{"schemaVersion":1,"id":"en-mechanism-balance","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-balance","slugPath":"body/systems/balance","title":"Balance","shortTitle":"Balance","summary":"Three independent sources of information the brain has to reconcile. Standing still is not a stable state: it is a permanent correction, and closing your eyes is enough to make that visible.","level":"intermediate","estimatedMinutes":8,"synonyms":["balance","vestibular system","inner ear","semicircular canals","posture","vertigo"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"Standing is not a state","anchor":"standing-is-not-a-state"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"The picture of a body \"in balance\" suggests a stable state, reached and then held. The measurements say otherwise: standing without moving, the body sways continuously, and those sways are constantly corrected."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"What we call balance is therefore a process, not a position. It never stops, including when nothing seems to be happening."}]},{"id":"b4","type":"heading","level":2,"text":"Three sources, one conclusion","anchor":"three-sources-one-conclusion"},{"id":"b5","type":"paragraph","content":[{"kind":"text","value":"The nervous system has three channels of information about the position of the body, and they are independent."}]},{"id":"b6","type":"comparison","title":"Where the information comes from","columns":["Source","What it reports","What defeats it"],"rows":[{"label":"Vision","cells":["The position of the body relative to the surroundings","Darkness, surroundings that are themselves moving"]},{"label":"Proprioception","cells":["The position of the segments relative to one another","An unstable surface, fatigue"]},{"label":"Vestibular apparatus","cells":["The movements and orientation of the head","Damage to the inner ear"]}]},{"id":"b7","type":"paragraph","content":[{"kind":"link","value":"Proprioception","href":"/en/body/systems/proprioception","external":false},{"kind":"text","value":" has its own page. Vision is the most obvious channel. The third is the least known, and it is housed inside bone."}]},{"id":"b8","type":"heading","level":2,"text":"The vestibular apparatus","anchor":"the-vestibular-apparatus"},{"id":"b9","type":"definition","term":"Vestibular apparatus","definition":"A set of fluid-filled cavities in the inner ear that detect the movements and orientation of the head. It plays no part in hearing, although it shares the same bony region.","alsoKnownAs":["organ of balance"]},{"id":"b10","type":"paragraph","content":[{"kind":"text","value":"It is built in two parts, which do not measure the same thing."}]},{"id":"b11","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"strong","value":"semicircular canals"},{"kind":"text","value":" are three tubes set in three different planes. When the head turns, the fluid inside them lags behind the wall, and that lag is detected. They therefore signal "},{"kind":"strong","value":"rotations"},{"kind":"text","value":"."}]},{"id":"b12","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"strong","value":"otolith organs"},{"kind":"text","value":" contain minute crystals resting on a sensitive membrane. Their weight tips them when the head tilts, and their inertia shifts them when it accelerates in a straight line. They therefore signal "},{"kind":"strong","value":"orientation relative to vertical"},{"kind":"text","value":" and linear accelerations."}]},{"id":"b13","type":"evidence","level":"established","statement":"Balance control rests on integrating visual, proprioceptive and vestibular information, whose relative contribution varies with conditions.","detail":"This is agreed across the reference works in neuroscience, established notably by removing or perturbing one source at a time. The exact proportions depend on the task, the supporting surface, the lighting and the person: no single split holds everywhere.","referenceIds":["kandel2021","purves2018"]},{"id":"b14","type":"heading","level":2,"text":"The experiment everybody has already done","anchor":"the-experiment-everybody-has-already-done"},{"id":"b15","type":"paragraph","content":[{"kind":"text","value":"Stand on one leg, then close your eyes. The difficulty rises immediately and markedly."}]},{"id":"b16","type":"paragraph","content":[{"kind":"text","value":"Nothing has changed in the leg: not the strength, not the position, not the surface. What changed is the number of channels available. The system was working with three sources, two remain, and it has to lean harder on them."}]},{"id":"b17","type":"paragraph","content":[{"kind":"text","value":"That is the simplest demonstration of the principle: balance is not a quality of the body, it is the result of sufficient information and fast correction."}]},{"id":"b18","type":"heading","level":2,"text":"When the sources disagree","anchor":"when-the-sources-disagree"},{"id":"b19","type":"paragraph","content":[{"kind":"text","value":"The three channels do not always say the same thing, and the system then has no way of knowing which is right."}]},{"id":"b20","type":"paragraph","content":[{"kind":"text","value":"This is what happens in a ship's cabin: the vestibular apparatus signals movement, vision sees only a still room. The conflict between the two is the mechanism most often invoked to account for motion sickness."}]},{"id":"b21","type":"heading","level":2,"text":"What this illuminates elsewhere in the corpus","anchor":"what-this-illuminates-elsewhere-in-the-corpus"},{"id":"b22","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"link","value":"walking","href":"/en/movements/walking","external":false},{"kind":"text","value":" page describes a single-leg stance occupying more than a third of the cycle, during which the pelvis must be held. The "},{"kind":"link","value":"lunge","href":"/en/movements/lunge","external":false},{"kind":"text","value":" page describes a narrow base that makes sideways control a constraint in its own right."}]},{"id":"b23","type":"paragraph","content":[{"kind":"text","value":"In both cases the muscular command presupposes reliable position information. "},{"kind":"link","value":"Gluteus medius","href":"/en/body/muscles/gluteus-medius","external":false},{"kind":"text","value":" can only correct a pelvis if something has signalled that it was tipping."}]},{"id":"b24","type":"paragraph","content":[{"kind":"text","value":"And the sense of effort itself is not a reliable gauge."