Checchin, M. Grassellino, A. Martinello, M. Romanenko, A. JACoW Geneva, Switzerland In the present study the radio-frequency complex response of trapped vortices in superconductors is calculated. The motion equation for a magnetic flux line is solved assuming a bi-dimensional and mean-free-path-dependent Lorentzian-shaped pinning potential. The resulting surface resistance shows the unprecedented bell-shaped trend as a function of the mean-free-path observed in our previous experimental work. We demonstrate that such bell-shaped trend of the surface resistance, as a function of the mean-free-path, may be described as the interplay of the two limiting regimes of the surface resistance, for low and large mean-free-path values: pinning and flux-flow regimes respectively. By tackling the frequency dependence of the surface resistance, we also demonstrate that the separation between pinning- and flux-flow-dominated regimes cannot be determined only by the depinning frequency. The dissipation regime can be indeed tuned either by acting on the frequency or on the mean-free-path value.