Your thoracic spine—the upper back or mid-back region—is the most stable part of your spine. It is very stiff, and the thoracic spine has a limited range of motion. It is like that for several reasons, all of which you can understand by looking at the anatomy of the thoracic spine.

Vertebrae
There are 12 vertebrae in the thoracic spine. They are labeled T1 through T12; the T stands for thoracic. You have more vertebrae in your thoracic spine than you do in any other spinal region. (The cervical spine—the neck—has 7 vertebrae, and the lumbar spine—the low back—has 5 vertebrae. There's also the sacrum and coccyx, which are 5 fused vertebrae and your tailbone.) The thoracic spine extends from your shoulders to your waist.

Vertebrae are the building blocks of your spine, and they stack one on top of each other, getting progressively larger from the cervical spine to the lumbar spine. Vertebrae help your body carry its weight efficiently, in addition to protecting the spinal cord and internal organs.

Thoracic vertebrae have a few unique characteristics, as demonstrated in the illustration below.

The spinous processes are very long in the thoracic spine. They also point downwards, as opposed to cervical and lumbar spinous processes, which point more horizontally. The thoracic spinous processes may even overlap each other, and because of that, it's difficult to arch your thoracic spine very much. The long spinous processes limit movement in the thoracic spine.

The intervertebral foramen in the thoracic vertebrae are where the spinal nerve roots exits the spinal canal and go out to the rest of the body. In the thoracic spine, though, the nerve roots don't take up much room in the foramen, so that's why you're less likely to have nerve compression problems in the thoracic spine: there's more space for the nerves to travel through.

The ribs attach to the thoracic vertebrae at the costovertebral joints, another unique characteristic of thoracic vertebrae. Everybody usually has 12 pairs of ribs—one for each thoracic vertebrae. The ribs attach in two places on the vertebrae: at the costal facet, which is on the "block" part of the vertebra, and at the costotransverse facet, which is on the transverse process.

Because the thoracic vertebrae are part of the rib cage, the thoracic spine doesn't move as much as the other spinal regions.

Intervertebral Discs
In between your vertebrae, you have intervertebral discs. These act like pads or shock absorbers for your spine as it moves. Each disc is made up of a tire-like outer band called the annulus fibrosus and a gel-like inner substance called the nucleus pulposus.

The proportion of annulus fibrosus to nucleus pulposus is greatest in the thoracic intervertebral discs. So—relative to the other intervertebral discs, thoracic discs have more of the tire-like outer band. This also adds to the limited mobility of the thoracic spine. Together, the vertebrae and the discs provide a protective tunnel (the spinal canal) to house the spinal cord and spinal nerves. These nerves run down the center of the vertebrae and exit to various parts of the body, where they help you feel and move.

Muscles, Ligaments, and Tendons
Your thoracic spine has a lot of soft tissues that support it. Muscles, ligaments, and tendons help your body move: twist, lift, throw, etc. Muscles are strands of tissues that power your movement. Ligaments are the strong, flexible bands of tissue that link bones, and tendons connect muscles to bones. Muscles, ligaments, and tendons all work to control your movements so that you don't hyperextend (bend backwards too much), hyperflex (bend forwards too much), or over-twist your spine.