The seven ton reptile, which was 13 feet tall and 40 feet long, was able to outpace any prey due to giant muscles located at the top of its tail.
Previously the tail had been thought of merely as a counterbalance to the weight of the beast’s giant head.
Palaeontologist Scott Persons, of the University of Alberta in Canada, said: “Contrary to earlier theories, T-Rex had more than just junk in its trunk.”
He compared the tails of modern-day reptiles like crocodiles and Komodo dragons with that of T Rex, which lived about 65 million years ago, and found the biggest muscles in all of them are attached to upper leg bones.
These ‘caudofemoralis’ muscles provide the power stroke allowing fast forward movement but Mr Persons discovered one crucial difference in the T Rex structure.
Animal tails get their shape and strength from rib bones attached to the vertebrae. Persons found that the ribs in the tail of T Rex are located much higher up.
That leaves much more room along the lower end for the caudofemoralis muscles to bulk-up and expand. Without rib bones to limit their size, they became a robust power-plant enabling the dinosaur to run.