When you stand, your spine can curve naturally in a S shape known as the ‘neutral’ spine. This is the healthiest shape for your back.
When seated, your spine should remain in this neutral shape. This requires a good, supportive chair that fits the shape of your back.
Spinal joints have a small range of motion – just 7 degrees. If you sit in a chair that doesn’t give you good back support, your spine is able to slump back into a C curve. In this shape, your spinal joints are forced outside their natural range.
Compared to most other joints, spinal joints are relatively slow to register pain. This is why things that are bad for your back don’t always hurt at the time, instead you might be sore afterwards. It’s also why your back may be stiff after a long drive, though you didn’t register the seat as uncomfortable when you were sitting in it.
Spinal joints are designed to flex by only a small amount. Flexing beyond this range by slumping your spine eventually leads to pain.