Do you need to know anatomy when doing yoga? Not really.
If you've already got good body awareness and control, if you are good at clearly defining what you feel and experience, then you don't really need to learn anatomy to do yoga.
However, if you are going on a long cross country drive, it can help to have a map so that you know where you are going and so that you can figure out where you are.
And that's what anatomy is like, it's a potential road map to the experiences of your body.
Now if you are experiencing problems with your body, pain, an inability to do certain actions or poses, anatomy is doubly useful because it can be used to guide how you explore your body.
But it helps to have some basic understanding of how the body works as an integrated whole.
Part of what is important with respect to yoga anatomy, and in particular the joints, is the idea that ligaments are active structures.
Ligaments and tendons are all part of the same structure until an anatomists knife separates ligaments from tendons. It's that cutting that leads to the idea that only tendons are active while ligaments are passive. The understanding that this gives us, of different muscles and how they work, is still valid, but it is important to understand that ligaments are just as much subject to muscle activity as tendons are.
Ligaments are important as active structures because they help keep the joint safe.
One way in which they do that is that they help to maintain space between bones. One reason that space is required between bones is that it helps keep joints lubricated. If bones are pressing against each other, then lubricating fluid is squeezed out from between them. The lubricant can't do its job meaning other methods have to be relied on to keep the joint lubricated.
Ligament tension directly affects joint capsule tension, and tension in the joint capsule is what pressurizes synovial fluid in such a way that bone ends are prevented from pressing against each other. At the same time, tension in the joint capsule also prevents the bones from moving too far apart. With muscle activation directly affecting ligaments, the required balance between pressure and tension can be maintained at each joint helping to keep the joint intact.
When space between the bones at each joint is maintained, another benefit is that it allows tension to be distributed evenly via not just the joint capsule but via all other tension transmitting connective tissue structures that cross that joint.
If you think of tension not just as a means of transferring work, but of transfering information, then maintaining space at the joints allows the parts of the musculoskeletal system to keep in real-time contact.
Joints are critical because there is no overlap. Once a joint is gone it's gone, but muscles have overlap. They can compensate for each other like alternate branches of a network. So when there is a lack of flexibility or pain, it may be a result of movement patterns instigated by the brain to protect joints.
Part of the challenge is removing these habits when they are no longer required. And this is where understanding joint mechanics can be helpful. If you understand that a particular pattern of muscle activation developed in order to protect a joint, then you can figure out how to remove the pattern in such a way that the joint is still protected but optimal movement is restored.
Part of the reason that muscle functions overlap is to allow us more freedom of movement. This overlap allows one group of muscles to work effectively in one range of motion. Then as that range of motion is exceeded, other muscles, better placed to handle it, can then take over control of the movement.
This same redundancy is useful when a muscle isn't functioning properly. If this happens then it potentially endagers the joint or joints that it works on. And so other muscles are called in to keep the joint safe, or failing that, then muscles work in such a way to prevent any movements that might endanger a joint. (Pain could be used to prevent such movements, or muscles may "lock" to prevent potentially dangerous movements, or a mixture of both might occur).
With this understanding it can be possible to figure out the reason behind whatever movement problem you have. And in the absence of problems it may also be useful to help you figure out how to improve performance. (Which is simply another type of problem.)
Sometimes maps aren't accurate. Sometimes there are mistakes. And sometimes maps can contain only so much information. And a very important point to note is that pointing your finger at the location of London England on the map is not the same as actually going there.
With that in mind it is important to realize that names of muscles are not the same as the actual muscles. Just because you can name all the muscles of the body doesn't mean you understand anatomy (or bio mechanics). The reason for learning anatomy as a yogi (or as a sportsperson) is to better know our body and to better use it. To that end, you can learn to feel many of your muscles and those that you can't directly feel you can learn to infer when they are active. Learning to feel and control your muscles will actually give you a better feel for your body. And that's where an anatomy book is useful. It helps you figure out where to put your attention so that you can directly experience the muscles and bones of your body.
Learning to feel and control your anatomy is a recursive process. You can use bones as references for feeling and controlling muscles. Muscles in turn help you to locate bony landmarks which in turn can then be used to fine muscle awareness.
In addition, at a particular level of muscle control you run into circular problems where you chase problems only to arrive back at the original problem. That's a sign that you need to go deeper.
While initial muscle control may focus on learning to feel larger muscles, this is simply because they are easier to feel and control. As you get better at feeling your body you can begin fine tuning your awareness noticing the more subtle changes in tension that occur in connective tissue and are the result of smaller, thinner or less numerous muscle tissue activation.
This is when you can begin working towards doing poses with full integration and minimum effort.