Why learn muscle anatomy?
So that you have a better idea of how and where to direct your sensing and controlling effort while learning to sense and control your muscles.
When doing yoga poses, muscle anatomy is helpful because it can help you decide where to squeeze or activate in order to help support you in the pose or take you deeper.
In this variation of side plank, I'm deliberately moving the inner edge of my shoulder blades away from my spine to activate serratus anterior. I'm also activating the muscles of the right side of my waist to slightly bend my spine to the right.
When doing back bending yoga poses, you may want to activate the muscles at the back of the body in order to help bend the body backwards.
To bend back at the hips you can use the gluteus maximus muscle, the hip extensors, and even the hamstrings (depending on whether the knee is straight or bent.)
To bend back at the knees you can use the hamstrings and a portion of the calf muscles. To bend back at the lumbar spine, thoracic spine and cervical spine you can use the spinal erectors.
While back bending often (but not always) uses the muscles at the back of the body to help bend backwards, they also at the same time stretch the front of the body.
And so backbending at the hips stretches the hip flexors including the psoas muscle. Backbending (or bending back) at the knee stretches the quadriceps.
Back bending the spine stretches the rectus abdominus and the intercostals at the front of the ribcage. Back bending the neck stretches the neck flexors.
(I sometimes consider wheel pose a back bend for the shoulders. A forward bend for the shoulders would then be any pose where the arms move down and back, like table top.)
When doing forward bending yoga poses you may want to activate the muscles that act on the front of the body.
Bending forwards at the hips you can use the psoas major and minor muscles and other hip flexors. You can't really bend the knee forwards, but you can straighten it, and for that you can use the quadriceps. To bend the lumbar spine forwards you can use the abdominals, particularly the rectus abdominus but also the obliques. (These muscles, in combination with the intercostal muscles also help to bend the thoracic spine forwards.) To bend the neck forwards you can use the neck flexors.
While bending forwards can exercise or use the muscles at the front of the body front bends can also stretch the back of the body. Forward bending at the hips stretches the glutes and other hip extensors, including the hamstrings if the knees are straight. Forward bending at the spine stretches the spinal erectors, the muscles that extend or bend the spine backwards. (This includes the muscles at the back of the neck, or the neck flexors.)
Like forward bends and back bends, side bending can happen at the hips and spine. It doesn't happen at the knees.
Side bending to the left stretches the muscles on the right side and vice versa.
Side bending the spine uses (and stretches) the obliques, intercostals and quadratus femoris. It may also use the psoas.
Side bending at the hip uses (and stretches) the gluteus medius and minimus. Note that side bending at the hip stretches the leg adductors. Moving the leg inwards uses this muscle and stretches the gluteus medius and minimus.
When doing twisting yoga poses, the twist might be passive or it may be active. In active twists the abs and intercostals are active. In passive twists, the pose isn't necessarily passive. Instead the muscles that normally are used to twist are passive. You may instead be using your arms and/or legs to drive the twist. The main muscles that can be used in active spinal twisting of the thoracic spine are the obliques and intercostals. These muscles can be used to stretch each other. And in the case of passive twisting, you use the arms to help stretch these muscles.
Muscles don't just bend the body in different directions, then can also be used against each other (or against some external force) to stabilize the joints that they work across.
As an example, to stabilize the spine, the abs and spinal erectors can be used against each other.
Note that stabilizing a joint can happen at any position. You can stabilize the spine and keep it straight or you can stabilize it while it is bend forwards or backwards or to either side. And you can stabilize it while it is in a twisted position.
Likewise with the hips, shoulders, wrists, ankles and elbows.
For any muscle that you wish to get familiar with, learn the location of the belly of the muscle as well as the points of attachment of that muscle to bone. The muscle belly is important because use can learn to recognize when it is contracted and when it is relaxed. The points of attachment are important because you can learn to recognize pulling sensations at these points.
More than being descriptions or definitions of muscles in an anatomy book, these are things that you can learn to feel and identify.
Use an understanding of muscle anatomy so that you can better control your muscles.
If you know where the belly of a particular muscle is with respect to other muscles in its neighborhood, in addition to points of attachment, you'll more likely be able to determine which muscle is activating and also choose which muscle to activate if you desire.
The belly of the muscle is the part of the muscle that actually does the work. This is important because you can learn to feel when a particular muscle is working and when it is relaxed by focusing on the muscle belly.
The sensations it creates when it relaxes or activates are not only important for giving your feedback as to whether your body is responding as desired but also to help you fine tune your mental map of where the muscle belly actually is. The better your ability to know where each muscle is, the easier you will simply be able to put your awareness on that muscle and willingly contract it.
It's a little like driving a car. If you want to accelerate you put your mind and your foot on the accelerator and press down. If you want to slow down, then you put your foot on the brake pedal.
In the case of particular muscles, if you want to contact or relax them, assuming your body is in a position that allows them to do so, then simply put your mind on them and "squeeze" or "relax" as required.
Another important aspect of muscle anatomy is knowing the points of attachment.
The belly of a muscle is attached to the bones that it works on via tendons.
If you know the points of attachment of a particular muscle you have another tool for more finely developing your body knowledge. You can use points of attachment to determine which muscle is activating or being stretched and also to help you actively contract a particular muscle if you desire.
