It was big and it was mean, but Tyrannosaurus rex wasn't fast. In fact, the dinosaur we most love to hate may not have even been able to run at all. So say researchers writing in the current issue of the journal Nature. Contrary to previous suggestions that the beast could reach running speeds of up to 45 miles per hour (mph), the new findings indicate that such swiftness would have been biomechanically unfeasible.

To test the idea that T.rex could move quickly, Stanford University postdoctoral researcher John Hutchinson and engineer Mariano Garcia of Borg-Warner Automotive created a computer model capable of estimating the amount of leg muscle a given animal needs to run fast. After verifying the model's accuracy using living chickens and alligators, the investigators plugged in the parameters for T. rex. They found that in order for the beast to run 45 mph, it would have needed supportive muscles in each leg amounting to nearly 43 percent of its body weight. "That is ridiculous," Hutchinson declares, "because it would leave very little room for anything else in the body--a skeleton, other muscles, et cetera!"

The findings shouldn't necessarily come as a surprise. "When you get down to the science of how animals move, relatively speaking, big things really don't move fast," Hutchinson points out. Further illustration of this point comes from the team's work on chickens, which are not only good runners but also one of the descendants of dinosaurs. Using the computer model to scale a chicken up to the size of T. rex (that's a 13,228-pound chicken), Hutchinson and Garcia showed that such an animal would require about 99 percent of its body mass in each leg as muscle in order to sprint. "That's far more than is possible," Hutchinson observes. "A giant chicken could not even walk."

Exactly how fast T. rex could move remains unknown, but the team proposes that roughly 11 mph--the pace of a leisurely stroll for these behemoths--might be a more reasonable estimate. In any event, even a ponderous T. rex could still hunt: body size would have likewise limited the ability of its large potential prey animals--duckbill dinosaurs and Triceratops among them--to flee.