}]},{"id":"b25","type":"evidence","level":"probable","statement":"The perception of the force produced depends partly on signals tied to the command sent, and not only on peripheral receptors.","detail":"The review gathers work on perceived position, movement and force. The respective share of central and peripheral origins remains debated across tasks, but the existence of a central contribution is widely accepted.","referenceIds":["proske2012"]},{"id":"b26","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b27","type":"callout","tone":"limit","title":"An organ inside bone","content":[[{"kind":"text","value":"The vestibular apparatus sits within the thickness of the temporal bone, and its cavities measure a few millimetres. The skull does appear in Body Lab's scenes; what it contains does not."}],[{"kind":"text","value":"Showing a skull on a page devoted to balance would mean showing a container for its contents — the same rule as for the "},{"kind":"link","value":"brain","href":"/en/body/systems/the-brain","external":false},{"kind":"text","value":" page."}]]},{"id":"b28","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b29","type":"callout","tone":"caution","title":"No exercise, no assessment","content":[[{"kind":"text","value":"This page describes a sensory system. It proposes no balance exercise, no protocol, and assesses no individual situation — the practices presented under that name cover very heterogeneous content."}],[{"kind":"text","value":"Vertigo, a spinning sensation, instability that settles in, repeated falls or an associated hearing loss belongs to a health professional."}]]},{"id":"b30","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b31","type":"sourceList","title":"Main sources","referenceIds":["kandel2021","purves2018","proske2012"]},{"id":"b32","type":"shapierAction","actionId":"conseils-exercices-mobilite","label":"Explore mobility exercises","description":"Working on body control in movement happens in Shapier; Body Lab only explains where the information that makes this control possible comes from.","webTarget":"https://shapier.app/en/conseils/exercices-mobilite","appTarget":"shapier://conseils/exercices-mobilite"},{"id":"b33","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"Standing without moving, what is the body actually doing?","choices":[{"id":"q1c1","label":"It sways continuously and those sways are constantly corrected","correct":true,"explanation":"That is what the measurements say: balance is a process that never stops, not a stable position reached and then held."},{"id":"q1c2","label":"It stays still as long as no movement is visible","correct":false,"explanation":"The picture of a stable balance is intuitive but wrong: the apparent stillness hides permanent sway and its correction."},{"id":"q1c3","label":"It only tips when fatigue sets in","correct":false,"explanation":"The sways are not a sign of fatigue: they exist all the time, including when nothing seems to be happening."}]},{"id":"q2","prompt":"Why does closing your eyes make standing on one leg immediately harder?","choices":[{"id":"q2c1","label":"The system loses one of its three information channels and has to lean harder on the two remaining","correct":true,"explanation":"The page is explicit: neither the strength, nor the position, nor the surface changed in the leg; only the number of information sources dropped."},{"id":"q2c2","label":"The leg loses strength when you stop looking at it","correct":false,"explanation":"Nothing changes in the leg itself; what changes is the available information, not the muscular capacities."},{"id":"q2c3","label":"The inner ear stops working without vision","correct":false,"explanation":"The three channels are independent: closing the eyes removes vision, but proprioception and the vestibular apparatus keep working."}]},{"id":"q3","prompt":"In a ship's cabin, what happens to the balance system?","choices":[{"id":"q3c1","label":"The vestibular apparatus signals movement while vision sees a still room, with no way of knowing which is right","correct":true,"explanation":"This conflict between the two channels is the mechanism most often invoked to account for motion sickness."},{"id":"q3c2","label":"Vision becomes blurry because of the ship's motion","correct":false,"explanation":"Vision works normally: it simply sees a still room, and it is precisely this contradiction with the inner ear that causes the problem."},{"id":"q3c3","label":"The otolith organs detect a continuous rotation","correct":false,"explanation":"The otolith organs signal orientation relative to vertical and linear accelerations, not rotations, which belong to the semicircular canals."}]}]},{"id":"b34","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-proprioception","en-movement-walking","en-mechanism-motor-control"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-proprioception"},{"type":"explains","targetId":"en-movement-walking"},{"type":"explains","targetId":"en-movement-lunge"},{"type":"next-step","targetId":"en-mechanism-motor-control"}],"authors":["equipe-editoriale-shapier"],"reviewers":["Thanh Chau"],"references":[{"id":"kandel2021","authors":"Kandel ER, Koester JD, Mack SH, Siegelbaum SA","year":2021,"title":"Principles of Neural Science, 6th edition","source":"McGraw Hill","kind":"book","url":"https://www.mheducation.com/highered/mhp/product/principles-neural-science-sixth-edition.html?viewOption=student"},{"id":"purves2018","authors":"Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE","year":2018,"title":"Neuroscience, 6th edition","source":"Oxford University Press","kind":"book","url":"https://lccn.loc.gov/2017028094"},{"id":"proske2012","authors":"Proske U, Gandevia SC","year":2012,"title":"The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force","source":"Physiological Reviews","kind":"review","doi":"10.1152/physrev.00048.2011"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["No scene accompanies this page: the vestibular apparatus is buried in the temporal bone and appears in none of the anatomical sources Body Lab uses.","The respective share of the three information sources varies with the task, the surface, the lighting and the person: no single split holds everywhere.","This page proposes no balance exercise and assesses no individual situation. Practices called balance training cover heterogeneous content.","Vertigo, instability that settles in, repeated falls or an associated hearing loss belongs to a health professional."],"seo":{"title":"Balance — three pieces of information the brain must reconcile","description":"Vision, proprioception and the inner ear: where balance comes from, why standing is a permanent correction, and what happens when the three disagree.","canonicalPath":"/en/body/systems/balance","image":"/og/en/mechanism-balance.png"},"app":{"offlineEligible":true},"version":1}