A QUASI-LINEAR VISCOELASTIC MODEL FOR THE PASSIVE PROPERTIES OF THE HUMAN HIP JOINT

Properties of passive elastic structures constituting the human hip joint can be exploited to increase efficiency of human locomotion. As studies estimating the passive contributions to the net joint moment often disregard damping properties of the joint such contributions overestimate the energy gained during leg retraction within swing and stance phase. We built an experimental apparatus to measure moment-angle-relations during motor guided cyclic movements over a wide range of angular velocities and step-like changes in hip angle. On the basis of the experimentally gained data set the objective of this study was to model the elastic as well as the damping characteristics of the joint in the sagittal plane utilizing the Quasi-Linear Viscoelastic theory (QLV). A double exponential function was conveniently employed to describe the elastic response. The dependency of the hip joint stiffness on biarticular muscles was incorporated by repeating the measurement protocol for different knee angles. Due to the fact that the stiffness characteristics of the elastic response were merely shifted over knee angles we introduced an equilibrium angle at the hip joint as exponential function of the knee angle eventually yielding an elastic response as a function of hip and knee angle. In order to cover the damping characteristics the reduced relaxation function comprising a continuous spectrum of relaxation was utilized. We exemplify the applicability of the QLV model on published kinematic data on human walking and estimated that approximately 27% of the energy passively stored at the hip dissipates during the gait cycle.