Try this experiment—this works best if you have two identical chairs. If you only have one you can work with that. The chair should have a relatively firm seat—a typical kitchen chair works well.
Determine if the chair seat (a) slopes backward (the front of the seat is higher than the back), (b) is level or (c) slopes forward (the front of the seat is lower than the back).
You will create two different seats so you can contrast them—one that slopes backward and one that is level or slopes forward a bit. You will need several same sized books to adjust the chair seats.
If your chair is level put two same sized books under the front legs so that the seat slopes backward. If your chair slopes backward put two same sized books under the back legs so that the seat is level or slopes slightly forward.
Sit on both chairs. Sit toward the front of the chair seat and don’t lean back. This is called active or task sitting (as contrasted with restive sitting, when you lean against the back of a chair). Don’t think too much. See if your body prefers one seat over the other.
Which one would you prefer to sit on if you were going to work on your laptop at the table, eat your lunch or write a letter?
Most people find is that they prefer the chair with the seat that is flat or sloped slightly forward.
Why would this be?
Sit on your chair that has the level or slightly forward sloping seat. What are you sitting on? If you have your bottom behind you, you will be sitting on your sitting bones. Remember that these sitting bones are rounded or rocker shaped.
The fact that your sitting bones are rounded is very important. What happens when you put a round object on a backward sloping seat? It rolls backward!
When the sitting bones roll backward it pulls the pelvis backward and causes the whole trunk to collapse into a c-shape, putting pressure on the lower back. It is nearly impossible at this point then to bend forward using the hip joints. Instead you are left to bend at your waist, which is not a joint at all.
A recipe for disaster!
To deal with backward sloping chairs I often fold up my jacket and use it as a pillow to level out the seat as best I can.
There are commercially made wedge shaped cushions to help correct backward sloping seats. If you invest in a wedge cushion I suggest a rather firm one.
Take a survey of how many chairs you see that slope backward, even slightly.
You will be surprised how ubiquitous they are.
So, why do so many chairs have backward sloping seats? One reason is that they are easy to stack.
Our bodies are paying a high price for the ease of storing a bunch of chairs!
If this topic is of particular interest to you check out this podcast with Galen Cranz, author of The Chair: Rethinking Culture, Body and Design