By Matt Kaplan of Nature magazine

The image of a Tyrannosaurus rex racing after a jeep in the 1993 film Jurassic Park inspired a generation's ideas about the extinct predator, but for decades studies have concluded that dinosaurs could not move quickly.

An analysis now suggests that although big dinosaurs are unlikely to have been able to run, the animals could instead have reached a fair clip by power-walking.

Heinrich Mallison, a palaeontologist at the Museum of Natural History in Berlin, presented his theory last week at the Society of Vertebrate Paleontology's annual conference in Las Vegas, Nevada.

Mallison takes issue with a key formula used in previous studies to deduce that dinosaurs moved slowly. This equation--first used by the zoologist R. McNeill Alexander in 1976--is based on observations of modern mammals and terrestrial birds. It assumes that the faster an animal walks or runs, the longer its strides will be.

Palaeontologists apply the formula to fossilized dinosaur tracks, to calculate the speed of the animal that left the imprints.

But Mallison wasn't convinced that a formula based on observations of existing animals would necessarily work with dinosaurs. "Most dinosaurs had hind limbs that differed significantly from living mammals and birds," he says.

So Mallison conducted a detailed assessment of the size, shape and muscle mass of dinosaur limbs, then used engineering software to build striding models, taking gravity, mass distribution and inertia into account. For most dinosaur species, he found that stride length was extremely limited by the conformation of the skeleton. But he also saw that in comparison with mammals, dinosaurs had relatively large muscles in their buttocks, which would have changed their gait, so that the Alexander formula would not apply.

Formula faux pas

Although for mammals the most efficient way to move quickly is to run, dinosaurs could instead have used their powerful buttocks to take short but very rapid strides. "These huge animals may have been able to move quite quickly by walking much as race-walkers do," says Mallison.

"Many palaeontologists have just been plugging their data into the formula and expecting to get a reasonable ballpark figure of an extinct animal's speed," says John Hutchinson, who studies evolutionary biomechanics at the Royal Veterinary College, part of the University of London. "They figure this equation is bulletproof and this shows it clearly is not."

Mallison's theory also solves a problem raised by Hutchinson in 2002. Hutchinson argued, without relying on the Alexander formula, that T. rex could not have run much faster than a human, because it did not have enough muscle mass, especially in its ankles. However, this wouldn't have been an issue if the dinosaur was a power-walker. "Race-walkers get big butts and little muscle associated with their ankles, and this is exactly what we see in dinosaurs," says Mallison.

Hutchinson is more cautious. "This idea is interesting, but first we have to take a closer look at muscle physiology and work out if dinosaur muscles could have contracted fast enough to create strides with such frequency," he says.

As for exactly how fast dinosaurs could have travelled, Mallison's theory raises more questions than it answers. If the Alexander formula doesn't apply, it is currently impossible to determine an animal's speed from its footprints without knowing the frequency of its strides. But one thing seems certain: even if a T. rex could have kept up with a jeep by means of dinosaur power-walking, it would surely have looked too absurd to make it into a Hollywood film.

This article is reproduced with permission from the magazine Nature. The article was first published on November 7, 2011.