The main goal of this book is to introduce the required coefficient of friction (RCOF) as a new variable to characterize the normal and pathological gait; also, its might be used in the prediction of the real fall propensity of patients with stroke and Parkinsons Disease (PD). In the First Chapter the literature review of Tribology (the study of friction) and the RCOF during the gait are presented. The Second Chapter discusses the methodology to calculate the RCOF based on the ground reaction forces (GRF). This variable is calculated as the ratio between the shear of the horizontal GRF...