A trade-off between kinematic and dynamic control of bimanual reaching in virtual reality

Abstract

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 3-dimensional virtual reality environment. They controlled a shared cursor, located at the midpoint between the hands, and reached to 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 with 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 tradeoff between these two parameters, and we propose this tradeoff as a potential neuromuscular mechanism of cooperative bimanual reaching.

Responsiveness to exoskeleton loading during bimanual reaching is associated with corticospinal tract integrity in stroke

“…However, it is common for stroke survivors to display variable behavior, likely due to the heterogeneous nature of their lesion size and location, or the use of compensatory movements ( Jayasinghe et al, 2021 ). Therefore, we used a sensorimotor tradeoff analysis developed by Brunfeldt et al (2022) to explore individual differences in motor performance.…”

“…We previously found a tradeoff between the kinematic and dynamic control of reaching in healthy individuals ( Brunfeldt et al, 2022 ). Specifically, when we asked healthy young adults to perform our shared cursor reaching task with a wrist weight attached to one arm, RC increased and MC decreased in the non-weighted arm for all 12 participants.…”

“…Specifically, when we asked healthy young adults to perform our shared cursor reaching task with a wrist weight attached to one arm, RC increased and MC decreased in the non-weighted arm for all 12 participants. Using data from Brunfeldt et al (2022) , we show that when plotting the change in RC (ΔRC) from baseline to wrist weight loading vs. the change in MC (ΔMC), all healthy participants’ behavior occupied the second quadrant ( Figures 5C , D ). In contrast, our stroke participants display a constellation of responses that occupy nearly all four quadrants ( Figures 5A , B ).…”

“…Participants reached for virtual cube-shaped targets located at 70% of their maximum arm length. Targets appeared one at a time, at body midline, at either eye or chest level ( Figure 1 ) ( Wang et al, 2021 ; Brunfeldt et al, 2022 ). Participants controlled a spherical cursor in one of two control modes.…”

“…Our previous work in shared cursor tasks revealed that the optimal reaching strategy resulted in a tradeoff between kinematic and dynamic control of the arms ( Brunfeldt et al, 2022 ). In the current study, we hypothesized that stroke participants would adopt this same optimal reaching strategy.…”