What you see of a movement is already late

A sprint ground contact: a tenth of a second.

That is too short for the brain to correct it along the way.

This week, I reread the article we wrote on the sensorimotor loop, and I came across a number that stops me every time. In a top-level sprinter at full speed, the foot stays on the ground for about a tenth of a second, sometimes less.

The faster you run, the shorter this contact gets, because the distance covered during ground contact stays roughly the same, around one meter (Weyand and his colleagues measured it as early as 2000).

Put this number next to two others, and you get a real problem:

• ground contact lasts about 100 milliseconds

• correcting a movement based on what you feel takes 100 to 200 milliseconds

• and yet, the sprinter does not collapse at every stride

So the foot has left the ground before a correction based on sensation can arrive.

To understand this, we first need to pin down two terms. The sensorimotor loop is the full cycle of any movement: the body senses, the nervous system decides, the muscles execute, then the body senses the result, which is used to adjust the next movement.

Proprioception is the sense that tells you where your limbs are and what they are doing without looking at them, thanks to sensors housed in the muscles, tendons and joints. If the sprinter waited for proprioception to signal that the foot is slipping before correcting, the correction would arrive when the foot has already left the ground.

The body does have faster circuits: reflexes. A stretch reflex goes through the spinal cord without climbing up to the brain, and responds in 20 to 30 milliseconds in the leg.

A second response, which probably makes a round trip through the cortex, arrives at around 50 to 60 milliseconds, and the first voluntary muscle signal only appears at around 170 milliseconds (these are the figures from Roger Enoka's reference textbook). These reflexes are valuable, but they always give the same response: they do not know whether the ground is wet, sloped, or whether you are on your fifteenth sprint of the session. We went through the difference between a reflex and the sensorimotor loop in a separate article.

The solution the nervous system found is prediction. Before each ground contact, the brain runs what researchers call an internal model: a simulation that forecasts what the command sent to the muscles will produce, and what the foot should feel at contact.

When it sends that command, it keeps a copy for itself, the efference copy, which tells it what sensation to expect and lets it spot any gap between what was planned and what actually happened. The leg muscles, for their part, already contract before the foot touches the ground: this is pre-activation, which prepares the stiffness of the leg during the flight phase.

Shadmehr and his colleagues produced a reference synthesis of this in 2010: it is this gap between what was planned and what happened, far more than repetition, that makes a movement improve.

You produce this kind of preparation thousands of times a day, without ever thinking about it. Standing, raise one arm as fast as you can: the muscles at the back of the thigh activate a few tens of milliseconds before the shoulder, to anchor the body to the ground and absorb in advance the imbalance the arm is about to create. This is called an anticipatory postural adjustment.

For years, I shouted "absorb it!" at athletes at the moment of contact, without seeing that I was speaking about an instant when nothing can be decided anymore.

You may know this, for yourself or for someone you work with: a movement you correct ten times, that seems learned in training, and that comes back as soon as the exercise gets fast. If that movement is decided before you see it, watching it while it happens is not enough to understand it.

What is the person in front of you preparing, in the two tenths of a second before ground contact?

Two experiments to try tomorrow, with no special equipment:

• unloading, in pairs, in thirty seconds

• the two tenths of a second before ground contact, filmed in slow motion

Unloading first. Hold an object of one or two kilos in your hand, elbow bent at a right angle, then remove it yourself with the other hand: the forearm that was carrying it hardly moves.

Then ask someone to remove it without warning you: this time, the forearm jumps up suddenly. The load is the same in both cases, and the only difference is that the first time, your nervous system knew when the weight was going to leave and had prepared the movement accordingly.

Then the slow motion. Film a start, a jump or a landing with your phone's slow-motion mode, then watch frame by frame the two or three tenths of a second that precede contact.

Observe the tilt of the pelvis, the direction of the gaze, the tensing of the trunk, and listen to the contact: a short, sharp sound, or a dull sound that crashes down. This window is where the preparation can be seen, whereas ground contact only shows its consequence (we devoted an article to this decision window).

What I take from it is that the coach's eye always arrives after the nervous system's decision.

What you see of a movement, the nervous system decided about two tenths of a second earlier.

At LabO RNP, this is precisely what we teach you to read: the sensorimotor loop as a whole, and in particular what is prepared before the movement.

Our full article details the five stages of this loop, including the one that is almost always forgotten: the comparison between what was planned and what really happened.

If this kind of reading speaks to you, come and take a closer look: we have already trained more than 1,000 practitioners, across 12 professions. You are a practitioner: the introduction to RNP, Neuro-Postural Reprogramming, is free, as is the course "How we learn to move". You are looking for support: request an assessment.