The control of stable postures in the multijoint arm.

The stiffness that is measured at the hand of a multijoint arm emerges from the combined effects of the elastic properties of the muscles and joints, the geometry of the linkages and muscle attachments, and the neural control circuits that act on the arm. The effective stiffness of a nonlinear linkage such as a two-joint arm depends on the force acting on the system as well as the intrinsic stiffness of the actuators. This paper presents an analysis of the factors that affect limb stiffness, including the effects of external forces. Three potential strategies for controlling the stability of the limb are proposed and demonstrated by computer simulations. The predictions from the simulations are then compared experimentally with measured stiffness values for human subjects working against an external force. These experiments were directed toward understanding what strategies are used by the CNS to control limb stiffness and stability. The experimental evidence showed that human subjects must increase the stiffness at the joints in order to maintain limb stability in the presence of applied external forces at the hand. In the process we identified a precise role for muscles which span two or more joints in the control of over all limb stiffness. A local strategy may be used to achieve limb stability, in which the muscle stiffness increases with muscle force. Multijoint muscles are shown to provide mechanical couplings which are necessary for the maintenance of stability. By utilizing these muscles, the neuro-musculo-skeletal system can control a global property of the system (stability) with a passive local strategy.