Execution of natural manipulation in the air enhances the beta-rhythm intermuscular coherences of the human arm depending on muscle pairs.

Natural manipulation tasks in air consist of two kinematic components: a grasping component, with activation of the hand muscles, and a lifting component, with activation of the proximal muscles. However, it remains unclear whether the synchronized motor commands to the hand/proximal arm muscles are divergently controlled during the task. Therefore, we examined how intermuscular coherence was modulated depending on the muscle combinations during grip and lift (G&L) tasks. Electromyograms (EMGs) were recorded from the biceps brachii (BB), triceps brachii (TB), flexor digitorum superficialis (FDS), and extensor digitorum communis (EDC) muscles. The participants were required to maintain G&L tasks involving a small cubical box with the thumb and index and middle fingers. Consequently, we found that the beta-rhythm coherence (15-35 Hz) in BB-TB, BB-FDS, and TB-EDC pairs during G&L was significantly larger than that during the isolated task with cocontraction of the two target muscles but not BB-EDC, TB-FDS, and FDS-EDC (task and muscle pair specificities). These increases in beta-rhythm coherence were also observed in intramuscular EMG recordings. Furthermore, the results from the execution of several mimic G&L tasks revealed that the separated task-related motor signals and combinations between the motor signals/sensations of the fingertips or object load had minor contributions to the increase in the coherence. These results suggest that during G&L the central nervous system regulates synchronous drive onto motoneurons depending on the muscle pairs and that the multiple combination effect of the sensations of touch/object load and motor signals in the task promotes the synchrony of these pairs.NEW & NOTEWORTHY Natural manipulation in air consists of two kinematic components: grasping, with activation of hand muscles, and lifting, with activation of proximal muscles. We show that during the maintenance of object manipulation in air the central nervous system regulates the synchronous drive onto human motoneuron pools depending on the hand/proximal muscle pairs and that the multiple combination effect of the sensations of touch/object load and motor signals in the task promotes the synchrony of these pairs.