We tend to think of the ribcage as a protective structure. (It protects the heart and lungs). In terms of movement and posture it may be more helpful to think of the ribs as lever elements.
It's because of the ribs that the intercostal muscles can be used to stabilize, bend and twist the thoracic spine.
Looking at the ribcage with a little bit more detail, what's the purpose of the sternum?
If you look at the skeleton of a snake, it has ribs but no sternum.
Look at the skeleton of an ostrich or emu and it has ribs and a very large and overly developed chest plate or sternum.
Why the difference?
A snake doesn't have limbs.
An ostrich or emu (I'm not sure which of the two I saw the skeleton of) has big wings. The sternum gives the muscles that operate those wings a stable foundation from which to work.
In the same vein, a particular type of turtle was studied and it was realized that the reason for its shell wasn't defense, but to enable it to dig more powerfully with its forelimbs.
So what's the takeaway for us humans?
The arms attach to the sternum via the shoulder blades which attach to the collar bones which in turn attach to the top of the sternum. An interesting point is that the bottom of the shoulder blades tends to lie even with the bottom of the sternum. This will shift as the shoulder blade moves relative to the ribcage, still it means that the shoulder blades can be used as a reference for helping to differentiate the upper and lower ribcage.
The main reason we have a sternum may be to give the arms a more stable foundation. And the reason the lower ribs don't attach to it is to give the lower ribcage a little bit more flexibility. (Going back to the turtle shell article linked to above, that may be in part so that we have better breathing capacity.)
Note that there are arm and shoulder muscles that attach to the ribs of the lower ribcage (and to the lumbar spine, cervical spine, hip bones and skull). But the sternum is there to give the upper ribcage that extra bit of stiffness it needs so that the arms can be used powerfully.
So if the upper ribcage is fairly stable, what is there to worry about?
To give the arms a more stable foundation, so that we can use them even more powerfully, it may be helpful to think about how to stabilize the lower ribcage. As noted, it may be more flexible so that we can can breathe more effectively. However, it may be that by stabilizing the lower ribs, we can use our arms with greater strength. We do so by giving the "arm" muscles that attach to the lower ribcage a stable foundation or anchor point. But before going into the details, a quick look at stability and mobility.
If you look at any camera tripod, they generally have the option of undoing a lever so that you can turn the camera in any direction. Then you can lock it once you have your subject is suitably framed.
With the ribcage and shoulders, we have the same options (if we choose to work at it.)
We can shape the ribcage appropriately to how we want to use our arms. Then, if we need to use our arms powerfully, we can then stabilize the ribcage so that our arms have a stable foundation from which to work. The point here is that stability isn't due to positioning but rather due to the action of muscles working against each other to create a stabilizing effect.
Note, depending on how much power we wish to exert and in what direction, we may also need to stabilize the neck and/or waist.
Focusing on the ribcage though, what can we do to make it more stable so that we can use our arms with less effort?
One idea is to use the internal and external intercostals against each other. And indeed that can be useful for both shaping the ribcage and maintaining that shape despite external forces acting on it.
These pictures could have been labelled a little better.
The intercostals are the muscles between the ribs.
The obliques are positioned between the ribcage and the pelvis.
Another idea focuses specifically on the lower ribcage.
This is the part that corresponds to the arch of the ribcage (the arch that houses the upper abdominal muscles.)
This part of the ribcage has the lower 5 ribs, the top 3 of which are false ribs, the bottom two of which are "floating" ribs. It also has the lower 5 thoracic vertebrae. These are the ribs that lack a direct connection to the sternum and so this part of the ribcage tends to be more flexible and thus requires more effort to make stable.
So what muscles can we use to stabilize this part of the ribcage (or at least a portion of it).
The Serratus Posterior Inferior muscle attaches to backs of the lower four ribs. From these ribs, the fibers (or fascicles) of this muscle angle downwards and inwards to attach to the lower two thoracic vertebrae and the upper two lumbar vertebrae.
The upper band of the Transverse Abdominis attaches to the front of these same ribs.
My own experience has shown that these two sets of muscles can work against each other to help stabilize the lower ribcage.
Note that because the serratus posterior inferior create a downwards pull on the backs of the lower ribs, it can help to first lift the ribs to give the fibers of this muscle room to contract and to give these fibers an opposing force to work against.
Another way to think of this is giving these muscles optimal length so that they can activate more effectively.
An upwards lift of the ribs may be created by use of the external intercostals (which attach between adjacent ribs) in addition to the levator costarum muscles (which reach downwards from thoracic vertebrae to attach to ribs one or two levels below).
One tip then for activating the serratus posterior inferior muscle fibers is to first lift the backs of the lower ribs and then create a downwards (and inwards) pull on these ribs, pulling them towards the upper lumbar spine.
So which "arm" muscles will stabilizing the lower ribcage affect the most?
Arm and shoulder muscles that have fibers that attach to the lower ribs include the serratus anterior and the latissimus dorsai.
The latissimus dorsai is generally though of as a pulling muscle while
the serratus anterior tends to be involved in pushing actions.
And so stabilizing the lower ribcage may be useful in both pushing actions (which generally require the serratus anterior) and pulling actions (which require the latissimus dorsai).
In the left picture you can see how some fibers of the latissimus dorsai attach to the lower three ribs.
In the right picture you can also see the serratus anterior muscle (unlabeled) in green.
(You can also see pectoralis minor, just above it!)
If you are into gym work, one way you can test this out, assuming you've got the ability to turn on your upper transverse abdominis and serratus posterior inferior muscles at will, is to try those muscle activations doing a bench press and while doing lat pull downs or chin ups or pull ups.
In either case, whether you focus on activation the serratus posterior inferior or the upper transverse abdominis, the opposing muscles will also tend to activate (since, as mentioned, muscles need an opposing force to act against.)
Even though the lower ribs don't attach directly to the sternum, the lower ribs are still closely tied to the upper ribs. Movement of the lower ribs tends to affect the upper ribs and vice versa.
The point here is that if you stabilize the lower ribs, it's probably going to have an effect on the upper ribs also. As an example, in the picture above, you can see another muscle that ties in to "arm strength", the pectoralis minor.
For arm over head positions, anchoring the lower ribs at both the front and back makes it easier to use your arm to pull with greater strength.
Note that since the upper fibers of the psoas attaches to the uppermost lumbar vertebrae, then stabilizing this region of the ribcage and the upper lumbar spine can help to give the psoas an anchor point for more effective functioning.
Thus if you think of the psoas as a possible factor in lower back pain then being able to stablize the lower ribcage and junction of the thoracic spine and lumbar spine via the upper transverse abdominis and serratus posterior inferior muscles may help.
Note that psoas interaction is a two way street. While stabilizing the lower ribcage can be used to give the upper fibers of the psoas a fixed anchor point, psoas activation can also affect the ability of the lower ribcage to stabilize.
Put another way:
Since the psoas activation is affected by quadratus lumborum activation and vice versa, and since the quadratus lumborum also attaches to the lowest pair of ribs, then hip activation may affect activation of the transverse abdominis and/or serratus posterior inferior.
One tip in this regard is to try to stabilize the hip, or both hips, when trying to stabilize the lower ribcage.
Why one hip or both hips?
It depends on whether you are using one arm or both arms. And since the latissimus dorsai also has fibers that are continuous with fibers of the opposite side gluteus maximus muscle, it may help to activate the hip on the opposite side to the arm you are using.