Bimanual coordination is an essential component of human movement. Cooperative bimanual reaching tasks are widely used to assess the optimal control of goal-directed reaching. However, little is known about the neuromuscular mechanisms governing these tasks. Twelve healthy, right-handed participants performed a bimanual reaching task in a three-dimensional virtual reality environment. They controlled a shared cursor, located at the midpoint between the hands, and reached targets located at 80% of full arm extension. Following a baseline of normal reaches, we placed a wrist weight on one arm and measured the change in coordination. Relative contribution (RC) was computed as the displacement of the right hand divided by the sum of displacements of both hands. We used surface electromyography placed over the anterior deltoid and biceps brachii to compute muscle contribution (MC) from root mean squared muscle activity data. We found RC was no different than 50% during baseline, indicating participants reached equal displacements when no weights were applied. Participants systematically altered limb coordination in response to altered limb dynamics. RC increased by 0.91% and MC decreased by 5.3% relative to baseline when the weight was applied to the left arm; RC decreased by 0.94% and MC increased by 6.3% when the weight was applied to the right arm. Participants adopted an optimal control strategy that attempted to minimize both kinematic and muscular asymmetries between limbs. What emerged was a trade-off between these two parameters, and we propose this trade-off as a potential neuromuscular mechanism of cooperative bimanual reaching.NEW & NOTEWORTHY This study is the first to propose a trade-off between kinematic and dynamic control parameters governing goal-directed reaching. We propose a straightforward tool to assess this trade-off without the need for computational modeling. The technologies and techniques developed in this study are discussed in the context of upper extremity rehabilitation.
Keywords: bimanual coordination; electromyography; goal-directed reaching; optimal control; virtual reality.