The Influence of Force Field Perturbations on Symmetric and Asymmetric Bimanual Reaching

Bimanual symmetric and asymmetric reaching movements require the motor system to coordinate the two limbs under varying spatial and physical demands. While prior research has shown a strong tendency for temporal synchronization between the hands, little is known about how bimanual reaching is organized when each limb is subjected to distinct types of force fields, such as viscous or elastic loads. This study systematically investigated how matched (viscous/viscous, elastic/elastic) and mismatched (e.g., viscous/no load, elastic/no load, viscous/elastic) load conditions influence key timing and kinematic parameters during bimanual reaching tasks across different spatial configurations and force magnitudes. Results show that kinematic measures for each arm, including peak velocity, peak acceleration, and peak deceleration, exhibit significant differences across both matched and mismatched load conditions, reflecting clear adaptations in limb behavior. In contrast, temporal aspects of movement--specifically the timepoints of peak velocity, peak acceleration, peak deceleration, and movement end time--remain coupled under matched loads but become decoupled under mismatched loads, with each limb operating more independently regardless of the spatial configuration of the task. Together, these findings clarify how the motor system balances the need for coordinated bimanual performance with the flexibility required to adapt to varying load conditions. This study extends prior work by demonstrating that not all bimanual reaches are temporally synchronized; instead, task-specific factors such as load type and symmetry critically shape how the limbs coordinate, revealing the motor systems capacity for flexible, context-dependent control.