Gravity and posture: how the body perceives the vertical

The most stable reference of your nervous system

Hello to you, the movement professional,

Six months in orbit, then the return. The astronaut steps out of the capsule supported by the ground team, and his first steps wobble even though his muscles were maintained on board throughout the stay. His inner ear has spent months without the weight that told it where down is. Before he can walk normally again, he has to relearn the vertical.

The body perceives gravity thanks to the otoliths of the inner ear, complemented by proprioception, vision and sensors located in the trunk. These signals build a reference vertical, on which the system sets the tone of the antigravity muscles at minimal cost. The RNP reading considers gravity the most stable landmark of posture: it is its loss that triggers the alert.

How does the body perceive gravity?

The gravitational vertical designates the direction of Earth's pull, the one a plumb line takes, and which the nervous system uses as a reference axis to organize posture. It never moves, while your head and your supports move constantly around it.

The first sensor of this direction is in the vestibular system. In the inner ear, next to the semicircular canals that read rotations, two small sensory surfaces read tilt and straight-line accelerations: these are the otoliths. Imagine a gelatinous membrane weighted with tiny calcium crystals, resting on hair cells. When the head tilts, the weight of the crystals slides the membrane and bends the hairs, and the brain receives a signal proportional to the tilt.

The two maculae share the work. The utricle and the saccule are oriented differently: the utricle, lying flat, mostly reads tilts and accelerations in the horizontal plane, while the saccule, vertical, picks up going up, going down, jumping, an elevator. Together, they constantly tell you where down is.

A short detour through physics, because it matters. An accelerometer cannot, on its own, tell a tilt from a straight-line acceleration. The brain resolves the ambiguity by cross-referencing the otoliths with the semicircular canals, vision and the supports. Perceiving gravity is therefore already constructing something.

Antigravity muscles: what holds the body upright?

Antigravity muscles are those that oppose the collapse of the body toward the ground: extensors of the neck and back, glutes, quadriceps, calves, in particular the soleus. They work continuously, quietly, as soon as you stand.

The otolithic signal reaches them by a direct route. From the lateral vestibular nucleus (Deiters' nucleus), the lateral vestibulospinal tract descends in the spinal cord on the same side and facilitates the motoneurons of the extensors, while inhibiting their antagonist flexors. If the head leans to one side, this tract reinforces extension on the side that risks giving way. The detail of these vestibulospinal pathways also sheds light on how the head prepares the force that arrives at the ground.

Feel it on yourself, right now if you are standing. Close your eyes, let your head drift very slightly forward: the calves and the back of the thighs tighten a notch, without your having decided it. That is the system resetting your center of mass onto its vertical.

This adjustment was built well before the weight room, on the floor, in the first months.

TLR, Landau and righting reactions: gravity organizes the infant

The first program that organizes the infant in relation to gravity is the tonic labyrinthine reflex (TLR). Its triggering stimulus is the position of the head relative to the vertical, read by the labyrinth: head forward, the body curls into flexion; head back, it opens into extension. The newborn then experiences gravity as a force that pins them to the ground, and the TLR gives them a first global organization of tone.

The righting reactions take over. The baby learns to lift the head against gravity, then to align the head with the trunk, then the trunk with the pelvis. Around three to four months the Landau reaction appears: held face down in suspension, the baby lifts the head, braces the back and extends the legs. The Landau belongs to the righting reactions, a family distinct from the archaic reflexes, and it persists in a functional form.

Where the TLR submits to gravity, the righting reactions learn to lean on it. The cortex learns to say no to the brainstem, the TLR goes under inhibition, and gravity becomes an axis on which the body builds itself instead of a force that crushes it. This is also why the floor movements of primary motor skills in adults remain a working base.

Postural tone, an economical background activity

Postural muscle tone designates the background activity, continuous and largely automatic, that keeps the body's segments aligned against gravity. Think of an engine idling, running permanently, tuned to the millimeter.

In a well-tuned adult, standing costs little. The available measurements put the cost of quiet standing a little above rest, in the order of ten percent compared with sitting, somewhat more compared with lying down. The body stands by stacking its segments on the vertical and letting the ligaments and background tone do most of the work, with small permanent corrections.

When this adjustment becomes blurry, the economy vanishes. You see tone that rises too high, supports that clench, breathing that locks with every effort. The article on postural tone, the silent base of movement, digs into this point.

Perceived vertical: a multi-input calculation

The vertical you feel combines several sources, like the three sensory systems of balance. The vestibular system gives head tilt, proprioception informs about the position of the neck, trunk and supports. Vision brings the lines of the surroundings, and a tilted room is enough to tilt the felt vertical. Ocular convergence and vision therefore also play on this adjustment.

