Some big dinosaurs often tripped and broke their ribs while chasing their prey, New Scientist magazine said Thursday.

Bruce Rothschild of the Arthritis Center of Northeast Ohio in Youngstown studied the family of two-legged dinosaurs known as Therapods, which includes the deadly Tyrannosaurus rex.He X-rayed fossil skeletons and found that ribs from some specimens of a smaller species known as Allosaurus showed the kind of fractures that would have been caused by a belly flop on to hard ground while running.

Rothschild will present his findings at a symposium in Philadelphia being held as part of the Dinofest exhibition this week.

He told New Scientist the results showed that some large members of the Therapod family were not the sluggish creatures that some paleontologists have suggested.

In 1995 James Farlow of Indiana-Purdue University argued that the Tyrannosaurus rex could not have run faster than 20 mph, because a fall would have probably killed the six-ton meat-eater.