Leeuwis, M. | Search bioRxiv

bioRxiv2026

Cortical responses to balance perturbations persist without active postural control

Standing balance relies on rapid reflexes as well as longer-latency subcortical and cortical processes to generate corrective responses to postural disturbances. Electroencephalography (EEG) studies consistently identify two perturbation-evoked markers of cortical activity, the balance N1 and midfrontal theta power, associated with changes in body orientation and corrective actions. It remains unclear, however, whether these markers depend on the nervous systems active control of posture or reflect a more general evaluation of unexpected sensory input. We tested this by measuring cortical and muscle activity during support-surface perturbations while systematically manipulating whether participants actively controlled posture. In Experiment 1 (n = 10), participants experienced identical perturbations while either actively balancing or being passively moved through equivalent motion. Despite large reductions in balance-correcting muscle activity during passive trials ([~]30-60%), N1 and theta responses persisted with only modest amplitude reductions ([~]10%). In Experiment 2 (n = 16), we created passive conditions increasingly removed from balance by varying sensory feedback (footplate + whole-body vs footplate-only motion) and motor engagement (isometric contraction vs. relaxed posture). Relaxed postures markedly suppressed muscle responses, yet cortical responses persisted, showing only modest modulation with sensory feedback (larger during footplate-only rotations) and no dependence on motor engagement. Together, these results indicate that N1 and midfrontal theta are not dependent on active postural control and persist even without matching sensory feedback or motor engagement. Rather than reflecting the generation or scaling of corrective actions, they index the early detection and evaluation of unexpected sensory events, consistent with prediction error or surprise processing. Key pointsO_LIWhen standing balance is disturbed by a perturbation, the brain shows characteristic electrical responses called the balance N1 and theta activity, which are thought to contribute to balance-correcting actions. C_LIO_LIWe tested whether these cortical responses depend on actively controlling posture or instead reflect the detection of unexpected motion irrespective of balance conditions. C_LIO_LIParticipants stood in a robotic balance simulator and experienced identical perturbations while actively balancing or being passively moved, and when whole-body sensory feedback and muscle engagement were removed. C_LIO_LIThe balance N1 and theta activity persisted in conditions where participants were not controlling their movement and even when whole-body sensory feedback and motor engagement were removed, whereas balance-correcting muscle responses were strongly diminished. C_LIO_LIThis shows that cortical responses to balance perturbations are not specific to active balance control but likely represent the brains detection and evaluation of unexpected sensory events. C_LI

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bioRxiv2025

The energetic cost of human standing balance and gait initiation over a range of natural postures

Humans typically select movements that minimize energetic cost, a principle most clearly observed during locomotion. Whether such optimization of energy expenditure also governs standing balance remains unclear because its energetic cost has not been systematically quantified across a range of natural postures. Moreover, because standing is often the resting state from which most walking begins, the optimization of posture may also reflect the energetic demands of initiating gait. In this study, we use a combination of indirect calorimetry and musculoskeletal simulations to characterize the energetic cost of standing and gait initiation across natural standing postures and investigate whether humans optimize energy expenditure under these conditions. In Experiment 1 (N = 13), we measured metabolic cost at preferred and six different prescribed whole-body orientations. Energy expenditure was lowest at a slight anterior orientation (1.15{degrees}) and increased monotonically with whole-body angle, rising twice as fast posteriorly compared to anteriorly. This asymmetry challenges the common modeling simplification that effort is symmetric and linear or quadratic with lean angle. Furthermore, participants preferred body orientations (1.50 {+/-} 0.73{degrees}) with similar energy expenditure to the minimum-cost orientation but with significantly more postural variability, suggesting that strict postural regulation was not necessary for energy-optimal control. In Experiment 2 (N = 20), participants initiated forward and backward walking from preferred or prescribed lean orientations. Participants did not alter their standing posture before expected gait initiations in the forward or backward direction, consistent with musculoskeletal simulations showing that leaning further in the anticipated direction did not significantly improve gait initiation time or energetic costs. Together, these findings suggest that postural strategies optimize energy efficiency when permitted by the demands of movement readiness. Our study quantifies the energetic cost landscape that governs human postural control, challenges widely used inverted pendulum estimations of this cost, and offers an empirical foundation for developing more accurate simulations of posture and energy expenditure. Author summaryHumans are thought to move in ways that save energy. This idea is well supported for walking, but it is not known whether we do the same during quiet standing. Furthermore, because standing is our idle state from which we initiate movement, we may optimize our posture to ease these transitions. In this study, we investigated whether humans stand in postures that minimize energy expenditure. First, we measured and simulated how the cost of posture changes over a range of natural whole-body orientations and determined that humans tend to choose postures close to the orientation with the lowest cost. Leaning backward incurs an additional energetic cost at twice the rate of forward-leaning postures. Second, we investigated whether expecting to walk in the forward or backward direction affects our preferred posture. Surprisingly, participants did not change their posture in preparation for the known direction of walking. Simulations demonstrated that the energetic benefit of doing so was small. Overall, our findings show that maintaining a slight forward lean results in optimal energy expenditure during standing and gait initiation. However, commonly used assumptions of how energy expenditure varies with lean angle do not match the measured cost distribution and those predicted by musculoskeletal simulation.

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