Many of my patients tell me the same thing in their first consultation: 'I'm not really sedentary, I run two or three times a week.' And yet, digging a little deeper, I usually find eight-to-ten-hour days spent sitting, in the same position, with very little variation. It's the most common misunderstanding I come across in practice — and it's worth clarifying, because it changes how we can act on it.
This month we're opening a new series on a topic I still find widely misunderstood: sedentary behaviour, not as an absence of physical activity, but as the accumulation of static mechanical load on the body.
What sedentary behaviour is not
Public health research has shown this for years: physical activity level and time spent sitting are largely independent variables. Someone can be very athletically active and still spend eight to ten hours a day seated — at a desk, in a car, in front of a screen in the evening. The body doesn't average the two out. An hour of intense exercise doesn't 'cancel out' nine hours of static positioning, in the sense that it doesn't undo the mechanical load built up during those nine hours.
What it actually is: mechanical load that accumulates
What I observe in clinic is that sedentary behaviour functions as a load — in the biomechanical sense of the word. Sitting isn't neutral for the body: the intervertebral discs are continuously compressed, certain muscles work in prolonged isometric contraction to hold posture, and circulation slows in the lower limbs. None of this is dramatic on its own, over a few minutes. The problem appears when this load repeats, hour after hour, day after day, without enough interruption.
Why duration matters more than intensity
This is a point I often explain differently from what patients expect: it's not sitting itself that's the problem, it's uninterrupted duration. Even a posture considered 'correct' — straight back, screen at eye level — ends up creating repetitive load if held without variation for hours. The body is built to change position regularly, not to hold an optimal posture continuously. In other words: the best posture is the one that changes.
Understanding this distinction already changes how the problem can be approached: it's not just about 'exercising more,' but about reducing the mechanical load built up during seated hours. In the next article, we'll look in detail at what actually happens in your tissues after 30 minutes of stillness — discs, muscles, circulation.
— Physical Therapist, founder of Reprogrammer Boutique