Before every movement, your brain plays the movie in advance

You have probably already observed it. A pianist who holds back a second longer before a difficult passage, an athlete who visualizes the jump before taking off, someone who suspends a movement because something tells them it will hurt, without having tried it. An elderly person who no longer climbs onto a chair because their brain has already decided they won't manage.

Every time, the same thing happens: before acting, the brain has run a movie, and you believe it more than you think.

Word of the day: the predictive internal model

The predictive internal model, or forward model, is a simulation the brain runs before every movement to predict how the body will feel, see and move in the instant that follows. It is not a distant forecast, but a very fast calculation that anticipates the state of the body a few tens of milliseconds later.

A large part of these simulations goes through the cerebellum, at the back of the skull, just above the neck. It represents only about a tenth of the brain's volume, but it contains the vast majority of its neurons. Its role is less to decide than to simulate, compare and adjust continuously.

Every time the cortex sends a command to the muscles, a copy of that command is used to build a prediction of what the body's sensors will send back (the efference copy). When the prediction matches what actually comes back (reafference), we speak of fluidity, technique, an inhabited movement. If the gap is small but regular, the system readjusts without our noticing. If it becomes too large, we feel friction: stiffness, hesitation, fear of falling, the sense that it won't come out (the mismatch between prediction and feedback).

When the movie is outdated

If the prediction is wrong but persistent, the body obeys an outdated simulation. Someone who had ankle pain for six months sometimes keeps the "fragile ankle" simulation long after the pain is gone: the brain holds the movement back before it starts, because the movie is still playing. An athlete returning from injury does not only recover muscles, they have to rewrite their internal model, and a person who has fallen once often walks differently months later.

This is why work on tissues benefits from being completed by an update of the internal model: as long as the movie predicts failure or pain, the body follows (the motor decision).

To change the movement, you sometimes have to change the movie that precedes it.

To understand how these predictions are learned and updated, the course "How we learn to move" is free, as is the introduction to RNP, Neuro-Postural Reprogramming.

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