Most anatomists talk about the body in terms of someone else's body. The original anatomists dissected cadavers to increase their own knowledge.
Anatomic yoga anatomy is intended to help you explore the anatomy of your own body without cutting it apart.
How then do we explore our own body? By feeling it. And by controlling it.
In a "controlled experiment" scientists limits what is happening so that they can understand how, for example, two chemicals react with each other. With the body we isolate parts of the body and practice rhythmically moving those isolated parts slowly and smoothly so that we can tune in to the sensations that are created by that movement.
We thus learn to feel our body and control it. The repeated movement also gives us the chance to calibrated sensation to what is actually happening so that we can accurately use our senses to feel what is happening.
Once we've learned to feel our body and have calibrated our sensations we can then begin experiencing our body, and work at better understanding it.
This experience and this understanding together can make it easier to improve the way we use our body and control it. It can also make it easier to learn new skills.
One way of thinking about an atom is as the smallest useful division of something that we are studying.
Reading a Chinese text on the origin of Chinese characters the author pointed out that for the purposes of that particular text, brush strokes are not a meaningful unit. Brush strokes might be akin to letters of the alphabet which by themselves aren't useful as units of meaning (a and i excepted.)
However there are larger elements which can be used as part of other characters that do have meaning in and of themselves. And this book would focus on those elements and the characters that they make up.
In the same way, with yoga anatomy it's a little bit pointless talking about muscle cells and how they work.
I'm not suggesting that this information isn't important and that you shouldn't read it.
What I am suggesting is that in the context of yoga that information (at least at my level of understanding) isn't always useful.
Anatomic yoga anatomy focuses on is anatomy that is useful in the context of a yoga practice. That means that it is something that you can feel and use to improve your yoga asana or help you work towards an asana or a variation that you don't otherwise know how to approach.
To back up this understanding anatomic articles focus on anatomy and yoga exercises that you can use to actually experience that anatomy or the effects of using that anatomy.
Part of experiencing our anatomy is learning to feel it.
The technical term oftend used for "feeling the body" is proprioception.
At the very basic level what we can learn to feel (proprioceive) is the activation are relaxation of muscle tissue. Generally we can learn to feel the larger "bulkier" muscles in this way since the sensation of activation and relaxation is easy to discern.
As an example we can learn to feel our buttocks contract and relax and we can also learn to deliberately contract and relax them.
We can also learn to feel our spinal erectors contract and relax. Although taken individually these muscle are small and thin, because there are so many of them as a group, there activation and relaxation is easy to feel.
Thinner and smaller muscles aren't so easy to feel because the sensation that is created when they relax or contract isn't so noticeable. However in the case of these muscles what is noticeable and controllable is the bone movements that they create. A prime example of this is the serratus anterior muscle. This muscle is very thin however it acts to protract the shoulder blade. As a result deliberately protracting the shoulder blades while focusing on the inner edge of the shoulder blades is one way to consciously control this muscle.
Note that it is easy to create protraction with a contraction of the pectoralis major muscle. However, this muscle is large and easy to feel when it is contracted and so also relatively easy to keep relaxed when protracting.
Another signal that we can notice when activating smaller/thinner muscles like the serratus anterior is a stretching or opening feeling between the shoulder blades. I strongly suspect that this is connective tissue that we are stretching, perhaps the connective tissue that lies within and also surrounds the rhomboids (which resist the serratus anterior and also can be used to retract the shoulder blades.)
Tai Ji and other "relaxed" or "internal" martial arts tend to focus on the use of these smaller muscles to move the body.
Ideally we learn to feel and control both the larger muscles and their smaller thinner counterparts.
The control of tension is the key to isolating, integrating, sensing and controlling the parts of the body.
One of the most important things that we can learn to control whether working on strength or flexibility is how to create stability. To create stability in a joint we can use muscles against each other or against some external resistance. In both cases, to create stability (as opposed to movement) the muscle force should be equal to the force it is resisting.
In terms of feeling and controlling our relationship with external bodies (including the earth) pressure awareness is key. This is covered in more detail in the section on yoga balance poses.
The actual state of being when we have just the right amount of tension throughout our body so that we can feel what is happening now as well as instantaneously respond could be called tensegrity.
