I will address the muscles of respiration and their functions and then talk about the concept of pan-costal breathing as related to singing.
The main muscle of inhalation is the diaphragm. It is a very complex muscle. Most importantly for inhalation, it is attached to the bottom of the lungs.
When the diaphragm contracts it pulls down on the lungs increasing their volume. This creates a negative pressure in the lungs relative to outside pressure (law of equilibrium) and pulls in air from the outside to equalize the pressure. If the mouth is open, the air will rush in. A desire to breath basically activates the diaphragm to contract and expand the lung space. In exhalation the diaphragm relaxes to reduce the lung space as air escapes.
In inhalation the ribs also expand to create space for the expanding lungs. Their action is principally controlled by the external intercostal muscles.
These muscles run diagonally from the spine to the top of each rib. When they contract, they lift the ribs up and outward creating greater space as in the diagram below:
In exhalations, this action is countered by the internal intercostals which run opposite, from the spine up to the bottom front of the ribs. Upon contraction, they pull the ribs down and in, reducing space.
Unfortunately the diagram that correspond to the internal intercostals (from the website that contained the previous diagram) was incorrect. It simulated the action of the externals. The theory in pan-costal breathing is that during singing, the external intercostals remain active. Their antagonistic function to the internals (which should dominate in exhalation and singing) helps to keep the ribcage stable such that the usage of air is more directly controlled by the rise of the diaphragm and remaining muscles of exhalation namely the rectus abdominus, transverse abdominus and the obliques.
The rectus abdominus originates from the pubic bone and inserts into the fifth, sixth and seventh rib. It pulls down on the ribcage during exhalation to reduce the space of the thorax.
The transverse abdominus is sometimes called the corset muscle because it goes totally around. It originates at the Iliac crest of the pelvis and the lower margin of the lower ribs, and inserts at the pubic crest among other points. It reduces the space of the lower torso to help push out air. It is also the principle muscle involved in giving birth. Below are two other views of the transverse.
The obliques running diagonally over the transversus and at the sides have the function of rotating the torso, among movements. They also help in reducing the space around the lower abdomen, pushing the viscera (abdominal organs) toward the diaphragm as it rises.
The external obliques originate at the Iliac crest of the pelvis and insert into the lower ribs. The internals run opposite. Their combined function help reduce the abdominal space.
What is interesting relative to singing is that all of this exhalation muscles have origin or insertion points at the pelvis. There is no need to attempt to activate the pelvic floor in singing. The natural function of these muscles require pelvic sensations. Indeed breathing is a natural function and does not change much for singing. Other than maintaining a sense of suspension in the ribcage during phonation, one must trust that the respiratory system responds to the singer’s desire to produce a certain sound. Rather than seeking to activate muscles (the brain knows better how to recruit muscles), one should make sure the muscles are in shape so they can do the work required of them.
Several other muscles are known to contribute in respiration, however their activity is not always conducive to efficient breathing for singing. Two muscles in particular, the Sternocleidomastoid and the Scalene group responsible primarily for head movement are attached to the upper ribs and can lift them during inhalation.
They are responsible for what is often called high chest breathing or clavicular breathing. They tend to be hyperactive when the principal muscles of breathing are otherwise insufficient in their functions.
Body alignment is also crucial to the correct function of these muscles. However, poor alignment is as much a result of weakness in those muscles. Posture improves with proper development of all muscles. Exercise is crucial for maintaining the voice. Alignment is never rigid. It is not something that can be dictated with a few words. Alignment takes time when the singer is not physically fit. Many muscles participate in creating proper posture without being rigid. They must all be appropriately developed to create the conditions for proper alignment.
Finally, we cannot talk about support, breath compression, sub-glottal pressure, etc, without talking about the source tone or glottal resistance. The appropriate closure of the vocal folds is directly involved in creating the breath pressure that drives it. In the end, breath is intimately tied to glottal function and vice versa. Likewise, resonance is mutually tied with glottal function. The instrument is one complex machine with many interdependent parts. It is always important that “the left hand knows what the right is doing.” Having an inner sense of dynamic and conscious coordination is crucial to vocal balance. It is also important to know what we are responsible for in the act of singing and what happens without our direct participation. Mostly the singer’s job is to imagine and desire the specific sound he or she wants to create. A well-tuned body will respond to fulfill those desires.