NEURARM: a dynamic robotic model of the human arm

A pair of muscles powering the human joint in an antagonistic configuration exemplifies the main difference between standard industrial robots and biological motor systems. Since muscles have a natural stiffness that varies with the muscle activation level, the central nervous system (CNS) can generate stable equilibrium postures, towards which the arm is attracted, by properly regulating the activation levels of antagonistic muscles (Hogan 1984) The elastic properties of muscles contribute to the finite stiffness/compliance properties of the limb, to the stability of the neuro-musculo-skeletal system in the face of significant feedback delays and even allow for the generation of target movements in absence of sensory feedback, by shifting the equilibrium point (Polit and Bizzi 1979). Moreover, the ability to modulate the stiffness of the limb is fundamental to the control of stable interactions with the environment leading to the theory of ‘impedance control’ (Hogan 1985). The intrinsic spring-like of muscles of the arm in theory allows the human motor system to generate stable postures and movements even in the absence of feedback control. Considering a single joint, the basic idea is that, working against each other, the two opposing spring-like muscles can establish a joint equilibrium position (EP). When the joint is at the EP, the net force and torque acting on it is zero. If moved to the equilibrium position and released, the joint will stay there. If displaced away from the EP by an external force and then released, the spring-like muscle properties will pull the limb back towards the EP. The EP is therefore a stable attractor. Control theories based on the presence of the EP in biological motor systems (Hogan 1984; Hogan et al. 1987, Bizzi et al. 1984; Feldman 1966) suggest that movements are programmed as a shift of equilibrium positions rather than through an explicit computation of forces. Thus, there is no need to solve the “inverse dynamics problem” for calculating the torque required to move the arm on the desired trajectory.