Chaturanga Dandasana. Otherwise known as the yogi push-up. We (yoga nerds) always talk about proper alignment and using the muscles in your back to support the posture instead of using your shoulders and chest muscles. Well...the question is: How the hell do you do that, it's a push-up right?
This pose is a challenge for nearly every practitioner at some point in their practice, so if you are new, don't get discouraged. And if you have been practicing for a while and still don't have it figured out...well, it's time to bunk up and get it right, or you're going to be in pain soon if you aren't already.
The Challenge. Half of the problem is that the posture isn't held for very long. It is typically part of a vinyasa sequence in a sun salutation, so it's easy to miss the technique and it's easy to fudge your way through and skip it all together if you are in a big class. True beginners have a tough time lowering down keeping the elbows close to the body, which is rule #1. Those with a little experience have a tough time keeping their head in line with the spine (sinking forehead syndrome), which is rule #2. Those with a little more experience than that, have a tough time engaging the muscles between the shoulder blades and not rounding the shoulders downward, rule #3. Actually, engaging the back muscles is a challenge for many that have been practicing for a while (years). Rule #4, to keep the shoulders broad and do not pinch the shoulder blades together.
Getting engaged. Have someone watch the muscles in your back between your shoulder blades to see if they are firing when you queue up a chaturanga.
Elevated Scapula. The rhomboids (major and minor, located beneath the trapezius) are supposed to retract the scapula when the trapezius contracts (they are antagonist). Often, students/practitioners have elevated scapulae along with the sinking forehead as they move through the push-up. This tells you that the rhomboids are not firing when called upon.
Another muscle that is involved in keeping the scapula "flat" on the back during the yogi push-up is the serratus anterior (SA). The SA attaches the ribs to the anterior medial border of the scapula (under the shoulder blade, the edge closest to the spine). The main function of the SA is move the shoulder blade up and laterally...wikipedia calls it the "boxer's muscle" because it gives the driving force behind the punch and it moves the shoulder blade laterally around the ribs. But here's the thing: when a person is punching they are not worried about engaging the rhomboids. So, when the arm is moving and pushing forward, the SA functions as the antagonist to the rhomboids, not necessarily a synergist. When doing chaturanga, you might need to retrain the muscle to function as a synergyst and keep the scapula in place while doing a pushing movement. When I say might, I would guess that about 80% of people might need to retrain the muscles. Note that you don't necessarily want to retract the scapula to the point of it moving closer to the spine, but that you want to just keep it firmly in its natural placement.
The main role of the rhomboids is hold the scapula onto the rib cage. They attach from the spine (T2-T5) to the scapula and are a superficial back muscle, the second layer right underneath the traps. Rowing uses the rhomboids primarily. The thing with chaturanga, is that to firm the blades to the back ribs while keeping the elbows close to the body you are asking your body to do a "rowing" movement combined with a pushing forward movement. When the rhomboids and the SA work synergistically, we get shoulder blades fixed firmly to the back ribs. In other words, when the SA pulls the blade away from the spine and the rhomboids pull toward the spine at the same time...viola...the perfect chaturanga happens. Two opposing and dynamic forces happening together...and the blades stay in place.
The bottom line, I believe, is that the SA and the rhomboids need to work together. I tend to focus more on the rhomboids as the primary force and let the body learn the secondary action of using the SA during scapula retraction. However, I know that it is possible to focus too much on the rhomboids which can lead to injury down the road.
Doing these simple exercises and being mindful of your own abilities will keep your shoulders safe and strong throughout your practice. Many people make the connection after one or all of these practices in a single session. For others who have been practicing it the wrong way for years, they might need more practice. Those patterns of movement are well established and may take more time to correct.
For those of you who have a lot of work to do on your SA, I have found a simple strengthening exercise that works really well. For example, if you have been engaging your rhomboids for years and need to play catch up on the SA...try the serratus anterior press with kettle bells or weights.
I hope you find these practices helpful...but more importantly... have fun doing them! If you are a yoga nerd, I know you will.