Many people in Germany are now investing a considerable amount of money in a "good" office chair – and yet still end up sitting there in the afternoon with a pulling sensation in their lower back or that dull feeling of heavy legs. This is frustrating because it feels as if ergonomics are an empty promise.
My theory is uncomfortable, but surprisingly often true in practice: The problem isn't primarily in the backrest. It's in the front – in the seat. More precisely: in the seat depth.
Because sitting isn't just a matter of comfort. Sitting is also a matter of blood circulation. If the backs of the knees aren't free, blood circulation in the legs can be impaired – and this is explicitly pointed out during the test sitting.
Anatomy & Ergonomics: The Chain of Effects
Here is the principle that I consider crucial: seat depth is the “starting point” of a chain reaction – and this chain often ends in the back, even though it starts in the front.
Too deep: When the seat "presses into the back of the knee"
If the seat is too low, the front edge presses towards the back of the knee. This sounds trivial, but it's a real ergonomic problem because blood vessels and nerves in that area don't react well to constant pressure. The VBG (German Social Accident Insurance Institution for the Trade and Distribution Sector) therefore explicitly recommends a gap of a few centimeters (approximately 5 cm, about three finger widths) to avoid compressing blood vessels.
And now comes the point that many underestimate: If the seat is too low, you unconsciously slide forward to avoid the pressure. Result:
- The pelvis moves forward.
- The lumbar spine loses contact with the backrest.
- The lumbar support no longer "hit" where it is supposed to work.
In short: The chair may have great lumbar support – but you sit in such a way that it becomes decoration.
Furthermore, the venous pump (the interplay between muscle activity and venous valves) is not improved if you are already sitting statically for extended periods and the back of the knee is also under pressure. To put it bluntly: this is prevention in reverse.
Too short: When the seat lets your thighs down
If the seat depth is too short, you lack sufficient support under your thighs. This results in too much pressure being concentrated on your sit bones – and this "small pressure point" becomes a major problem after two hours. It's not just uncomfortable: it often leads you to constantly change position, sit crookedly, or slouch forward.
And again, the following applies: The lumbar support can only contribute to relieving pressure on the intervertebral discs if you actually stay back in the chair – and for that, the seat depth must be correct.
The "standard chair" myth: Why "one size" doesn't fit.
In Germany, people love standards – and rightly so. But when it comes to sitting, "standard" is often a myth that makes people sick.
Yes, there are standards and measurement methods based on DIN EN 1335 – and seat depth is listed there as a relevant parameter. The problem is that many inexpensive or older, run-of-the-mill office chairs don't have a sliding seat (i.e., no seat depth adjustment). They then come with a fixed measurement – and this fixed measurement only fits a relatively small "standard range" in reality.
If you are on the shorter side (under about 1.65 m), there's a good chance the seat will feel too deep and press down on your knees. If you are very tall (over about 1.85 m), you often won't have enough support under your thighs – the seat will feel too short. In both cases, it's not your body that's the problem. It's the chair.
I'm deliberately phrasing this subjectively because I believe it's the most important shift in thinking: You don't have to adapt to the chair. The chair has to adapt to you – not the other way around.
The practical test: The 2-4 finger rule
If you take only one thing away from this text: Do this check now. Not later. Now.
Step 1: Sit upright, with your buttocks pushed back as far as possible so that you can actually reach the backrest.
Step 2: Check the distance between the front edge of the seat and the back of the knee/calf.
Standard: There should be a few centimeters of space between the back of the knee and the front edge of the seat – in practice, about 2 to 4 fingers' width (often described as "fist-width"). A very practical guideline is around 5 cm or about three finger-widths , to ensure that blood vessels are not compressed.
If you find that you can only sit "nicely" when you slide away from your back: then the problem is not your discipline, but the fit.
Solution & Conclusion: Investing in the Fit
My clear recommendation is: Don't judge your chair by price, brand or "how soft it feels", but by fit .
Your next step:
- Do you have back problems, heavy legs, or numbness?
- And can't the seat depth be adjusted?
Then this is not just a minor comfort issue, but a pretty clear signal: you are very likely sitting against your body – every day.
Modern ergonomic chairs solve this problem simply with a sliding seat. And interestingly, product specifications for adjustable seat depth specify a minimum adjustment range of 50 mm (5 cm) – this is precisely the measurement that often determines in practice whether the back of the knees remains free and whether you can sit back against the chair's backrest.
Furthermore, if a product is tested according to DIN EN 1335, this understandably reduces skepticism for many in Germany, because requirements for dimensions and verification are systematically considered. A recent example from the market even cites the newer DIN EN 1335-1:2023 standard and links it to a sliding seat with adjustable seat depth.
Finally, to reiterate without any evasiveness: I consider seat depth adjustment to be one of the most underrated features of an office chair. Not because it's "nice to have"—but because it determines whether your lumbar support can actually function and whether the backs of your knees remain free, so that blood circulation in your legs isn't unnecessarily restricted.
FAQs
Why is seat depth so important?
Because it keeps the backs of the knees free, distributes pressure, and enables the lumbar support to work correctly.
How do I check the seat depth correctly?
Sit all the way back and leave about 2-4 fingers' distance between the front edge of the seat and the back of your knee.
What should I do if my chair doesn't have seat depth adjustment?
If you experience pressure in the back of your knees, numbness, or back stress, a chair with a sliding seat is the more sensible solution.
