Middle Back pain or mid-back pain means different things to different people. On this website middle back means from your belly button up to the top of your "costal arch", the area shaded red in the diagram. Find where your ribs end and your soft belly begins on both sides, then feel your way up to where your ribs meet in the middle. That's the top of your costal arch.
If you've got "middle back pain" further up in your back, check the Upper Back Pain resources (when they become available.)
The mid-back can be where pregnant women have back pain during and after birth--the mid-back takes some of the strain of the baby's weight.
The middle back is in truth a strange and wonderful beast. It bears a lot of weight: heavy head, chest, shoulders and arms, plus anything you're carrying or doing with your arms. But it is also a very mobile, flexible part of the spine. It twists, it bends and it supports weight.
The lumbar curve of your spine (pointing forwards) turns into the rib spine curve (pointing backwards) and the mid-spine is, well, somewhere in the middle, doing neither one thing nor the other! And, in life, many people are vague and a bit unstable in this part of their bodies.
Mid-back pain is often caused by poor lifting-- some people bend and straighten tiny mid-back joints, rather than stabilising the spine and using big, powerful ankles, knees and hips to do most of the moving. If you look at someone lifting badly with their mid-back, it often looks as if there's a hinge in the middle of their back. It's as if their back folds above their belly button. It's very easily visible in many people.
Middle back pain is relatively difficult to "reach" with exercises. Here is a rolling exercise that is good for reminding the middle back about its flexible properties and crawling which reminds it of its strength. A lot of the lower back pain exercises are good for the middle back too. So is lying down.
"If you look at someone lifting badly...it's as if their back folds above their belly button."