Tyrannosaurus rex, the killer dinosaur that lived about 85 million years ago, was not the swift hunter depicted in Hollywood movies. Instead of chasing its prey at speeds of up to 45 mph, as some studies have suggested, these dinosaurs may not have been able to run at all.

?These animals were no speed demons,? says John Hutchinson of Stanford University. He specializes in the evolution of movement and says the science of how animals move shows that big creatures do not move quickly. At about 40 feet long, up to 20 feet tall and weighing about 13,000 pounds, Tyrannosaurus rex was very big.

Hutchinson and Mariano Garcia, of Borg-Warner Automotive in Ithaca, New York, have created a computer program to analyze animal motion and figure out how fast large dinosaurs could move. ?It has been known for a long time that as things get bigger, they don?t move as fast relative to their size and in fact as they get really, really big, they can?t run at all,? says Garcia. ?But until now, no one that I know of has tried to predict the cutoffs, which is what we are doing.?

Hutchinson and Garcia included the impact of posture, center of mass, leg weight, total weight and torque, the twisting force that muscles need to apply about the joints, into their program. They tested its accuracy by using data from living animals.

?Our model shows that these really fast speeds of 50 mph and probably down to even 25 mph just don?t hold up when you really scrutinize them and look at the physics,? Hutchinson says. ?It doesn?t make a lot of sense that these animals could go that fast. There?s really no good evidence that they could.?

Fossils of smaller dinosaurs indicate that they moved fast but now we know that large predators couldn?t catch up with them. If we were stuck in ?Jurassic Park,? we could outrun them too.

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