In this monograph, two novel mechanical models are presented: an analytical-kinetical model with regard to the kinetics of the human knee joint under non-standard squatting, and a numerical-kinematical model with regard to the local kinematics (sliding-rolling phenomenon) of the knee joint under standard squatting. The new analytical-kinetical model is capable to calculate the patellofemoral, tibiofemoral, patellar tendon and quadriceps tendon forces in the knee joint under different squatting motions. The model takes the effect of a moderate trunk motion (horizontal shift of the center of gravity) into account, thus the influence of this new parameter on the kinetics can be adequatly analysed. The second aim of the monograph is to unfold the sliding-rolling phenomenon related to the currently applied, commercial knee prostheses under standard squatting movement. The phenomenon, which so far has not been studied in such depth with regard to knee prostheses, is addressed by means of multibody models, which consider real three-dimensional geometries, the effect of friction between the condyles, and the collateral ligaments as well.