Bimanual force control differs between increment and decrement

Background: Everyday bimanual tasks require increasing and decreasing forces to manipulate objects. Optimal bimanual coordination during force increment and decrement is essential to complete a bimanual task. However, little is known about the differences in bimanual control during force increment and decrement. The purpose of this study was to 1) investigate whether task performance and bimanual coordination differ between force increment and force decrement in a bimanual task, and 2) determine the contribution of bimanual coordination to task performance during force increment and force decrement.

Methods: Seventeen right-handed young adults (24.10 ± 3.09 years) performed following tasks involving bimanual isometric index finger flexion: 1) maximum voluntary contractions and 2) visually-guided force tracking involving gradual force increment and decrement. The force tracking task involved controlled force increment and decrement while tracking a trapezoid trajectory. The task goal was to match the target force with the total force, i.e., sum of forces produced by both hands as accurately as possible. We quantified bimanual task performance with the accuracy and variability of total force in force increment and decrement phases. We measured bimanual coordination between two hand forces by computing time-series cross-correlation coefficient and amplitude of coherence in 0-1 Hz.

Results: We found decreased accuracy and increased variability of the total force in decrement compared with increment phase. Further, the cross-correlation coefficient and coherence amplitude were greater during force decrement than force increment phase. Finally, cross-correlation coefficient and coherence in 0.5-1 Hz predicted the accuracy and variability of total force.

Conclusions: We provide evidence that task performance is reduced during force decrement as compared with force increment, suggesting that force release is more challenging than force generation in bimanual tasks. Further, bimanual coupling of the forces was better during force decrement than force increment. Overall, the coordination of forces from both hands influences the task performance across combined increment and decrement phases. Specifically, decoupling of forces produced by both hands facilitates error compensation strategy to reach the task goal. Together, these findings highlight that the bimanual control of forces is task-dependent and emphasize the importance of collaboration between hands in achieving a common task goal. These results may have implications for understanding changes in bimanual control with aging and neurological disorders.