This paper describes computer simulations and experimental results of stress relaxation carried out in polymethyl methacrylate (PMMA). The results are analysed in terms of the RT model proposed earlier. The main theoretical result is that transitions of side-groups, or parts thereof, can account for approximately 10% of stress relaxation. The experimentally observed relaxation strength in a time of approximately 3 h at 50 K below T, is of the order of 40-60%. These facts provide conclusive evidence that cooperative motions of main chain segments are responsible for most of the stress relaxation. A qualitative model for stress relaxation due to chain twisting is described and shown capable of large stress losses.