We have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the Central American Volcanic Arc. The zone of interest is the segment of the arc on the Chortis Block, where the Cocos plate is subducted beneath this continental crust. As has been shown by different researchers, the strain in this part of the volcanic arc (at least in the El Salvador and Nicaragua area) is transtensional, and the majority of the focal mechanisms reported are of strike-slip motion. However, the geological characteristics vary along the arc and it can be divided at least in three clear segments: Guatemala, El Salvador and Nicaragua. We have developed different models testing the influence of the considered main forces: Subduction related arc-normal forces, subduction-related arc-parallel forces and Caribbean drift related forces. Our results show that, to achieve a transtensional state of stress on the volcanic arc, it must be modelled as a lithospheric weak zone, which is coherent with its thermal state, and the forces related with the eastward drift of the Caribbean plate must be higher than those related with the subduction. This means that the coupling on the subduction interface must be low, with or without strain partitioning due to the obliquity of the subduction. The peculiar characteristics of the arc in Guatemala are due to the geometry of the boundary between the plates of North America and Caribe, and the closeness to the Middle America Trench at this point. Here the western edge of the Chortis block is pinned against North America, even with low trench-normal forces, making this triple junction between the Cocos, North American and Caribbean plates a zone of diffuse deformation. The extension in the western part of the Chortis block, from Guatemala to the Honduras depression, is explained by the geometry of the North American - Caribbean plate boundary and the direction of motion of the Caribbean plate with respect to North America. The direction of extension is always E-W regardless the magnitude of the applied forces. In short, the state of stress in the northern part of the Central American Volcanic Arc is a combination of the stresses due to the Caribbean-drift and the stresses of the subduction, but the latter should not be high and therefore the coupling of the subduction interface should be low or very low.