This work proposes an original quantitative approach to understand the backward plastic strain resulting from the unloading of a polycrystalline metal during an interrupted tensile test. Based on a prior model by Salama and Roberts, the main assumption is that the relaxed plastic strain is due to the small relaxation of mobile dislocation loops, also responsible for the internal stresses. During unloading, the number of dislocation loops per active source remains constant, but they move backwards as the applied stress decreases. Thus, the main challenge consists in the quantitative estimation of each hardening component and their variations, upon loading and during the unloading sequences, without reverse straining. The model is applied in the case of an interstitial-free steel, with excellent agreement. This study provides a more consistent framework related to the evaluation of the Bauschinger effects in metals avoiding the ambiguity on the choice of the plastic strain offset.