Task-dependent changes in gain of the reflex response to imperceptible perturbations of joint position in man

1. It has been demonstrated recently that, when suitably instructed, subjects could alter the stiffness at the elbow in response to a slowly and imperceptibly changing elastic load. Although evidence was provided in favour of this occurring via changes in gain of the reflex response to stretch, changes in the degree of co-contraction could not be entirely ruled out. The major objective of the present experiments was to determine if subjects could alter stiffness at the wrist in a similar task, and then to determine whether they retained this ability when co-contraction was made impossible by anaesthetizing the nerve to the wrist extensors. A second objective was to determine if changes in stiffness could be controlled independently at the wrist and elbow. 2. Subjects, with eyes closed, initially held position constant against a constant force that loaded the flexors. For the wrist, they were instructed: (i) to keep the hand as still as possible (keep position constant) or (ii) to let the hand be moved by the perturbation (keep force constant). The perturbation was an initially imperceptible elastic load whose direction (loading or unloading) could not be predicted. Subjects were also asked to indicate when the perturbation was first perceived. 3. When asked to hold position constant or force constant at the wrist, subjects demonstrated task-dependent changes in stiffness prior to perception of the perturbation. These changes in stiffness were still achieved when the nerve to the wrist extensors was anesthetized and thus co-contraction was prevented. 4. Five subjects demonstrated the ability to control stiffness independently at the wrist and the elbow although most subjects had difficulty with the task we employed to demonstrate this. 5. The results demonstrate: (i) that for the wrist, set-dependent changes in stiffness that occur prior to perception of a slowly developing perturbation can be mediated by changes in gain of reflex responses to those perturbations, and (ii) that stiffness can be controlled independently at the wrist and elbow, presumably in part by changes in gain of stretch reflexes.