The thoracic spine is made up of the 12 vertebrae that your ribs connect to. The way
these vertebral bodies are shaped lend themselves specifically to rotation, but maintaining good range of motion with flexing and extending your spine is essential for a variety of other areas of the body.
We're all familiar with the rounded back, forward head posture that has been villainized for millennia (Did anyone else's grandma give you a firm thumb in-between your shoulder blades to get you to sit up straight while proclaiming, 'You'll get stuck like that!'). While having bad posture isn't the death sentence we once believed, limited thoracic extension can affect the muscle length-tension relationship in your upper traps and other neck muscles that can lead to tension headaches, neck pain and even TMJ issues.
Additionally, the single only bony attachment that your shoulder/arm has to your skeleton is at the acromioclavicular (or AC) joint - the little bump you feel at the end of your collar bone. Otherwise, your shoulder blade glides over your ribcage due to a complex relationship with the muscles that attach to it all working together. If your mid back is stiff (especially if it's stiff and rounded - think basically everyone who identifies as a swimmer) that will alter the way your shoulder blade moves over your ribcage. And altered mechanics usually mean uncomfortable movement patterns.
Have you heard the idiom, "Rob Peter to pay Paul"? Your body, and specifically your spine, and thus your whole self operate on a delicate balance between mobility and stability. Mobility is defined as your ability to control a range of motion and encapsulates strength, range of motion and flexibility, whereas stability is defined as your ability
to oppose external forces due to strength and joint congruency.
Think of a newly planted baby tree versus an old, established oak tree in a wind storm. The
newly planted tree that is more supple and flexible will sway and maybe bend in the wind, possibly needing to be staked for stability. The old, sturdy oak tree will stand tall in a heavy gust. The human body should display both stability and mobility; people are able to do back bends and touch their toes, but are also able to lift heavy loads.
What can happen (due to prolonged and sustained postures, disuse due to bed rest, immobilizations due to injuries such as fractures, or other causes including decreased elasticity of tissues with the normal aging process) is a stiffening of tissues and joints. This often occurs through the middle of the back (the thoracic spine). If this part of your spine gets very stiff, your body will find mobility elsewhere, often in the low back, which can lead to low back pain that can often be debilitating if not treated well.
Here are a couple exercises you can do to improve your thoracic mobility.
Complete 5-10 repetitions in each direction of each of these most days a week. Hold for 5-10 seconds per repetition, and to not push through pain. Cracks and pops might occur as you loosen up some tissues!