T.rex had large powerful legs in comparison to it's puny arms. The Ornithomimids, fast dinosaurs were capable of reaching great speeds. Their leg structure is similar to modern day fast animals like the cheetah. They had smaller upper leg bones in proportion to their lower leg. However, T.rex unlike the Ornithomimids had a stubby femur and tibia which were almost equal in length. This is a strong argument that T.rex was no sprinter but with it's long strides and large leg muscles it could be said that T.rex was faster than most of it's prey.