The choice of a qualitative or quantitative method in research is often a false debate. More than a methodological choice, this choice positions the researcher within a paradigm defining its disciplinary references. Studies truly linking quantitative and qualitative are still uncommon. Lack of time and financial resources are typically invoked for not using the combination. Mostly fundamentally, true limitations are linked to the complexity of endorsing such a dual approach from the research object construction to data gathering and analyses. Indeed, epistemological, theoretical and empirical difficulties are constant obstacles to be confronted. This communication presents a reflection at the heart of these challenges. It was developed in the context of a doctoral thesis that aimed at describing and understanding the evolution of mobility during the aging process, in both its behavioral and experiential dimensions. The research was conducted with a multiple epistemological and methodological standpoint, tackling the object of study at once from an individual and longitudinal perspective. The presentation discusses the challenges of crossing qualitative and quantitative approaches, namely during the formulation the research questions/hypotheses, the construction of the theoretical framework, as well as the empirical work itself. The relevance of such combined approached, but also their limitations and most needed improvements are discussed in the conclusion.