Nb3Sn is now commonly used in the design of high-field large-scale magnets. However, it is a brittle material, the superconducting properties of which degrade under mechanical strain. Both ITER TF and CS magnets make use of Nb3Sn strands in cable-in-conduit conductors. Experiments have been carried out in the TARSIS facility at University of Twente aiming at measuring the strand critical current as a function of periodically applied strain/stress. Until recently, these experiments have given good indications of the strand behavior, but they had not been fully understood because of the lack of an accurate description of the local strain along the tested strand. Furthermore, they cannot be extrapolated directly to a real cable-in-conduit conductor because they do not simulate the differential thermal contraction, which puts the strand under longitudinal compression. Using the mechanical code MULTIFIL developed at Ecole Centrale de Paris, associated with the electrical code CARMEN developed at CEA/IRFM, this paper aims at understanding the mechanisms of the critical current reduction during a TARSIS experiment by coupling the local strain map of the strand to the complex current paths between Nb3Sn filaments. Comparison with experimental results and with analytic limiting cases are presented and discussed.