The things that help control both stability and flexibility of the ribcage, as well as enabling you to feel it are the muscles that work between adjacent ribs, the intercostals, and the muscles that work on the back of the spine and the backs of the ribs, the spinal erectors. Additionally, the obliques can be useful in either function, playing a role similiar to the intercostals. However, instead of attaching between adjacent ribs, these muscles attach, generally speaking, between the ribs and the pelvis. While their roles aren't as clear cut, the transverse abdominis muscle, (in particular the upper band of the transverse abdominis, made up of fibers that attach to the ribcage), as well as the respiratory diaphragm can also have an important affect on ribcage control.
How is it that the ribcage can be both flexible and rigid
The ribcage is both a flexible and rigid structure. It is the joints between the vertebrae and between the vertebrae and the ribs and the muscles that act across these joints that allow these dual roles. These muscles can be used to vary the shape of the ribcage and they can be used to stabilize the shape of the ribcage.
In a way this functionality is similiar to that of a camera tripod.
A camera tripod allows you to vary the lengths of the legs (individually or together) and it allows you to swivel and/or turn the head of the tripod relative to the legs. However, you also have the necessary option of stabilizing or locking each element once it is in the desired position. The usefulness of the camera tripod is in its ability to be stable in a wide variety of positions.
One key difference between a camera tripod and the ribcage is that the ability to adjust the shape of the ribcage and stabilize it is built into the ribcage itself. And the same things that adjust the shape of the ribcage, also help to stabilize it.
Why do we have a ribcage?
If we look to the turtle as a model, one reason that turtles or tortoises may have developed shells is to enable them to dig more effectively. The shell provides a foundation or anchor for the muscles of the forelegs so that they can be used effectively for digging.
Note that the rigidity of the tortoise's shell also limits limb movements. And that's one advantage of having a flexible ribcage. It affords the muscles of the arms a foundation for the application of strength but at the same time allows us to use the strength of our arms in a more diverse way than simply digging.
The ability for the ribcage to change shape is a way of extending the range of movement of the arms.
Why have a ribcage that can be both flexible and rigid?
As well as offering the arm muscles a foundation for strength, the ribcage does offer protection. And as well as giving the arms greater range of expression, the flexibility of the ribcage also allows it to be used for respiration.
Using the ribcage for respiration
One of the side affects or bonuses of having a ribcage that can change shape is that we can use it to assist in breathing.
The intercostals are muscles that work between adjacent ribs. Meanwhile, the spinal erectors are muscles that attach to the backs of the ribs and spinal vertebrae. Used together, these two sets of muscles can change the shape of the ribcage to move a reasonably large volume of air in and out of the lungs.
Note that if you try just using your intercostals then the ability to change lung volume is significantly reduced.
So, if you want to breathe deeply using your ribcage bend your spine backwards and forwards while breathing while at the same time lifting and lowering your ribs.
One reason for mentioning this is that you can use ribcage breathing exercises as a way of improving thoracic or ribcage mobility.
The diaphragm and balanced ribcage mobility
The respiratory diaphragm is a domed umbrella like muscle whose bottom edge attaches to the bottom rim of the ribcage. This muscle can both affect the operation of the ribcage and be affected by it.
While it makes sense to view the diaphragm as a single central muscle, when working on balanced sensation on the right and left sides of the ribcage, it can help to divide the diaphragm into left and right halves.
Muscles that can work with the diaphragm include the the transverse abdominis, whose fibers run horizontally around the space between the ribcage and pelvis, and the serratus posterior inferior which attaches to the backs of the lower four ribs and from there reaches down to attach to the two lower thoracic and upper two lumbar vertebrae. It should be noted here that as well as attaching to the bottom circumference of the ribcage, the diaphragm also has attachments to the sides of the bodies of the upper two or three lumbar vertebrae.
Stabilizing the lower half of the ribcage
With respect to the application of strength via the arms, the upper portion of the transverse abdominis in particular in combination with the serratus posterior inferior, and possibly the diaphragm, can be important for stabilizing the bottom half of the ribcage. This can become more critical the more force you are trying to exert via the arms.
Using the camera tripod for inspiration, you might practice using these muscles to create stability through a ride range of positioning or shaping options for the ribcage. So for example, practice it with the ribcage "neutral". But also with the ribcage and thoracic spine bent forwards, backwards, to either side, and also while it is twisted.
What are some basic movement possibilities of the ribcage
If you understand the movement possibilities of the ribcage you can exercise them and thus improve or maintain ribcage flexibility.
The ribcage and thoracic spine can bend sideways as well as front to back. It can also twist. Another general movement is making the thoracic spine feel long.
Because the thoracic spine and ribcage are made up of a series of joints, there are range of other possible shapes.
Fine tuning or adjusting ribcage control
An important aspect of exercising the mobility of the ribcage is adjusting or fine tuning. Because you can use your ribcage and thoracic spine's inbuilt muscles to mobilize it, these same muscles also allow you to feel your thoracic spine and ribcage. You can then use the sensations generated by these muscles as a guide to adjusting how you use those same muscles. You can adjust to balance sensation throughout the ribcage. You can adjust to eliminate "empty" spots, areas where you lack sensation. You can also adjust to eliminate areas of excessive tension.
An another important idea when practicing ribcage mobility is that of maintaining connection. So that you can use your muscles continuously during the active phase of any ribcage mobility movement, it helps to continue to feel your muscles. If you can feel ribcage and spine, that means your muscles are active since it is muscles that allow you to feel your ribcage and spine.
