We present a simple constitutive model based on Kuhn segment alignment that predicts the observed monotonic extension thinning in steady state viscosity eta(E) even at extension rates above the inverse Rouse time ((epsilon) over dot > tau(-1)(R)) for entangled polystyrene melts [Bach et al., Macromolecules 36, 5174-5179 (2003)], while preserving the extension thickening typically seen in entangled solutions for (epsilon) over dot > tau(-1)(R) [Bhattacharjee et al., Macromolecules 35, 10131-10148 (2002)]. We tested two mechanisms by which Kuhn segment alignment can affect terminal relaxation time tau(d) and rheology within a modified Doi-Edwards-Marrucci-Grizzuti (DEMG) model, which is a simplified tube model. The first mechanism is an increase in the tube diameter, as is inferred from ideas of Doi and Edwards [The Theory of Polymer Dynamics (Clarendon, Oxford, 1986)] and Sussman and Schweizer [Macromolecules 45, 3270-3284 (2012a)], while the second mechanism is a decrease in segmental friction, as described in recent work of Yaoita et al. [Macromolecules 45, 2773-2782 (2012)]. We find that the DEMG model, modified to allow tube diameter enlargement and reduction of entanglements, when done self-consistently, fails to predict extension thinning for entangled melts. The DEMG model modified by the second mechanism, to allow Kuhn segment alignment effects on local friction, correctly predicts extension thinning for entangled melts, while retaining thickening in entangled solutions with high solvent volume fractions. (C) 2014 The Society of Rheology.