Co-speech gestures destabilize posture via shared motor-cognitive resources

Abstract Speaking and gesturing are fundamental aspects of human communication that commonly occur while maintaining upright posture. Although speech and postural control are known to compete for shared motor-cognitive resources, it remains unclear how co-speech gestures affect balance control and their underlying neural mechanisms. Using combined behavioral and neuroimaging approaches, we investigated whether co-speech gestures interfere with concurrent postural control, whether this interference depends on the spatial reference frame (egocentric vs. allocentric), and whether dual-task costs arise from competing motor demands, overlapping cognitive processes, or both. Forty-one right-handed adults (25.9 ± 6.8 years) maintained posture—monitored via a wearable Inertial Measurement Unit (IMU) sensor—while performing co-speech and no-speech gestures with right and left hands from allocentric or egocentric perspectives. Functional near infrared spectroscopy (fNIRS) was applied to measure cortical oxygenation in bilateral pre- and postcentral areas. Postural sway increased during co-speech versus no-speech gestures, right versus left hand, and egocentric versus allocentric gestures. Co-speech elicited increased left premotor and supplementary motor cortex (PMC) activation. Egocentric gestures produced greater frontal cortex and Broca's area activation than allocentric gestures. Co-speech gestures increase postural instability and recruit shared motor-cognitive resources within the left premotor cortex, suggesting that speech, gesture production, and balance control compete for overlapping neural networks. Right-hand and egocentric gestures produced the strongest interference effects, indicating that gesture characteristics modulate motor-cognitive demands during postural control.