Breathing
It is important to find the deeper intrinsic back muscles to do the action of spinal extension in this pose. Using the latissimus dorsi and other more superficial muscles affects the scapulae and rib cage and interferes with breathing by inhibiting the movement of the ribs.
In this pose, the serratus anterior is active to maintain a neutral position of the scapulae against the push of the arms. When the arms push, the shoulders don’t elevate but the spine is lifted.
The latissimus dorsi are not helpful as extensors of the spine, because they create flexion of the upper back and internal rotation in the arms.
Many people assume the legs should be passive in cobra, but numerous actions in the legs are required to keep the joints in alignment. The hamstrings, especially the semitendinosus and semimembranosus, extend the hips and maintain adduction and internal rotation. The extensor portion of the adductor magnus, along with the deep and medial fibers of the gluteus maximus, also extends the hips without externally rotating the legs. The vastus lateralis, vastus medialis, and vastus intermedius work to extend the knees. Weakness in the medial hamstrings can cause the gluteus maximus to do more than its share of hip extension, in which case the legs externally rotate or abduct, or both.
Weakness in the pronators of the forearms or shortness in the supinators (or interosseus membrane) makes the elbows flare out to the sides and affects both the elbow and shoulder joints. The forearms should stay parallel to each other for the best alignment of action through the arms into the spine.