How can poor posture result in back pain?

Simply like this, posture cause the muscles of the upper back to continually overwork to counterbalance the pull of gravity on the forward head. This position is often accompanied by forward shoulders and a rounded upper back, which not only feeds into tha neck problems, But can also cause shoulder pain .

 hope this helps :)

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(I took notes on this before and I'm not in highschool xD) This posture would cause the muscles of the upper back to continually overwork to counterbalance the pull of gravity on the forward head, which would start to hurt your back if you continue to do so. (Hope this helps :p)