The king of the Cretaceous Era, Tyrannosaurrus rex stood on two powerful hind limbs and terrorized potential prey with its huge size and lethal jaws. The dinosaur was big and bad, but was it also fast? That has long been a topic of scientific debate, with some paleontologists arguing that T-rex ran at a top speed of 45 miles per hour and others suggesting a more moderate 25 mph. Both estimates seemed too fast to John Hutchinson of Stanford (Calif.) University, who, as a graduate student at the University of California-Berkeley, set out with help from postdoctoral research Mariano Garcia, now of Borg-Warner Automotive, to test them using principles of biomechanics.
The research created a computer model to calculate how much leg muscle a land animal would need to support running fast. They determined that a T-rex probably could not run quickly. in fact, hindered by its size, it may not have been able to run at all. Although not enough is known to give an exact speed limit, a range of 10 to 20 mph is possible, they indicate.
"When you get down to the science of how animals move, relatively speaking, big things really don't move fast," explains Hutchinson. When small animals move quickly--rabbit jumping, monkeys climbing, birds flying, or cheetahs sprinting--they endure high physical forces for their body weights. Such forces are biomechanically impossible for large animals. Aquatic animals, such as whales, are less limited than land animals, like elephants, because water buoys them.
Skeletal muscle is built similarly in all vertebrate animals. The force that it can exert depends on its cross-sectional area--that is, two factors: muscle length and width. However, an animal's weight, or body mass, depends on three factors: length, width, and height. The math behind that physical reality results in limitations. "That's why, as animals get really enormous, eventually to support their weight, their muscles have to be bigger and bigger and bigger," Hutchinson points out. "But as they get bigger, they add more mass. So you run up against a problem as animals grow larger in that they need to be adding more muscle cross-sectional area to support their own weight, but the mere fact of adding that muscle adds weight. Eventually, something's got to give. No one's really ever tried to look at, or barely thought about, how much muscle a huge animal like a T-rex would need in order to run quickly. A lot of discussion has been over the bones--were they strong enough?--or other lines of evidence. But the main question to me is, could the muscles generate enough force to support the body during running?"
Finding an answer was tricky, as the researchers were studying something they couldn't observe directly. "We're looking at extinct animals, which we know very little about, and we're trying to understand their locomotion, which we have almost no evidence of directly," Hutchinson notes. …