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Bimanual control differs between force increment and force decrement




Alam, Tasnuva, author
Patel, Prakruti, author
Lodha, Neha, author

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Background: Everyday bimanual activities require increasing and decreasing forces to manipulate objects, for example, buttoning a shirt or tying a shoelace. Optimal coordination of increasing and decreasing bimanual forces is quintessential to achieve the overall goal of the bimanual task. However, little is known about differences in force control in bimanual force increment and decrement. Purpose: The purpose of our study was to 1) investigate whether bimanual task performance and force coordination differ in force increment versus force decrement and 2) identify the contribution of force coordination in bimanual task performance while increasing and decreasing forces. Methods: Seventeen right-handed young adults (24.10±3.09 years) performed following tasks involving isometric index finger flexion: 1) maximum voluntary contractions and 2) visually guided force tracking involving gradual force increment and force decrement. Each task was performed in unimanual with the right hand only i.e., control condition and bimanual with both hands together i.e., experimental condition. The force tracking task involved controlled force increment and decrement while tracking a trapezoid trajectory as accurately as possible. We quantified task performance with accuracy and variability of the total force in increment and decrement phases. We measured force coordination of the two forces by computing the time-series cross-correlation coefficient and amplitude of coherence in 0-1Hz. Results: We found reduced accuracy and increased the variability of the total force in force decrement compared with force increment. Further, the peak correlation and coherence amplitude was greater during force decrement than force increment. Finally, the peak correlation coefficient and coherence in 0.5-1Hz predicted total force accuracy and variability across the two phases. Conclusions: We provide evidence that performance (force accuracy and steadiness) in bimanual force decrement is reduced compared with force increment, highlighting that force release is more challenging than force generation in bimanual tasks. Overall, improved bimanual task performance is contributed by reduced coordination of two forces indicating that reduced constraint between the hands facilitates error compensation. However, the implicit strategy is to produce highly coordinated forces while executing controlled force release, impacting the task performance. Clinical implications: Considering the significance of increasing and decreasing forces to manipulate objects, our study provides insight into the fundamental differences in bimanual task performance and force coordination during dynamic force manipulation requiring increasing and decreasing forces. These findings can form the basis to understand how aging and neurological conditions impact bimanual function. Future endeavors should be targeted to evaluate more deep understanding of upper limb motor control and invent protocols for the rehabilitation.


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