Paleontologist Gregory M. Erickson of Florida State University provides the following explanation of "how a 5-ton teeter-totter gets up."
Scientific inquiry has focused on the utility of the diminutive arms of tyrannosaurs for nearly a century. Several theories, including some regarding the arms' role in raising these animals from the ground, have long been kicked around. American paleontologist Henry Fairfield Osborn, the first one to describe T. rex, initially expressed doubts that the relatively small humerus, or upper arm bone, associated with this enormous animal really belonged to it. Once convinced, however, he forwarded the first theory in 1906 of their utility--in grasping organs for copulation. In 1970 British paleontologist Barney Newman posited that the arms actually served as braces to prevent the front of the body from skidding forward as the animal rose from a prone position using its hindlimbs. During such activity, the forelimbs would have been extended in an action reminiscent of a push-up. Other competing theories contend that the arms are vestigial (degenerate organs that have lost much use) or that they functioned as meat hooks while the creature's teeth were employed.
Are any of these theories correct? We may never know the answer. Nevertheless the recent finding of the first specimens of complete T. rex forelimbs in Northern Montana has opened the door to biomechanical analyses and osteopathic observations from which new insights into the physical capacities of these structures have emerged. With this new data, arm function hypotheses are being reanalyzed.
It is now clear that T. rex's hands could not reach its mouth. The elbow could not be extended much beyond a 90-degree angle. The arms were very strong (perhaps capable of curling nearly 400 pounds) but had a very limited range of motion, both side-to-side and up-and-down. The wrists were considerably weaker and do not seem suited for supporting large mechanical loads. Like those of their albertosaur "cousins," the small T. rex arms were often broken during life. This fact suggests that they were poorly suited for whatever the dinosaurs were trying to use them for and, more importantly, that these animals could go without using their arms for periods of up to a month.
Collectively, these findings seem to fly in the face of just one of the aforementioned theories--Newman's push-up theory. If this is the case, then how then did T. rex get up? I think we can look to the birds (avian dinosaurs) for the answer as they can stand up without the aid of arms. It is simply a matter of getting one's limbs below the center of gravity before extending them. I am not aware of any studies suggesting tyrannosaurs could not do this. Furthermore, tyrannosaurs would have had the additional aid of their tails. From skeletal evidence and albertosaur trackways (in which the tails did not drag), it is clear tyrannosaur tails acted as counter-balances--10,000-pound walking, teeter-totters. The tail would have helped to keep the center of balance back on the body as the hindlimbs were moved into position underneath.
Clearly tyrannosaurs got up at least once during their lives (at birth) and there is no reason to believe they could not throughout life--armed with pathetic arms or not.