman).




Sources
All the articles quoted were published in Tyrannosaurus rex - The Tyrant King by Larson and Carpenter (editors)