Speakers of a given language vary with respect to their acoustics, articulation, and motor commands. This variation is driven by a variety of influences, such as emotional states, communicative interaction, and individual properties of the vocal tract. In this work we focus on the latter. First, we build speaker-specific biomechanical tongue models. Second, we discuss the impact of the relative position of the bending in the vocal tract on the basis of extensive simulations with two different models. We focus on /i,a,u/ by defining target regions in the acoustic space, and discuss the corresponding speaker-specific articulatory and motor command variability observed.