Line up Usain Bolt at the starting blocks with a Tyrannosaurus rex and the dinosaur would be left behind in the sprinter's dust, according to computer-assisted research that turns long-held assumptions on their head.

While the running ability of T-rex has been hotly debated among paleontologists for decades, the consensus from previous biomechanical models was that the Cretaceous-era carnivore could manage speeds of up to 75 km/h.

That's more than half as fast again as the quickest man in history.

But because of its size and weight, the predator would actually have broken its legs had it tried to break into a sprint, the University of Manchester research showed.

"The muscles need to be able to generate sufficient power to allow high-speed locomotion, but at the same time the skeleton has to be able to cope with the loads generated by the high speed," said Professor William Sellers from the university's School of Earth and Environmental Sciences.

"This is where it fails. T-rex's skeleton is simply not strong enough for running locomotion," he told Reuters.

Researchers used an engineering technique called multi-body dynamic analysis, coupled with machine learning, to produce what they say is the most accurate simulation of T-rex's gait and biomechanics to date.

They concluded that T-rex was limited to walking speeds of about five metres per second, equating to 18 km/h — less than half the speed of Usain Bolt's personal best of 44.7 km/h.

A study published in Royal Society journal Biological Sciences in 2007 suggested an at that time conservative top speed of 29 km/h — still fast enough to chase down top runners over longer distances.

Sellers said his study had forced a re-think on how T-rex caught its prey.

"It certainly would not have been able to chase down faster-moving prey animals," said Sellers. "That leaves other hunting options such as ambush, and of course it means that (discredited) ideas such as 'T-rex the scavenger' have to be reconsidered."