The origin of a dramatic loss of tensile strength in poly(4-methyl-1-pentene) [PMP] above a major dynamic mechanical transition that occurs around 45 degrees C : is explored. The usual mechanisms associated with loss of strength in polymers are shown to be absent in this case. Viscous dissipation disappears almost completely during deformation of PMP above the transition of interest here. A thermodynamic analysis of reversible deformation is shown to provide remarkably good predictions of the work of rupture under such conditions. The thermorheological responses of PMP suggest that the transition of concern here is not accompanied by the onset of rapid backbone motions but by pendant group motions. Indeed, 2D wideline separation (WISE) NMR, which correlates dynamics (via H-1 widelines) with structure (via C-13 chemical shifts), clearly reveals that this transition is primarily accompanied by the onset of rapid pendant group motions. Such motions, coupled with the relatively large inter-backbone distances in PMP, provide the conditions necessary for elimination of viscous dissipation. These findings require refinement of the previously held view that the transition around 45 degrees C in PMP denotes a glass transition associated with the onset of rapid backbone segmental motions.