Jack Horner and Don Lessem argued in 1993 that "Tyrannosaurus" was slow and probably could not run (no airborne phase in mid-stride). However, Holtz (1998) concluded that tyrannosaurids and their close relatives were the fastest large theropods. Christiansen (1998) estimated that the leg bones of "Tyrannosaurus" were not significantly stronger than those of elephants, which are relatively limited in their top speed and never actually run (there is no airborne phase), and hence proposed that the dinosaur's maximum speed would have been about , which is about the speed of a human sprinter. Farlow and colleagues (1995) have argued that a 6- to 8-ton "Tyrannosaurus" would have been critically or even fatally injured if it had fallen while moving quickly, since its torso would have slammed into the ground at a deceleration of 6 "g" (six times the acceleration due to gravity, or about 60 metres/s2) and its tiny arms could not have reduced the impact. However, giraffes have been known to gallop at , despite the risk that they might break a leg or worse, which can be fatal even in a "safe" environment such as a zoo. Thus it is quite possible that "Tyrannosaurus" also moved fast when necessary and had to accept such risks; this scenario has been studied for "Allosaurus" too. Most recent research on "Tyrannosaurus" locomotion does not narrow down speeds further than a range from , i.e. from walking or slow running to moderate-speed running. A computer model study in 2007 estimated running speeds, based on data taken directly from fossils, and claimed that "T. rex" had a top running speed of . (Probably a juvenile individual.)