The model proposed in [7] to calculate the broadening effect produced by elastic modes on NMR line shapes of nematic liquid crystals, in which all modes with relaxation times longer than a NMR time scale Δτ are static, and infinitely fast in the opposite case, is put on a more rigorous theoretical basis, by considering all modes with their actual relaxation times. The correlation function of the transverse magnetization is calculated and expressed in terms of the self-correlation function of the components nxandnyof the local director, assumed to be equivalent and independent gaussian random variables. Formal expressions are given for the general case, and in the one constant, cylindrical and spherical cases, approximations. The general procedure describing how to use this formalism for a NMR spectrum composed of many lines, is given. This formalism is then used to analyse the same data as in [7] concerning a main chain nematic polymer, in the spherical approximation. It is shown that fits with the same quality are obtained. These results provide (i) theoretical support for the model of [7], (ii) an operational way to define Δτ and (iii) a practical example for discussion of the controversial problem of the ‘cut-off wavevector(s)’ of the modes, which define the size of the elementary uniaxial object in the nematic medium. It is shown that, for this polymer, the smallest size corresponds to a volume between one and four repeat units. The analysis of line shapes provides the viscoelastic parametern½K3/2, wherenandKare the average viscosity and average elastic constant. Values ofnandKcan be deduced from the theory. The limitations of the model are discussed. It is shown that the present model and the one of [7] are complementary. It is argued that the present formalism may be useful to analyse NMR line shapes in conventional polymers.