"At mid-stance during fast running, a running biped pushes against the ground in order to support itself, exerting a force that is proportional to the animal's weight and the relative amount of time that the foot is on the ground," said Hutchinson. "In turn, the ground pushes backupwardson the leg with a vertical force."
That vertical force, called a ground reaction force, is about 2.5 times body weight during fast running.
Hutchinson and Garcia's model calculates the ground reaction force along with other necessary parameters such as limb lengths and posture to determine the amount of muscle mass (as a percentage of body mass) that would be needed to create an equal and opposite force to maintain equilibrium.
Since the force a muscle can generate increases less rapidly than body weight, the muscles of larger animals generate less force per unit of weight than the muscles of smaller animals, Andrew Biewener of Harvard University explained in an accompanying article in Nature.
Applied to living animals such as chickens and alligators, the model verifies that chickens are good runners whereas alligators cannot run on two feet. Applied to what is known about dinosaurs, it shows that large dinosaurs were probably not fleet of foot.
"Our research doesn't show that the larger animals had to be totally sluggish and restricted only to slow walking, but it does rule out that they could run at extremely high speeds," said Hutchinson.
The researchers say there are too many unknowns about the biomechanics of dinosaur muscles to say exactly how fast T. rex and other large dinosaurs could run. The computer model estimates a top speed between ten and 25 miles (16 and 40 kilometers) an hour.
While this is much slower than the speeds depicted in movies and suggested by some paleontologists, humans would still require some fast thinking to escape T. rex's wrath. "Because of scaling reasons, even when walking, [T. rex] moved fast by human standards," said Garcia.
Not So Fast
Movie depictions of dinosaurs as fleet-footed creatures is rooted in the "hot-blooded-dinosaur" theory advocated in the 1960s and 1970s by John Ostrom, an emeritus geologist at Yale University, and Robert Bakker, a freelance paleontologist who works in Wyoming.
Ostrom and Bakker theorized that birds evolved from dinosaurs and therefore dinosaurs were probably hot-blooded like their bird descendants. Artists, in turn, began to depict dinosaurs like birds, which are fast and active. So in Jurassic Park, T. rex was shown chasing a car traveling at 45 miles (72 kilometers) an hour.
To scientists seated in the theater, and even some animators working behind the scenes, the scene was not intuitively correct. The computer model designed by Hutchinson and Garcia adds to a list of studies that explain why.
Scientists such as Julia Day of the University of Cambridge study fossilized footprint tracks of dinosaurs to discern their speed. In a paper published in the January 31 issue of Nature, Day and her colleagues reported on a dinosaur footprint from a quarry in Oxfordshire, England, that shows dinosaurs could run at speeds upwards of 18 miles (30 kilometers) an hour.
This speed is at the upper end of Hutchinson and Garcia's estimate for the largest of the dinosaurs, and the track was probably left by a dinosaur three to five times smaller than T. rex, said Hutchinson. Keeping the size difference in perspective, Day's estimate is plausible, he said.
A 1995 study by James Farlow, a paleontologist at Indiana-Purdue University in Fort Wayne, Indiana, concluded that the strength of T. rex's thigh bone relative to its body mass was not strong enough to support fast running. That conclusion, said Farlow, is supported by Hutchinson and Garcia's research.
Farlow's paper also shows that T. rex was at serious risk of injury if it fell. "Because the chance of a fall would be greater at high speeds than while walking, we suggested that Tyrannosaurus might not have been smart to run quickly, even if it was structurally fit to do so," said Farlow.
The addition of Hutchinson and Garcia's work to this debate is a welcome perspective, said Biewener. The study of how fast dinosaurs could run will always "be an uncertain science, dependent on the availability of fragmentary, long-dead material," he said. "So it is welcome when new analytical approaches such as this are brought to bear on such problems."
Recent National Geographic News stories on dinosaurs:
Researchers Melt Polar Dinosaur Mysteries
Scientist's Finds Spur New Thinking on Dino Evolution
Dino-Era Vomit Fossil Found in England
Study Paints New Picture of Dinosaur's Nose
Skeleton of New Dinosaur "Titan" Found in Madagascar
"Tidal Giant" Roamed Coastal Swamps of Ancient Africa
"Feathered" Fossil Bolsters Changing Image of Dinosaurs
Oddly Angled Teeth Make Masiakasaurus Stick Out
Additional dinosaur resources from National Geographic:
Paul Sereno: NG explorer-in-residence and dinosaur hunter
Destinations: Dinosaur National Monument
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