Mobility impairment is a major challenge for the healthcare systems of developed countries. Balance deterio-ration is a physiological consequence of the ageing process, which makes it an endemic problem in ageing societies. Despite the importance of this issue, the development of appropriate therapies and protocols is challenging due to our limited understanding of human locomotion and equilibrium. This paper presents a technique that can track the BoS geometry from the feet' posture and its validation with healthy subjects. The proposed model uses a posture dependent reference frame called Saddle Space, which is aligned to the principal direction of the potential energy surface. The target application for the model described in this paper is the development of sensors for the evaluation of balance during activity of daily living. However, the proposed methodology can also be applied to bipedal robots, and the saddle space can also be employed to describe other posture dependent variables.