The moveable handhold: A new paradigm to study visual contributions to the control of balance-recovery reactions

Abstract Balance-recovery reactions that involve rapid step or reach-to-grasp movements are prevalent and functionally important responses to instability. Successful use of these reactions to recover balance in daily life requires a capacity to modulate the reaction to deal with the continual variation in environmental constraints that occurs as the person moves, i.e. location of objects that can obstruct limb movements or serve as handholds to grasp. The most direct approach to study this involves applying balance perturbations as subjects move within a visually complex environment; however, this approach does not allow precise control over kinematic variables or visual inputs, and is susceptible to strong learning effects. We have therefore developed an alternate approach, wherein the subject is stationary and the relative motion between subject and constraints that normally occurs as a result of ambulation is instead introduced via movement of the surrounding obstacles or handholds. We previously developed a motor-driven “obstacle-mover” to manipulate constraints on step reactions, and now describe an analogous approach to study reach-to-grasp reactions, using a motor-driven “handhold-mover”. We anticipate that this paradigm will provide new opportunities to probe CNS control of upright stance, by providing a sensitive indicator of limitations in the neuromusculoskeletal systems. It can also be used to test perturbation-evoked reactions in seated subjects, thereby allowing testing or training of persons who are unable to stand and use of techniques (e.g. TMS, EEG) that can be difficult to perform in free-standing subjects.