In the 1980s and 1990s, the German physiologist Horst Mittelstaedt added two pieces to the puzzle. The brain would have an internal preference for the body axis, which pulls the perceived vertical toward the head-to-feet line. It would also have a visceral graviception: sensors located in the trunk, linked notably to the kidneys and blood mass, report on the direction of gravity independently of the inner ear.

Back to the astronaut from the start. In weightlessness, his otoliths keep responding to accelerations, but the gravity signal disappears. Up and down lose their obviousness, the brain reinterprets its sensors, and some astronauts describe orientation illusions at the start of a mission. On return to Earth, standing and walking require postural readaptation, from a few days to a few weeks depending on the length of the stay.

The nervous system relies on gravity because it never moves: it is the loss of this landmark that puts it on alert.

Is gravity the first stress on the human body?

A widespread phrase presents gravity as the first stress of the human being. The data invite a calmer reading. Constant, predictable, present from day one, gravity provides the system with its most reliable reference. What triggers the alert is its disappearance or its rupture: a free fall provokes, within a few tens of milliseconds, a muscle response linked to the labyrinthine input, described as early as the 1970s by Greenwood and Hopkins. Weightlessness disorients. A support that gives way makes you startle.

There is still a correct part in this phrase. Working on the floor reduces balance demands: the support surface is wide, the center of mass is low, you cannot fall any lower. The system can then explore head and trunk movements without threat, and it is a good entry point to stimulate the vestibular system gently, before moving back up toward standing and then the sport movement.

Keep one question for your next session: what happens in your athlete when gravity changes its relative direction, at the precise moment they leave the ground?

Gravity and performance: weightlifting, jumps and landings

In weightlifting, the bar adds a vertical load that the system must align on the same reference. The position of the head and gaze influences the engagement of the posterior chain: a stable head, oriented relative to the vertical, lets the vestibulospinal pathway facilitate the extensors of the back, hips and knees at the moment the load is at its maximum.

During a jump, the otoliths register the rise, the apex, then the descent. The landing is prepared in the air: the leg muscles pre-activate before contact, and this pre-activation relies in part on otolithic information. A landing that slams, a knee that collapses inward or a trunk that caves in can signal a reading of the vertical that arrives late. Protective reactions, such as the parachute reaction, extend this logic when balance is clearly lost.

On the RNP reading side, you read the output (the movement, the support, the landing), you test the vestibular input and its cooperation with proprioception and vision, then you stimulate and retest according to the test, stimulation, retest cycle. The image of the body in the dark applies here as is: relight the vertical before loading the output.

What vestibular stimulation does not guarantee

No floor exercise and no vestibular stimulation on its own guarantees a better landing or a heavier bar. The links between perceived vertical and performance are solid in physiology, more indirect at the scale of a given athlete.

One can still test a falsifiable prediction. In an athlete whose stability clearly degrades when vision is removed and the support is made unstable, a few weeks of progressive otolithic stimulation should first improve this test, then the landing quality measured with the same protocol. If the landing does not move, the priority input was elsewhere.

A body that reads gravity well leans on it with every movement.

👉 I want to intervene in the right place.

Frequently asked questions about gravity and posture

How does the body perceive gravity?

Mainly through the otoliths of the inner ear, the utricle and the saccule, which encode the tilt of the head relative to the vertical. Proprioception, vision and sensors located in the trunk complete this signal, and the brain combines everything to build a reference vertical.

What are the antigravity muscles?

They are the muscles that oppose the collapse of the body toward the ground: extensors of the neck and back, glutes, quadriceps, calves. They are notably influenced by the lateral vestibulospinal tract, which facilitates the extensors from the vestibular signal.

What is the gravitational vertical?

The gravitational vertical designates the direction of Earth's pull, that of a plumb line. The nervous system uses it as a reference axis to organize posture. The perceived vertical, for its part, results from a calculation that blends vestibular input, proprioception, vision and visceral graviception.

Is gravity a stress for the body?

The data show rather the opposite: gravity is the most stable reference of the nervous system. It is its loss, during a fall or in weightlessness, that triggers an alert response. In a well-tuned adult, standing costs little energy.

Why work on the floor for posture?

On the floor, the support surface is wide and the risk of falling disappears. The system can explore head and trunk movements without threat, which makes it a good entry point before returning to standing and movement.

What happens to astronauts' posture?

In weightlessness, the gravity signal disappears and the brain has to reinterpret its sensors. On return, standing and walking require a postural readaptation that takes from a few days to a few weeks depending on the length of the stay.

Is working on gravity and posture a form of healthcare?

No. LabO RNP is neither medical nor paramedical, and is a complement to a care pathway, never a replacement for it.

By the LabO RNP team