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No. 485:
Fleeing a Dinosaur

Today, we outrun a dinosaur. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

It's unlikely you'll really ever to have to outrun a dinosaur; but suppose you did. Could you? Questions like that are deadly serious business for Neill Alexander at Leeds University.

You see, there's a lot we don't know about joints -- about ankles, wrists, and knees. Is springiness part of their motion? Or are they just hinges that go where our muscles put them?

Both Alexander and a local biologist work with the Zoo. When an animal dies, the biologist gets it first and flays off the fat. She studies the way mammals use their fat. Then she sends the lean meat and bone to Alexander. "We have a Jack Sprat relationship," he murmurs quietly, as he cuts in with hacksaw and scalpel.

Occasionally he lays his hands on a human limb. But animal size presents a baffling array of questions. To understand humans, he must bracket them. He studies mice on the one side and hippos on the other. That's where the question about fleeing a dinosaur comes in. Naturally, his lab -- his charnel house -- includes dinosaur bones.

But let's look at the question of springiness first: Alexander prides himself on the simplicity of his methods. The only hi-tech instrument in his lab is a tension-testing machine. He uses it to measure elasticity in tendons and ligaments. The human foot, it seems, is really a big spring. It returns 70 percent of the energy we put into it, but only when we run. That's why we shift to a run when we want to go more than 5 miles an hour.

Running is a series of falls. It's fall and jump, fall and jump. Running means using gravity, and gravity doesn't treat an elephant the same way it treats a mouse. Horses are especially interesting. A horse breaks from a walk into a trot to capitalize on the spring of his ankles. But then he shifts gears a second time. He breaks from a trot into a gallop. Why?

Alexander found the answer in films of galloping horses. A galloping horse flexes his back. The tendons of the back form another huge spring. At 11 miles an hour, the horse begins using it to conserve still more energy as he runs.

So what about that Tyrannosaurus rex who's chasing you? His legs are three times longer than yours. But he cannot leap. His bones can't take a fall. You could almost surely get away from him. Next time you have such a nightmare, you can rest easier.

And we see that the question isn't frivolous after all. Alexander tells us what any good experimental scientist knows. The world is a strange and wonderful place. If we mean to understand reality, we must be willing to test it all the way -- to absurdity.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Kunzig, R., The Man Who Weighs Dinosaurs. Discover, Special 10th Anniversary Issue, October 1990, pp. 76-83.

The measure of dynamic similarity for running animals is the Froude Number. The Froude Number is the velocity of motion divided by the square root of the product of the acceleration of gravity and some characteristic length of the animal. Two animals in motion can be compared when their Froude Numbers are the same. If your motion is to resemble the motion of a dog, then, because you are longer, you have to run faster than the dog.