What is good posture?
If you look at the picture below, I think we can all agree that the first picture shows a girl slouching, and that this is “bad” posture.
But what about the second picture? Her eat is over her shoulders, her shoulder is over her hips. She looks pretty lined up. Btu when I look at her lower back, I can see a big curve there. What is tells me is that her hip flexors are shortened or tight, pulling her pelvis forward. Her lower back is lordotic, or rounded. Her lower back erector spinae muscles are working really hard to hold her up.
The third photo shows good sitting posture. While we don't want a flat spine, the muscles along the spine will be less active, less tight, less fatigued, less painful.
Below I included a picture of how to sit correctly at a computer.
— Korey Pieper, DPT, OCS, CCI