When looking at breathing anatomy for yoga teachers it helps to understand the muscles that are involved in the act of breathing and how they act in relative isolation. As an example, in Diaphragmatic or Belly Breathing:
Other muscles that can be involved in breathing include:
The obliques and intercostals can be used to expand and contract the ribcage causing inhales and exhales respectively.
Internal Intercostals and Internal Obliques.
External Intercostals and External Obliques.
If the spine is kept still during these movements then the movement and breath created is quite small which often leads to the idea that the costal muscles are only accessory muscles of respiration. However if ribcage movements are combined with spinal movements, say bending the spine backwards along with opening or expanding the chest followed by bending the spine forwards and sinking and contracting the ribcage, the resulting breath can be quite large and quite satisfying. In this case the spinal erectors are used with the external obliques and intercostals to help expand the ribcage while inhaling.
Spine bend forwards (exhaling)
Spine bent backwards (inhaling).
It can also be used to increase pelvic awareness.
This type of breath is often used in an on-all-fours position: cat pose. I prefer to teach it either while sitting or standing upright.
Another group of muscles that can be used for creating inhales is the Levator Costarum which act to lift the backs of the ribs. They attach to the spinal vertebrae and reach down to attach to the ribs either one or two levels down.
When contracted they pull upwards on the rear of the ribs and this can cause an inhale while the exhale can be gravity driven while standing upright. If using the levator costalis with the spine horizontal then a muscular action would be required to pull down on the rear of the ribs to cause an exhale. In this case the Serratus Posterior inferior could be used to pull down on the lower three or four ribs. But also the longissimus, which are the inner most of the spinal erectors, could be used.
Note that used in relative isolation the respiratory effect using just these muscles is quite small. I often use them more as an aid for thoracic extension. And so these muscles can be used to augment easy breathing. In this case, while bending the thoracic spine rearwards, pull upwards on the backs of the ribs to engage these muscles. With experience you may find that lifting the back ribs seems to create space at the back of the ribcage making it easier to bend it backwards, and thus making it possible to deepen the inhale while bending backwards.
Another breathing method that involves opening the back of the ribcage (as opposed to the front) uses a combination of the intercostals, obliques and the levator costalis. The feeling is of opening or expanding the back of the ribcage instead of the front (which is more usual.)
In this type of breath the middle of the thoracic spine seems to push rearwards relative to the pelvis so that both the upper and lower thoracic spine bend forwards causing the rear of the thoracic spine and ribcage to open.
If you are familiar with the feeling that is created between the shoulder blades when you retract them using the serratus anterior, the feeling of breathing into the back of the ribcage is similiar. And actually opening up the back of the ribcage may cause the shoulder blades to spread. At the same time the levator costalis can pull upwards on the ribs further creating an "opening" sensation at the back of the ribcage.
The respiratory diaphragm is a dome shaped muscle situated below the lungs. It actually supports the bottom of the heart and lungs while the liver, kidneys and stomach are suspended form its underside. It attaches to the bottom edge of the ribcage as well as to the upper vertebrae of the lumbar spine. There are openings in the diaphragm for the passage of the blood vessels as well as the esophagus.
The diaphragm has fibers that attach to the kidneys as well as the 12 pair of ribs. It may be possible to isolate these fibers by anchoring the diaphragm where it connects to the front of the ribcage. These fibers can then be activated by pulling upwards the rear of the kidneys which are located at about the level of the 12th ribs.
I often like to combine the above action, lifting the backs of the kidneys, with a drawing in of the lower band of the Transverse Abdominis.
Respiratory Diaphragm and Transverse Abdominus
The Transverse Abdominis (TA) whose fibers groups can be activated in isolation. The lower belly corresponds to the triangle formed by the ASIS (the forward pointing peaks of the hip bones) and the pubic bone. The inguinal ligament is the ligament which connects ASIS to pubic bone and also creates the visible line that separates the inner thigh from the belly.
The lower fibers of the Transverse Abdominis attach to both the ASIS and the inguinal ligament,
Transverse Abdominis contracted.
Transverse Abdominis relaxed.
A simple breathing exercise is to focus on drawing in the lower belly (below the belly button) and then relax and repeat. Then do the same but focus on the upper belly. Then practice drawing the lower belly in first followed by the upper belly.
I'll often have students draw in the lower belly, then pull up on the backs of their kidneys to create an inhale.
When activated the lower portion of the Transverse Abdominis pulls in on the ASIS which tends to cause the sitting bones to move outwards effectively distorting the pelvis and causing the sacrum to nutate (nod forward) at the SI Joints.
To keep the pelvis from distorting, and to keep the SI Joint stable, the pelvic floor muscles could activate to oppose the transverse Abdominis and stabilize the bowl of the pelvis. This isn't the same as stabilizing the hip where the femur is stabilized relative to the pelvis. Instead it is an action that prevents the pelvis from distorting by stabilizing the SI Joints.
One of the ways that I like to think of the transverse Abdominis is as a tension control mechanism for the Abdominis rectus.
If only the lower belly is pulled in the transverse Abdominis actually adds some slack to the portion of the rectus that crosses the upper belly. This then makes it easier to lift and expand the front of the ribcage. Also it means that if the diaphragm contracts downwards to cause an inhale then the upper belly can expand outwards to allow this to happen.
