The measurements of the smectic reflection intensities of side-chain liquid-crystal polymers give access, after inverse Fourier transform, to the profile of coherent scattering length density along the normal to the layers. We give here three backbone density profiles associated to three liquid-crystal polymers : two polymethacrylates and one polyacrylate differing only by the nature of the backbone or by the mesogen extremity. It is shown that the backbones are always confined between the mesogen layers, and could be even localized right in the middle of the mesogen layer. In addition, this study demonstrates that small modifications of the liquid-crystal polymer formula are sufficient to induce very different backbone density profiles.