Brett Jone's wrote an excellent post, Thoracic Mobility..., on June 14th.
To add to it:
Range of motion of lumbar and thoracic spine
The lumber spine has greater flexion/extension range-of-motion (ROM) based on disc height and spinous processes which project almost straight posteriorly, while the thoracic discs are smaller and the spinous processes project more inferiorly, which decreases flexion/extension ROM.
The architecture of the thoracic spine lends itself to greater rotational ROM based on the position of facet joints, and the lumbar spine's facet joints decrease rotational ROM when compared to the thoracic vertebrae. (check out a good anatomy/biomechanics text)
With less flexion/extension ROM in the thoracic spine, there needs to be optimal alignment of the lumbar spine, so thoracic extension end ROM isn't limited by the spinous processes and anterior connective tissue structures. Which then lends itself to optimal pelvis, hip, knee, foot, and ankle alignment.
All this is obvious when you understand the cascading effects of every anatomical structure's alignment on every anatomical structure's alignment. The important thing is to have adequate coordinated mobility to allow for quality movement and positioning...
Breathing is something often taken for granted, which it probably should be ok, but...
As Brett said in his post,
"...if we are breathing correctly and using the diaphragm and intercostals for respiration there is a tremendous amount of movement in the thoracic spine. However, most people breath through their traps and upper chest which facilitates the development of kyphosis (rounding of the upper back) and a rigid rib cage."
This creates a "flip-flop" of what is supposed to occur. We breathe through the chest/thoracic region which "tenses" all the surrounding muscles which are supposed to be used to control movement of our arms, shoulders, scapula, and neck.
How can this happen? We have been breathing our entire life, how can it change?
This can be stress in any form, both psychological or physical. Don't forget that LACK of movement is also stress to our bodies. When we are under stress we go into what is called the startle reflex; our "fight or flight" response of the sympathetic nervous system. What happens when we are under stress, acute or chronic, is we tense up the muscles of the upper body, shrugging the shoulders, bringing the head forward, etc, basically going into the fetal position. Let me be clear however, the startle reflex usually isn't as blatantly obvious as a full grown adult going into full fetal (Lloyd Christmas in the rest stop bathroom when C-Bass stops by)... most of the time it is shrugging of the shoulders and bringing the head forward, or more often just "tensing" of the muscles which create those actions.
Regardless, from this position our breathing gets altered. We begin to use our diaphragm and intercostals less and less, as Brett stated. What happens from here could potentially be a cascade of effects that may potentially lead to everything and anything that could go wrong in the body.
When we have good diaphragmatic breathing, the diaphragm (again, refer to a good functional anatomy text) pushes down on the viscera. This "pushing" down creates pressure on the pelvic floor, which creates reflexive activity of the deep abdominal muscles... something to think about. From this we potentially improve:
1. Lumbar spine muscle function and control leading to:
2. Improved hip and lower body function
3. Increased thoracic spine ROM leading to:
4. Improve scapula and glenohumeral function and ROM
5. Improved function of all muscles, especially scapular, shoulder, and hip muscles
6. Improved movement efficiency
Not to mention all the other systems affected by increased sympathetic nervous system activity: cardiovascular, endorcrine, immune, etc., ... Focused Breathing techniques can also reduce the sympathetic nervous system effects, and as with anything, proper breathing can be relearned.
I guess you could say breathing might be kind of important. No wonder the upper body muscles start to limit range of motion of the thoracic spine, scapula, and shoulder, think of how many reps these not-for-breathing muscles might be getting; we breathe every second of our life!