Shared premotor resources underlie dual-task interference between speech and balance

Humans frequently speak while standing, potentially engaging overlapping motor-cognitive resources. Dual-task paradigms with increased motor-cognitive demands reliably impair postural control, presumably due to competition for shared neural resources. The premotor cortex is critically involved in balance stabilization and in resolving competing motor plans and is also engaged during processing of action-related language. We therefore hypothesized that a central processing bottleneck emerges in the PMC when speaking and controlling posture simultaneously. Forty-one right-handed healthy adults (25 female, 16 male) maintained an upright stance while either verbally describing or mentally imagining navigation through a visually presented maze from an egocentric or allocentric perspective. Postural sway was quantified using inertial measurement units (IMU), while cortical activity was recorded with functional near-infrared spectroscopy (fNIRS). Postural sway increased during verbal description compared to imagery, whereas imagery did not compromise balance. Verbalization was associated with increased activation in bilateral premotor and supplementary motor cortices (PMC) relative to imagery. Across conditions, cortical activation was greater in the PMC and parietal cortex than in the frontal cortex and Broca's area, with a predominantly left-hemispheric pattern. Allocentric navigation elicited greater overall cortical activation than egocentric navigation but did not impair postural stability. These findings demonstrate that verbalizing motor actions destabilizes posture by overloading shared motor-cortical resources, revealing a premotor bottleneck underlying dual-task interference (DTi). In contrast, increased cognitive demand alone does not necessarily impair balance. Action verbalization thus represents a critical source of postural interference during everyday dual-task behavior.