This paper describes the development of a theoretical model for the strength of mechanical relaxation in terms of micromechanics of deformation. Polymethyl methacrylate (PMMA) is used as the model amorphous polymer. The internal molecular rearrangements during relaxation are identified and accounted for by the rotation of specific atomic groups. Voronoi tessellation is used as a method to characterise the nanostructure in amorphous glassy polymer below T-g. A theoretical model is postulated and shown to provide a limited quantitative prediction capacity of the anelastic deformation and the corresponding stress relaxation based on measurable molecular parameters without adjustable factors.