Systematic nonlinear relations between displacement amplitude and joint mechanics at the human wrist.

This study quantified the systematic effects on wrist joint mechanics of changes in amplitude of displacement ranging from within the region of short-range stiffness (0.2% of resting muscle length) up to 3% of resting muscle length. The joint mechanics were modelled using a second-order system from which estimates of joint stiffness, viscosity, inertia, natural resonant frequency and damping ratio were obtained. With increasing amplitude of displacement, the stiffness decreased by 31%, the viscosity decreased by 73%, the damping ratio decreased by 71% and the resonant frequency decreased from 10.5 to 7.3 Hz. The patterns of change in joint mechanics with displacement amplitude were nonlinear but systematic and were well described by power relationships with high R(2) values. These relationships provide normative data for the adult population and may be used in the modelling of human movement, in the study of neurological disorders and in robotics where human movement is simulated. The observed patterns of high initial stiffness and viscosity, decreasing progressively as displacement amplitude increases, may provide a good compromise between postural stability and liveliness of voluntary movement.