Sit, stand and sway: postural state modulates visual influence on sway.

Human locomotion and navigation involve an interrelationship between individuals, tasks, and the environment. For example, when balance is challenged, visual input helps fine-tune postural control responses. Perception-Action (P-A) theory emphasizes the critical feedforward role of optic flow in locomotor control and adaptation. While P-A coupling has been previously studied, the nature of this relationship, sex differences, and its impact on movement control, particularly muscle activity, require further investigation. Participants (N = 24; 23 ± 3.4 years) were instrumented with upper-body kinematic markers, and right-leg muscle activation was recorded using electromyography (EMG). Three postures were assumed (seated with feet planted, seated with feet dangling, standing) while viewing three virtual optic flow fields (hallway, boardwalk, trail) on a projector screen, along with a no video control condition. A weighted center of mass (COMHTP) model assessed displacement and acceleration range, and segmental analyses calculated trunk and head range of motion (ROM). EMG signals were filtered, normalized to the no video condition, and integrated using trapezoidal methods. Significant vision × posture interactions were found for COMHTP displacement in both sexes, with greater sway during optic flow conditions, particularly when standing. Posture consistently influenced COMHTP outcomes, with standing eliciting greater displacement and acceleration. Head and trunk ROM showed selective interactions in pitch and yaw. EMG results indicated posture-related increases in vastus lateralis (females) and gastrocnemius (males). Findings suggest visual cue complexity and posture interact to influence balance-related motor behaviours and highlight the importance of considering sex in visuomotor research.