For a long period of time it was believed that T-Rex was a very slow giant, but when a recent study was made in consideration to Rex' legs, the result was not what was expected. Palaeontologist Phil Currie studied the proportions of the hind legs of T-Rex; the study revealed a strange fact. Proportions of the individual bones of hind leg gives us an idea of how fast an animal could be. Fast running birds such as Ostriches have short upper leg bones or the femur and longer shin bone or the lower leg bone. In T-Rex we come across the same patterns, lower leg bones are long or longer than the upper leg bones. When we see the anatomy of the hind legs of T-Rex, inspite of its huge size it was built for agility, Phil concludes.

But Jack Horner too studies the same and presents a different answer, he finds T-Rex to be a walker. When we look at the proportions of the hind legs of a fast moving predatory dinosaur by name Troödon, we see that the lengths of the bones are not same. We see that the thighbone is shorter than the shinbone, and this is true in each and every non-tyrannosaurid carnivore. We see the same in Ostriches and all the other running birds. The primitive function of the legs was running that means you have to evolve legs for walking. In T-Rex the thighbone and the shinbone have equal length or the thighbone is slightly longer than the shinbone. These are the features that are good for walking and not running, says Jack.

A fossilised footprint discovered in New Mexico has been made a guarded secret due to its uniqueness. The footprint was probably made on the banks of the river, where the soil was damp enough to make a print. The soil hardened up and was washed away by floods, later wet sand deposited over the print forming sandstone cast. Examining the footprint it can be drawn out that the animal running to make a 3 foot long, 2 feet wide and a foot deep footprint, would weigh a minimum of 5 tonnes, and that would be T-Rex. The footprint is sitting on a 9 feet long block and bares no other footprint on it; this shows that the distance between each footstep would be atleast 9 feet. With this, we calculate the minimum speed of the animal to be as much as 11km/h.

Birds are one of the many animals closely related to dinosaurs. Their movement can also shed light on the movement of dinosaurs. Jim studies the similarities between T-Rex and modern Emus. Jim describes that Emus are swift walkers, they don't hop like kangaroos, but put their feet one in front of the other. He finds the same in T-Rex too. X-rays reveal that the presence of a tail can make a great change in the movement of an animal. But modern birds have no tail, so scientists study crocodiles, as they have a long muscular tail, much like T-Rex. Crocodiles and birds have distinct ways of walking, but in T-Rex there is a combination of bot, it has crocodilian hips and bird like legs, which isn't present in any of the living animal today.