Noelia Samartin, a doctor of neurology and clinical psychologist, says the thing no fitness marketing wants to hear: most of the damage does not come from not training, it comes from sitting. And those are two different things we have been mixing up for years.

She explains the difference with an example that stings: a person who exercises two hours a day at the gym is still sedentary if the rest of the day is spent sitting uninterrupted for more than four hours. Those two hours do not buy off the remaining twelve hours of stillness. Movement spread through the day, she says, is worth more than concentrated effort in the evening.

What actually happens when you walk

The mechanism is not abstract. While we walk, the body releases molecules such as IGF-1 which cross the blood-brain barrier and stimulate BDNF - a factor Samartin describes as fertiliser for the nervous system. It strengthens the connections between neurons and helps form memories in the hippocampus.

On top of that comes the chemistry you feel the same day: walking raises serotonin, which keeps mood stable, dopamine, which carries motivation, and endorphins. Noradrenaline in turn lifts alertness and attention. That is why a conversation that will not get going at home unties itself after ten minutes of walking.

The figure that demolishes the ten thousand myth

Research has shown that maximum protection against dementia is reached at around 9,826 steps a day - but also that a significant protective effect begins at roughly 3,800 steps. Above ten thousand there is no additional benefit. So the number 10,000, which every wrist device writes in red when you have not reached it, is closer to marketing than to science.

That is good news for anyone living a normal life rather than a life designed around an app. Three thousand eight hundred steps is the market and back, or two stops on foot instead of the bus. The brain, as Samartin puts it, thanks you for consistency far more than for sporadic effort.

The question is not whether we have time for the gym. The question is why we built cities in which the walk to the bakery is something you have to plan for separately.