2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/109160We formulate a reduced model to analyze the motion of the core of a twisted scroll wave. The model is first shown to provide a simple description of the onset and nonlinear evolution of the helical state appearing in the sproing bifurcation of scroll waves. It then serves to examine the experimentally studied case of a medium with spatially-varying excitability. The model shows the role of sproing in this more complex setting and highlights the differences between the convective and absolute sproing instabilities.5 pages, 4 figures + 1 figure as auxiliary material, to appear in Phys. Rev. LettPattern Formation and SolitonsSoft Condensed Mattertext