On the role of feedback and feedforward mechanisms in the reflex control of posture and movement in human multi-jointed limb

Two modes of reflex control of human limb position are described. When confronted with unexpected perturbations, the system uses a feedback mode which takes into account the dynamic coupling between different limb joints. This mode is appropriate for online control of the limb trajectory during reaching movements or during the maintenance of stable posture. Although the performance of such low-stiffness feedbacks in the control of position is modest, the system can generally tolerate substantial margins of error and still achieve the desired goal. However, when a forthcoming perturbation can be anticipated, the system can switch to a feedforward mode of control, with the advantage of avoiding neural conduction delays. This does not imply that the two modes, feedback and feedforward, are always mutually exclusive.