A slumped posture does not allow us to be alert and to remain aware while meditating. When you slump, you may also experience tension in the neck and shoulders (look at how the back of the neck is short and compressed, rather than long and open).
There are three causes of slumping:
sitting too low
having insufficient tilt in your seat
habitual slumping
In the photograph, the meditator is sitting too low. When you sit too low, this tilts the pelvis backwards, and so the upper back has to slump forwards so that you can stay in balance. This has the effect of closing the chest and reduces your ability to breathe freely. If you sit in this posture you'll tend to feel rather dull and may even fall asleep because of the constriction in your chest.
Having your seat at the right height, but having a flat or insufficiently tilted seat, is also a cause of slumping. And sometimes slumping is just a habit.
After we've had a look at over-arching we'll look more closely at how to deal with slumping. But what we most often do is to try to hold ourselves upright by force of will. This can result in a posture that "looks" okay. If you slump, and then force yourself to sit upright, your posture might look okay from the outside, but before long those clenched muscles will start to feel pretty painful from the inside.
