Back pain is one of the number one complaints in the US , and wouldn't it be great if there was a simple daily activity you could do to stay pain free,*cough*yoga*cough cough*? More than just a physical practice that will reduce pain but a mental practice that can reduce stress and therefor stress related tension.
The asana, or pastures/poses, of yoga are active stretches and mobilizations of the muscles and connective tissues in our body. This helps loosen and lengthen tense parts of our bodies that may be cause structural imbalances cause discomfort or even pain. Think of your muscles at ropes tied to an accordion tube, if some of the rope are pulling more than other it would change the Curve of the tube, changing it''s alignment, just like tight muscle in you back could do to your spine. Regularly moving your muscles in there fullest range of motion can keep them loose and balanced.
The stretching aspect of the asanas can also increase circulation to the body, and better circulation mean healthier tissue, and that means all kind of wonderful things, better mobility, betterfunction, less pain...
As well at stretching, yoga postures also strengthen the body. When your muscles are stronger the can stabilize your structure (your bones) better, preventing injury and reducing pain.
Another gift yoga gives is a better awareness of breathing. Helping you focus on breathing into your stomach rather than chest so you use fewer accessory muscles that are inducing unnecessary tension. This is very beneficial for those with upper middle back discomfort.
And as many know already yoga can be very relaxing for the mind as well. Reducing mental stress can also induce relaxation in muscle because you are now no longer clenching on contracting from a mental discomfort such as worry or anger.
All of these together in your daily yoga practice will help reduce and maybe even elimination your back pain.