One of the reasons that diaphragmatic breathing can be difficult to learn is that if it is done with the ribcage lifted then tension is added to the front of the belly making it difficult to push the belly forwards. And so I'll often first teaching diaphragmatic breathing with the ribcage down (slouching) so that the belly is relaxed. It is then easier to move the belly in and out. Then once this has been learned the next exercise is to do the same with the ribcage lifted gradually more and more.
The next stage is to practice contract the diaphragm while gradually lifting the ribcage. In this case tension is gradually added to the wall of the belly at the same time as the diaphragm contracts downwards. Done smoothly and slowly it can feel quite meditative, particularly if you focus on keeping constant tension in the abs as the chest is lifted.
Another technique is to pull the lower belly inwards prior to contracting the diaphragm downwards. If the ribcage is kept still the upper belly will expand as the diaphragm is contracted.
A simple instruction for activating the Transverse Abdominis is to pull the belly inwards towards the spine.
Pulling belly inwards using Transverse Abdominis.
Because the fibers of the transverse Abdominis run horizontally around the waist (like a belt), it makes it easy for these fibers to contract the belly to the point that its diameter is smaller than that of the ribcage and pelvis.
(Note that the lower fibers of the TA connect to the ASIS and inguinal ligament, however the affect of their contraction can be to marginally reduce the diameter of the upper opening of the pelvis. Even if this amount is so small as to be negligeable one possible queue for teaching how to activate the lower transverse Abdominis is to pull inwards on the ASIS and/or inguinal ligaments.)
Pulling the ribcage upwards while exhaling to suck the belly in.
Some students may lift their ribcage to make their waist smaller. If the ribcage is pulled up away from the pelvis without an inhale then suction causes the waist to pull in. The action of pulling in the belly using the Transverse Abdominis looks different and feels different to vacuuming the belly.
An easy way to detect when students are using the former as opposed to the latter is to watch their ribcage (and shoulders) and also to listen to their breath.
Some students may be well versed in engaging their abs i.e. obliques and rectus, and that generally involves a hardening of the ab wall. Pulling inwards using the transverse Abdominis is a different feeling. And actually pulling in on the entire transverse Abdominis may actually add tension (by lengthening) to the obliques as well as the rectus Abdominis giving those muscles pre-tension which then makes it easier to activate them.
One way that students can activate or engage the abs is to pull down on the ribcage of pull the ribcage towards the pelvis. For transverse Abdominis activation the focus is on pulling the waist inwards and rearwards towards the spine. The ribcage and pelvis can be kept still during this action.
Pulling the transverse Abdominis inwards without moving the ribcage generally causes an exhale.
Belly pulls in (exhale)
Chest lifts (inhale)
Relax both (exhale)
Keeping the transverse abdominis engaged, the ribcage can be lifted and expanded to cause an inhale. One possible breathing pattern is:
Someone asked me about the anatomy of reverse breathing.
In this breathing method the lower back is expanded (lengthening or opening vertically so that more space is created between the back of the ribcage and rear of the pelvis) during an inhale.
In this instance the rectus Abdominis and or obliques could be used to anchor the front of the ribcage so that it doesn't lift while inhaling. What may then happen (I'm not positive on this) is that both the quadratus lumborum (which attaches to the back of the pelvis and from there to the transverse processes of the lumbar spine (l1-l4) and the twelfth rib on each side) and the rear-middle fibers of the respiratory diaphragm activate together.
The quadratus lumborum pulls down on the 12th ribs while the respective fibers of the diaphragm pull upwards on it. During the course of the inhale when the focus is on expanding the lower back the diaphragm exerts a slightly greater force than the quadratus lumborum. Since the front of the ribcage is anchored the added tension to the diaphragm pushes upwards on the bottom of the back of the ribcage. Because the diaphragm is exerting more force than the quadratus lumborum the quadratus lumborum gradually lengthens allowing the rear of the ribcage to lift upwards away from the rear of the pelvis.
It may actually be the case that the whole diaphragm is active in this action otherwise there might be "leaks" places where the diaphragm gives in, and so possibly the whole diaphragm is engaged thought it might be the case that certain fibers are more active than others.
For the exhale, the tension in the diaphragm can be gradually reduced so that the rear of the ribcage sinks down due to gravity or due to residual tension in the quadratus lumborum. There may be a corresponding slackening at the front of the belly since the rectus and/or obliques don't have to exert so much effort to keep the front of the ribcage anchored.
This is my best guess as to what happens based on my experience and understanding of reverse breathing. I'd suggest using it as a starting reference.
On further feeling around, I'd suggest that it is the more forward fibers of the internal and external obliques that activate to keep the front of the ribcage anchored. But it may be that the transverse Abdominis also activates to pull in the belly while the external obliques activate to keep the fronts of the lower ribs activated and the internal obliques activate to negate the forward pull of the external obliques on the ribcage.
The external obliques create a forwards and/or downwards pull on the ribcage relative to the pelvis. The internal obliques can be used to create a rearwards and/or downwards pull on the ribcage. The intercostals may also be involved for further stabilizing of the ribcage.
Also note that if the transverse Abdominis are activating, particularly the lower fibers, the pelvic floor muscles may also activate to stabilize the pelvic bowl via the SI Joint.
With all these breathing variations:
There are also twisting actions, side bending or side snaking actions that can be assisted or caused by the act of breathing or during which breathing can occur.
The simplest way to explore these options is to learn to feel the pelvis, spine and ribcage as well as the muscles that move them: the obliques, intercostals, spinal erectors, transverse Abdominis, pelvic floor muscles and respiratory diaphragm. And one of the simplest ways to learn to feel these muscles is to practice using them while breathing.