The usual causes of middle back pain are poor posture, muscle strain, or disc injuries in the spine, according to WebMD. The other possible causes are overuse of muscles, ligament injury and myofascial pain, which affect a group of muscles or connective tissue in the back.
Aside from these causes, any condition that puts pressure on the spinal nerves also triggers mid-back pain. These serious back conditions include osteoarthritis, compression fracture, scoliosis or kyphosis, herniated disc, spinal stenosis and degenerative disease, according to WebMD. However, there are cases when middle back pain may be caused by cancer, gallbladder disease or an infection.