A distinguishing feature on T.rex was it's abnormally small forelimbs. These arms were only three feet in length and could not reach the mouth, so they were not used directly for eating.
Using muscle scars on T.rex's bones it is possible to reconstruct the muscle size and structure of the animal. The biceps attached far down the forearm providing more leverage. It can be estimated that these strong forearms could lift 200 times that of a human arm. The teres and latissimus dorsi shoulder muscles were very large and would have made T.rex's three foot arms appear even stubbier on the forty foot body.