Gravity support from a robotic exoskeleton increases spontaneous use of the nondominant upper extremity during a choice reaching task

Abstract

The objective was to determine if gravity support for the left arm of right-handed participants would increase left arm use during a 3D reaching task in virtual reality. Twelve healthy control participants each completed 630 reaching movements broken into 6 blocks. The majority of targets were placed close to the mid-sagittal plane at three heights and participants were free to use either limb when reaching for targets. The hand had to stay in the target for a prescribed dwell time before the target disappeared. For all reaching tasks within a block, the left arm gravity support was set to either 0% or 75% of full arm support. The blocks also varied in the dwell time (2, 4, or 6 sec). The order of blocks was balanced across participants in terms of gravity support level and dwell time. EMG level in the left medial deltoid decreased with increasing gravity support (p<.001) and was higher for higher targets compared to lower targets (p<.001). The odds of using the left arm were 1.95 times higher with gravity support compared to no support (p<.001). With gravity support, we expected greater shifts towards the left arm in tasks that were more energetically demanding. This was not the case, as the increased use was evenly distributed across all target heights, and use decreased or remained unchanged with increasing dwell time. Results are discussed relative to current models of limb choice and the potential use of robotic gravity support to overcome learned non-use in stroke patients.