There are 3 regions of the spine (cervical, thoracic, lumbar); the mid back is considered the thoracic spine and is comprised of 12 individual vertebra. Beside the vertebra, the health of the mid back is highly effected by the low back, neck and shoulder joints; any tightness, weakness or imbalance within these other joints places stress on the mid back.
Kyphosis is the postural name given to the mid back when someone slouches. Not only will this cause the joints and muscles of the mid back to ache, but will eventually lead to shoulder, neck and low back difficulties.
With Mid back pain and altered postures, you can also begin to notice some difficulties at the shoulder. As shown in the below picture, if you have limitations in your mid back it will prevent you from being able to raise your arms above your head. In order to fully lift your arms, you mid back and neck joints must be able to extend or slide backwards. A forward bent posture prevents this which begins negatively affecting your shoulders.
Our back alignment is greatly dependent on our muscle strength. The stronger our back muscles the better our posture. The back muscles help pull the head and the shoulder back and up giving us more of an upright posture, but as our back muscles weaken the shoulder and the head will begin to slouch forward.