I had bet on the spiky ball

"She did what?"

That is all I managed to write when I saw the two photos.

In October 2023, Seb, who founded LabO RNP with Adrien and me, posted a before/after in our internal group. It showed a person he was working with, photographed a few weeks apart, and the difference was visible from across the room.

He had combined three things:

• proprioception work for the wrist and the ankle

• a spiky ball rolled under the foot

• three exercises: rounded back and arched back, head nods, rocking

I bet on the ball.

The bet made sense. The sole of the foot is covered in mechanoreceptors, tiny sensors that turn pressure, stretch or vibration into an electrical signal for the nervous system.

There are four families: one reads sustained pressure and the texture of the ground, another reads skin stretch. The last two react to change: one to contact or the start of a slip, the other to vibration.

When you impoverish this information, with cold, local anesthesia or thick foam, the body sways more when standing: that is an established result of balance research. At LabO RNP, we therefore regard these sensors in the sole of the foot as one of the main entry points of the sensorimotor loop, and a spiky ball as a simple way to send them a clear signal.

Seb answered: "No, it's your exercises that worked best." These three exercises have one thing in common: they move the head and neck far more than they stimulate the foot.

So they mostly engage the vestibular system, a balance sensor housed in the inner ear, which includes two types of organs. The otoliths are small crystals resting on a membrane, which slide when the head tilts or accelerates in a straight line, something the back-and-forth rocking engages directly. The semicircular canals are three fluid-filled tubes in each ear, which detect head rotations, like those of a nod.

These movements also engage neck proprioception, that is, the sensors in the cervical muscles and joints that constantly signal the position of the head relative to the trunk.

The real lesson of this story is about how the nervous system chooses its information. To stay upright, it combines three sources: vision, the vestibular system and somatosensation, which covers everything the body feels through the skin, muscles and joints.

It does not give them equal trust: this is what is called sensory weighting. In Peterka's work (2002), on a firm surface, somatosensation provides about 70% of balance information, and as soon as the ground starts to move, the vestibular system takes the biggest share. This setting varies from person to person and from moment to moment, so the most useful door in a given person is not necessarily the one you imagine (we have covered what happens when a sensor lies).

I had bet on the sensor I could see best, and I would never have known if I had not asked the question.

Since then, at our open days, we had 31 participants test two minutes of head nods: their distance between fingertips and floor, legs straight, improved by 2.8 centimeters on average. There was no control group, so it is a clue and not proof, but it confirms what we see in the field: the right door has to be measured.

You may know this, for yourself or for someone you work with: an exercise that works wonders for one person and does nothing for another. We insist, then switch exercises at random.

Through which door does the body of the person in front of you agree to let you in?

Here is the test, stimulation, retest protocol we use so we no longer have to bet:

• measure a reference movement, three times, and keep the best trial

• stimulate a single door, for one to two minutes

• measure again in exactly the same way, then compare

For the reference movement, take something simple and quantifiable. For example, the time held with feet together and eyes closed, the time held on one foot, or the distance between fingertips and floor, legs straight.

For the door, choose only one at a time. It can be the spiky ball under a bare foot for one to two minutes, on a hard floor, two minutes of head nods, or ten to twenty slow rocks on all fours.

If the measure improves clearly, that door mattered; if nothing moves, test another one; if the measure gets worse, that is useful information too. Two limits to keep in mind: a gain obtained within the session is not yet a lasting adaptation, so retest a few days later. And if you stack three stimulations before retesting, you will never know which one acted.

A variant lets you find out which source a person relies on. Time them with feet together on a hard floor, eyes open, then eyes closed, then on foam with eyes open, then on foam with eyes closed: this is an enriched version of the Romberg test, and we explained what these tests really measure.

If they drop off as soon as they close their eyes, they rely heavily on vision; if they drop off on the foam, they rely heavily on the support surface. The foam is here to read, not to train, and we explained why standing on one foot or on an unstable surface does not prepare you for the field.

And if an area of the sole of the foot feels nothing at all, the exercise gives way to medical advice. To go further on the foot, we detailed what barefoot walking and minimalist shoes change in the signal from the ground.

What I take from it is that a good exercise is not enough: it needs the right entry point, and that entry point can be measured.

The body chooses its own entry point. Our job is to measure it before insisting.

Testing the doors one by one and reading the result is what the introduction to RNP lays out, free and to follow at your own pace.

If this kind of reading speaks to you, come and look more closely: we have already trained more than 1,000 practitioners, across 12 professions. You're a practitioner: the introduction to RNP, Neuro-Postural Reprogramming, is free, as is the vestibular guide. You're looking for support yourself: request an assessment.