Primitive chain network (PCN) and non-equilibrium molecular dynamics (NEMD) simulations indicate that the nonuniversality seen in the elongational rheology of linear polymer melts and concentrated solutions is caused by monomeric friction reduction. We present, in the context of tube models with varying tube diameter, a new constitutive equation based on enhanced relaxation of stretch (ERS) and find good agreement with experimental evidence and with predictions of the extended interchain pressure (EIP) model. The stretch evolution equations of the ERS and the EIP model can be converted in a form, which allows expressing them in terms of monomeric friction reduction. Monomeric friction coefficients obtained from the ERS model and those used in PCN and NEMD simulations show at least qualitative agreement. While the reduction of monomeric friction is incorporated in PCN and NEMD simulations by empirical correlations between friction coefficient and segmental orientation or elongational stress fitted to elongational viscosity data, the EIP and ERS models provide analytical and parameter-free relations of monomeric friction reduction as a function of chain stretch.