Control of an unstable load using visual feedback

Joint stiffness defines the dynamic relationship between the position of the joint and the torque acting about it. Joint stiffness is viewed as consisting of two components: reflex and intrinsic stiffness. However, when subjects attempt to maintain the position of a joint constant, such as in upright stance, voluntary torque will contribute to the measured joint stiffness. This voluntary torque may arise from visual, vestibular and proprioceptive sources. In this study, subjects were given the task of maintaining the position of a large inertial load constant, using only visual feedback. We found that subjects produced torques which were delayed, low-pass filtered versions of the load velocity. We also showed that the use of velocity information is critical, because it is not possible to stabilize the system without it.