Scientists: T-Rex couldn't move fast

New research suggests the fleet-footed T-Rex portrayed in movies such as "Jurassic Park," above, may not have been able to run at all.

STANFORD, California (CNN) -- New models of the leg muscles of Tyrannosaurus Rex suggest that a real T-Rex might not have passed the screen test for "Jurassic Park." Stanford University researchers writing in the British journal Nature this week suggest that a T-Rex could not have been able to run as fast as the one in the movie -- and might not have been able to run at all.

"There is no way you could fit enough muscle into its body for that kind of locomotion," said John Hutchinson, co-author of the Nature article. "You wouldn't have enough room left over for all the other body parts."

The speed of T-Rex has been debated for years. Some paleontologists compare the dinosaur's long, slender-looking legs with those of living animals such as ostriches and horses, and argue that the dinosaur ran at a top speed of 45 miles per hour.

That would fit the scene in "Jurassic Park," in which a T-Rex keeps pace with an escaping jeep.

VIDEO CNN's Jeff Flock visits Chicago's Field Museum for a look at the latest theories about the speed of tyrannosaurus rex (February 28) Play video (QuickTime, Real or Windows Media)

The Stanford researchers thought that estimate was much too fast considering the dinosaur's enormous size, and believed anatomy comparisons alone were not producing accurate speed estimates. Using biomechanics, they created a computer model to analyze how much leg muscle mass is necessary for running.

"It is a simple model, although realistic enough to capture the principles of locomotion," said Hutchinson. "First, you have a stick figure model, with a bunch of segments joined by joints. Then you assign weights to those segments and compute the physics of the posture. When the foot is joined to the ground, you can compute the forces."

The computer model showed that in order for T-Rex to run 45 miles per hour, as much as 86 percent of its weight might have had to be leg muscle mass. That, according to Hutchinson, is ridiculous.

Hutchinson said a general principle known about animals for decades is that a creature's body weight increases faster than does the force muscles can exert. In other words, the ability of muscles to exert force at very large sizes is outstripped by the actual weight of the body that those muscles need to support.

"As animals get bigger, their muscles have to be bigger and bigger to support their own weight. But adding muscle adds weight, and eventually something has to give," Hutchinson said.

Most biomechanical research on locomotion has focused on animals up to the size of a horse, according to Hutchinson, and very little is known about big animals today. He is currently working on the biomechanics of elephant locomotion, and he hopes that biomechanical modeling will reveal insights into how living big animals work.