The ribcage as a foundation for the arms (and thus a potential source and solution to shoulder problems)
As mentioned, one reason for having a reasonably rigid or stable ribcage is so that greater forces can be exerted via the arms.
Comparative anatomy (snakes, emus, humans)
To help understand this, if you look at the skeleton of a snake, it has lots of ribs, but no sternum. And that corresponds to the fact that it doesn't have limbs. An emu or Ostrich has forelimbs in the shape of powerful wings. Correspondingly, it's chest plate is massive. The chest plate acts as a foundation for the muscles that act to move the wings.
And so whether looking at a turtle, a bird, or ourselves, it helps to think of the forelimbs, or wings or, for ourselves, our arms, as beginning at the ribcage. And so if you have shoulder problems, a good place to start dealing with them is to look at the ribcage (and neck) since it is literally the foundation for the shoulders and arms.
Should your focus on ribcage mobility or ribcage stability
Depending on the nature of your shoulder problems, it could help to work on ribcage mobility so that you can extend the movement range of your arms. It can also help to improve thoracic stability so that you give your arm muscle a stable foundation. In either case you can use the inbuilt muscles of your spine and ribcage to control your ribcage whether trying to mobilize it or stabilize it. And you can use these same muscles to help feel your ribcage so that you know that your mobilizing or stabilizing efforts are succesful.
Note here that if you work on muscle control, which involves feeling muscles as well as controlling them, it's easy to work on both mobility and stability. You actually can get both as a result of muscle control.
Work on ribcage stability first, and then scapular stability
When working on the arms, the next step from working on the ribcage can be to focus on the muscles that move the shoulder blades and collar bones relative to the ribcage. As with the muscles that work on the ribcage, these muscles can be used to move the shoulder blades and collar bones to help extend the reach of the arms (downwards, forwards, upwards, to the sides and back) but also to help stabilize the shoulder blades once they are in the desired position.
Why practice feeling and controlling your ribcage first if you have shoulder problems?
Because the muscles that move the scapula relative to the ribcage and that help to stabilize it in most instances originate from either the ribs or the thoracic spine. In some cases they also originate from the neck and base of your skull as well as the lumbar spine, sacrum and hip bones.
The point is, if you work on feeling and controlling your ribcage so that you can stabilize it in a variety of shapes, you then give your scapular stabilizer muscles a stable foundation from which to act. It's a lot like building a building by starting with the foundation. Each succesive floor then acts as the foundation for the one above it.
Twisting the ribcage
One of the most notable things that the ribcage can do is twist. You could think of the ribs as levers put in place to give the obliques and intercostal muscles leverage for turning each of the thoracic vertebrae relative to each other (and relative to the lumbar vertebrae and hip bones).
Using the arms to generate a twist
A common approach to spinal twisting is to use the arms to generate the twist. This is the equivalent of giving the muscles of the spine and ribcage a passive stretch. Unless you resist the stretch using the muscles that you are trying to stretch. And so one option for twisting is to resist the stretch when using the arms.
In general, when working to create stability, you can use opposing muscles against each other to create that stability. As an example, to stabilize the spine and ribcage, in particular the thoracic spine and lumbar spine, you can use the external obliques (and possibly the external intercostals and rectus abdominis) against the iliocostalis. This is a branch of the spinal erectors that attach from the hip bone to the backs of the ribs. You can use these muscles to stabilize the spine and ribcage in a forwards/backwards direction.
Resisting the twist, using obliques and arm muscles against each other
Using the arms to twist the ribcage you can use your obliques and intercostals against the arm muscles to resist the twist. Once you are resisting the twist, you can vary the resistance so that you gradually go deeper into the twist while keeping both sets of muscles active.
This is a lot like slowly squatting down. Your quads or your butt (or both) will be active to control your descent, but as you go deeper they lengthen while staying active.
Using the arms to resist coming out of a twist
Note that as well as being able to use the arm and spine muscles against each other while deepening the twist, you can also use them against each other when trying to come out of the twist.
Twisting using the muscles of the spine and ribcage
Another approach is to solely use muscles that are integral to the ribcage and spine. In this case you don't use the arms at all. Instead you solely use the muscles that work on the spine and ribcage to drive the twist.
As an initial step for activating muscles of the spine and ribcage you can work at making your spine feel long and then from there adjust your ribcage so that the front, sides and back of your ribcage have feel also. In this case, "feel" equates to muscle activation.
Generating sensation while twisting
Muscles that work on the spine and ribcage can act against each other to generate sensation (muscular activation sensation and connective tissue tension). And since they are acting against each other they also create stability. Staying tuned in to the feel of your spine and ribcage, you can then use these muscles to twist your spine, turning your ribcage relative to your hips and from there turning your ribs relative to each other.
Maintaining sensation while twisting
If you maintain the feel of your spine and ribcage while doing this, that means that your spine and ribcage muscles are still active and you can continue to use them to deepen the twist.
If you loose "feel" then you can try adjusting so that you regain it. And so a good idea is to focus on moving slowly and smoothly so that as you deepen the twist you can continually adjust to maintain feel and thus muscle control.
Muscle control for the spine, get a better feel for your spine and ribcage (while learning to better control it)
Need more detail on exercises for learning to feel your spine and ribcage? Muscle control: spine, hip bones, ribcage gives you just that. It's like driving lessons for your body with a focus on your spine and ribcage. You'll learn the ABC's of feeling your spine and ribcage and controlling them. And as with driving lessons, you'll learn to feel and control your spine and ribcage in such a way that you don't continually need an instructor or teacher next to you telling you what to do.
Find out more about Muscle control: spine, hip bones, ribcage.