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A Simple Model for Estimating the Effect of Joint Stiffness on Impact Ground Reaction Forces During Landing

Joint stiffness, which is a result of muscle functioning, plays an important role on the contact event in drop landing. It is important to determine how the elasticity and the damping of the joint can change the magnitude and the time of impact, from two perspectives: 1) to characterize features in human landing and accordingly develop an accurate theoretical model of landing consistent with experimental data. 2) to understand whether and how changing the joint stiffness by an external component can reduce the risk of landing injury. The aim of this study is to better understand the role of joint stiffness on impact ground reaction force (GRF) and its time in drop landing, by using a very simple model. To this end, a two-mass model is considered in which the stiffness of the joint is represented by a spring and a damper connection. The elasticity between the shoe and the ground is also modeled by a spring and a damper. The effect of the joint stiffness on the magnitude of the impact force and the time of impact is then studied by changing the spring and damper coefficients of the joint. Moreover, the problem is further studied in terms of the poles of the system to see how the impact GRF can be reduced by changing the poles. The obtained results can be of practical importance in developing orthoses for reducing the joint forces in patients with joint problems.