Human running behavior has been modeled as bouncing ball. In this regard, mass-spring model has been employed to study the mechanical characteristics of lower extremity during running. The mechanical adaptation of the lower extremity to the environment during running presents significant considerations in the understanding of the mechanism that governing the mass-spring system. A mass-spring model with a changing equilibrium point is developed in this study. The model predicts that the knee joint will experience three different stages during running stance phase. Each stage (phase) is associated with different joint stiffness, which is constant with in the each phase. The model further suggests that this unique stiffness could be estimated by using kinematics of knee joint only without kinetic measures. This model provides information that improves our understanding of knee joint mechanical behavior and neuromuscular control mechanism. It also has important clinical applications.