In yoga there is a lot of focus on heart opening, which usually means expanding the upper chest. This is critical for back bending, in which the easy bend comes from the low back and the challenge is to spread the curve evenly along the spine. Heart opening also relates to emotional gripping — it’s common to tighten the chest in response to stress or anxiety. As a result of spending so much effort on opening the front of the upper torso, sometimes the back side of that same region of the spine is compromised and forgotten.
For a lot of us, opening the chest means puffing out the pectorals, rolling the shoulders back, and squeezing the shoulder blades together, like a soldier. To the contrary, a full opening of the chest has to include softness around the armpits and space for the side ribs, which means that the shoulders get wider and not just driven back. In child’s pose, sense the breath moving into the back, spreading the shoulder blades and allowing the backs of the armpits to both spread and lengthen toward the crown of the head.
Next, try a more subtle feeling of opening in the upper back. Stand in tadasana, cross your arms and hold opposite elbows, and allow the forearms to rest fully on the front ribs or solar plexus. Allow the hands to draw the elbows toward each other as the armpits spread. Inhale more space between the shoulder blades. See if you can still lift the sternum here. The key point is that the opening in the front and back of the chest, while opposite in direction, can coexist.
In fact, when you think about lengthening the spine, or anything else for that matter, you only find the full length when both sides are long — otherwise you’re just bending. While of course opening the chest maximally is inconsistent with opening the upper back maximally, see if these two movements can support each other like the two sides of an arch. Taking the fight out of opposition is one of the most joyful reliefs life offers!