To actively contract a particular muscle , you can focus on pulling or drawing the muscles points of attachment towards each other.
While stretching, and trying to relax, you can use the location of any pulling sensations to figure out which muscle is hindering you.
For example, when doing side to side splits or even half splits, I generally feel a pulling sensation on my inner knee, below the knee joint (so on the lower leg.) I feel a corresponding pull close to my pubic bone. I can thus infer that in this case it's my gracilis that is tightening up.
To help loosen it I can then try to deliberately contract it with greater force, and then relax it.
This is very much like releasing a ratchet, you have to pull it tighter so that you can then let it go.
If you try this yourself, make sure that you focus on slowly and smoothly contracting and slowly and smoothly relaxing. Also, practice this while your feet are fairly close together so that you have a better idea of what to expect as you move deeper.
When learning to feel the muscles in your body, you may find it helpful to redefine (or define) muscles and muscle groups outside of "normal definitions" given in anatomy books.
Generally, you can refer to muscle anatomy books to learn the whereabouts of a muscle belly its points of attachment and its function. The names of muscles don't necessarily correspond to their function and sometimes muscles are grouped together because they share a location in the body but not a function.
The way around this is to learn the names of muscles as a way of labeling and identifying them, but also feel free to create your own muscle anatomy classifications based on where the muscle is and what it does.
Even if you don't know the technical name, the interesting thing is that the name tells you where to direct your awareness. Back benders are generally on the back of the body.
To bend backwards, you can focus on activating the muscles along the back of your spine and legs. If you only want to bend your spine backwards then contract only the muscles along the back of the spine. To bend forwards focus on the forward bending spinal muscles, the abs, or the forward bending hip muscles, (the hip flexors) or both.
And rather than saying a muscle can only be in one group, be open to the idea that any single muscle can have multiple functions.
Renaming can also be used to name muscle fibers separately.
As an example you could break down the external oblique into fibers according to which rib they attach to and whether they attach to the pelvis or the connective tissue that wraps around the rectus abdominus.
So that if you are doing an active twist, using your obliques and intercostals to turn your ribs and ribcage relative to your pelvis, you can focus on activating the fibers that act on ribs 9 and 8, or on the 7th pair of ribs if you choose. You might want to do this if one set of ribs feels "dead" in a pose. To wake up the muscles that act on those ribs, you can focus specifically on moving those ribs.
As another example, the glutues maximus, has attachments from the sacrum to the thigh bone. It also has attachments from the pelvis to the IT (illiotibial) band. These groups of fibers have different lines of pull and if you can learn to distinguish them you can separate the functions of the gluteus maximus. You can learn to activate your butt muscle in such a way that your leg moves back or in such a way that it moves back and externally rotates.
The first step towards developing this control is understanding first that you can actually separate the two actions. (Like learning that a car has an accelerator, a brake and a steering wheel.) Then you can practice distinguishing the two actions. You then can go about experiencing your muscle anatomy so that you better understand it.
While learning where the belly of a particular muscle is situated is important, another thing that you can learn or become cognizant of with respect to muscle anatomy is the position of a particular muscle belly with respect to the bones and joints that it works on.
As an example, the rotator cuff muscles have their bellies all on the shoulder blade. These muscles (there are four) all act on the shoulder joint. The deltoid, biceps and triceps long head also act on the shoulder joint. However the bellies of these muscles are either on top of the shoulder joint or on the upper arm bone.
If you consciously pull your shoulder joint towards your elbow joint and vice versa you can activate muscles that act on your shoulder and elbow. You can then adjust the squeeze so that the squeezing sensation is isolated to your shoulder blade or it moves to your shoulder or to your upper arm. In the first instance you are more than likely engaging the rotator cuff muscles, in the second case it is your deltoid, latissimus dorsai and pectoralis major that activate. In the third case it is your biceps and triceps that you are activating.
By understanding how a set of muscles and their muscle bellies relate to the joint that they work on you can more easily distinguish which muscles are active and which are not. You can also more easily control which muscles you are activating.
Learning your muscle anatomy, and learning how to feel and control your body is an iterative process. You get better at it the more you do it. Eventually you'll reach a point where you can feel which muscles are activating and which aren't.
Or if you aren't sure but you want to know, you'll be able to break down what you are doing and have a good guess at what is activating.
By learning to feel and control your muscles to greater and greater degrees, you can use this understanding as a diagnostic tool to identify problem areas in either yourself or your students or clients. You could also use it to experiment. Try different muscle activations to see if one set of muscles can be used more effectively than another.
Another reason for getting to know your muscles is so that you can simply let them get on with their job as they are designed to do.
In this case you could treat your muscles like the employees of a company. The better you know your employees and what they are capable of doing, the better you can assign jobs or tasks, and then you can let them get on with it.
By becoming aware of your muscle anatomy you do a similar thing, you get to know your body as a whole. As a result, you can then learn to lead your entire body with a clear idea of what you are trying to do. Your muscles can then get on with doing it.
For more on learning to feel your body and control it, the Driving Lessons for your Body series of PDFs includes exercises for improving body awareness and muscle control.