Imagine driving a car with a steering that doesn’t respond instantly and a GPS that always reflects where you were a second ago. To stay on course, you must constantly infer how to steer the wheel from outdated information.Our brains do exactly that every time we move: sensory signals reach the brain tens of milliseconds after an event and motor commands take similar time to travel to the muscles, which then need extra time to generate force. In other words, the brain is always working with “old news” and must predict the future outcome of every action.This predictive ability is most impressive when we stand upright because it requires keeping a tall, top‑heavy body balanced on two small feet.Balance challengesScientists have long known that neural delays make balance hard to control. Even in healthy young adults, it takes about one-sixth of a second for information from the feet, muscles and inner ears to reach the brain and for a corrective signal to return to the muscles. Simple physics models treat the body as a mass balanced around the ankles and predict that if the delay is too long, standing becomes impossible. The physical properties of our bodies similarly shape how we move. Just as a large van steers more sluggishly than a compact car, a large person standing upright resists motion and feels sudden pushes or bumps less sharply.To test whether the brain treats delayed signals similar to changes in body mechanics, a team at the University of British Columbia and the Erasmus University Medical Centre in the Netherlands built a life‑size “body‑swap” robot. A participant stands in the ‘body-swap’ robot at the University of British Columbia. (Sensorimotor Physiology Lab/UBC), CC BY-NC-SA Participants stand on two force‑sensing footplates and are secured to a padded frame. Motors move the frame in response to the forces they generate, making the whole system behave like their real body swaying under gravity. Crucially, the robot can alter the simulated body mechanics on the fly: it can make you feel lighter or heavier, add or remove energy from your motion, or insert a delay between your forces and the motion you feel — mimicking the brain’s own sensory‑motor lag.Three experimentsWith this tool, researchers asked whether the brain treats time (delay) and space (body dynamics) independently, under three experiments:1. Changing body dynamics and delays alter balance similarly: Participants stood while the robot inserted a 0.2‑second lag between their commands and resulting motion. That pause — a blink of an eye — caused larger sway and pushed many participants to a virtual “fall” boundary. Similarly, sway increased when the robot made the body feel lighter or added energy to the motion, much like a gust of wind pushes you forward.2. Delays feel like altered body mechanics: With the delay turned off, participants adjusted their bodies’ mechanical properties until their sensation matched the delayed condition they had just experienced. They chose a lighter body or a setting that added energy. When they were asked to make the delayed condition feel “natural,” participants selected a heavier body or a setting that dissipated energy from the motion. Hence, tweaking the body’s mechanical properties can recreate or cancel the feeling of delayed information. 3. Improving balance under delay: Volunteers who never experienced the robot stood on it with the 0.2‑second delay present, combined with a heavier body or one that dissipated energy from the motion. Their balance improved instantly: sway dropped by up to 80 per cent and most participants no longer reached the virtual fall boundary. Blending time and spaceTaken together, the three experiments support one conclusion: the brain does not store separate solutions for “late information” and an “unstable body.”Instead, it maintains a unified internal model that blends time and space into one representation of movement. When sensory feedback is outdated and the body feels unstable, adding heaviness and dissipating energy from the motion restores balance. Conversely, making the body lighter or adding energy reproduces the instability caused by delays. In either case, a unified representation of balance is used to keep you upright.These findings are more than a laboratory curiosity. As we age or when diseases damage long nerves, signals travel slower and are more disrupted, leading to balance deficits and a higher risk of falls. According to the World Health Organization, about one in three older adults falls each year, and falls are the leading cause of injury‑related hospital stays, costing health systems billions of dollars. The body‑swap robot offers a new perspective to this problem: assistive devices and wearable exoskeletons that supply just enough “helpful resistance” the moment a person begins to sway can counteract the destabilizing effects of neural delays. They also raise a broader question: have the body sizes of animals and the mechanics that compensate for neural delays evolved to enhance their survival? The next time you lean over a sink or chat in a doorway, remember that your brain is quietly juggling time‑and‑body representations in the background. The fact that you never notice this balancing act may be the most astonishing finding of all.Jean-Sébastien Blouin receives funding from the Natural Sciences and Engineering Research Council of Canada. Patrick A. Forbes receives funding from the Dutch Research Council (NWO).