Personally when I first started yoga, and for a long while afterwards, I was only vaguely aware of my shoulder blades. They used to wing out alot and still do when I'm not sitting with good posture.
As I began exploring handstand and down dog and even headstand I began to explore shoulder exercises that can be used the develop conscious control over the serratus anterior and trapezius muscles. I then used these shoulder exercises as part of a preparation for poses like plank pose, Warrior 1, Down dog, Handstand and even Wheel pose.
At one time I was even using a very simple shoulder exercise to teach students how to relax the trapezius. (I'd forgotten about it for a long time.) Big tip for relaxing the trapezius, keep your neck straight and you head pulled back as you do this shoulder exercise.
Meanwhile I learned how the rhomboids can be used in positions like shoulder stand and plow pose to help bring the shoulder blades together. The trouble is, it's hard to learn to use them in those poses so I figured out some simple shoulder exercises to help activate them before they have to work against the weight of the body.
Even later I began learning about the pectoralis minor and how it can be important in arm positions like that used in Prasaritta Padottanasana C.
You can also control the spine indirectly using the pelvis, ribcage and skull as levers.
All of the anatomy drawings you see on this website have been drawn by myself. And that's another part of how I learn to better understand anatomic yoga anatomy, by trying to draw it. And that's one suggestion I would make to you, try drawing your anatomy if you want an easy way to better understand it. You may find that it helps your yoga practice. Drawing, or learning to draw you look for relationships and you try to capture those relationships on a piece of paper. And initially your drawings might not be that good. But that's alright. You look and learn and then you draw again.
I do the same thing with Chinese calligraphy. I practice over and over again.
One key thing to avoid disappointment (whether trying to learn to write Chinese characters or trying to draw parts of the body) is to focus on drawing isolated elements.
I focus on the shoulder blade or the side view of a pelvis until I get it right. And then I move on to something else so that I rest from what I've done. I do the same thing with calligraphy, and you can do the same thing when actually doing yoga.
Do a pose. Study it during or afterwards and ask yourself how it can be made better or ask yourself which relationships you don't understand. You can then work on understanding that particular relationship next time you do the pose.
Learning to feel your pelvis can be helpful because with it you not also help to control your center of gravity, you also control the base of your spine. With deeper awareness of the pelvis, learning it's landmarks you have a reference for all of the muscles that attach to it. And that leads to the hip muscles, including the psoas major, which doesn't actually attach to the pelvis but does cross it as well as the other hip flexors (which bend the hip forwards) and hip extensors like the gluteus maximus (which bend it backwards.)
One of the keys for doing yoga is knowing the possibilities of each joint.
If you are doing your own practice, when you know the possibilities for each joint you can experiment to see what helps best. There are macro movements such as having the leg forwards or back, in hero pose or lotus etc. But there are also micro actions that you can do that don't alter the position but do change the feel of the pose ideally making it feel more integrated or even just more comfortable.
At the hip joint three actions that you can do are compressing it, opening or creating space within it and also simply relaxing it. Two other actions are internally rotating it and externally rotating it. These micro actions are actions that you can do whether the macro position of the thigh is internally rotated such as in hero pose or externally rotated such as in lotus pose or pigeon.
In all cases these are actions or movements that you can learn to feel. You can either learn to feel the bones moving or you can feel the muscles that do the movement either contracting or relaxing. Ultimately you learn to feel both. In either case, when you are able to focus on feeling your body and responding to instead of thinking about what you have to do, then you are possibly doing yoga.
As for the SI joint, I'm still exploring that but from what I understand so far, mula bandha and the pelvic floor muscles may be the key controlling elements for more delicate positioning of the sacrum relative to the pelvis.
Mula bandha might be useful in maintain the integrity of the pelvis at the SI joint and also a key component in helping to manage backbending and forwards bending of the spine.
Two final body parts that I'll mention here are the elbows and knees.
Until relatively recently I only paid passing attention to these body parts. You may find that understanding these parts, and deliberately activating the muscles that act on them can help your yoga pose.
In the case of the elbows I found that activating them and deliberately rotating them can make poses like handstand easier. If nothing else it gives something else to focus on while doing this inverted yoga pose.
As for feeling the knee joint, and deliberately activating them, well that may help in making your hamstring stretches a little bit more easier.