A crucial element of backbend practice in yoga is to increase the mobility of the triceps and muscles of the shoulder, the neck and the upper back, such as deltoid, rhomboid, trapezius, splenius, supraspinatus, teres, infraspinatus ect. Naturally the upper and lower back also has their part in every backbend but the activation and the intensity of the movement of the different muscle groups are very diverse. Another important aspect is the flexibility of the practicioner. The practitioners genetics, muscle and bone structure, body bulk, connective tissue, femur lenght, background in movement, activities in daily life (such as his/her job etc.) and even gender can influance the flexibility of the upper back and shoulder muscles. This post is an intruduction, talking about the different type of practicioners and their different activations in backbend practice, to provide effective shoulder and upper back activational and mobilization techniques.
Lets see a basic type of practicioner, whom I teach and watch practice a lot. A beginner female yoga practitioner in her 20s, without any background in crossfit, weightlifting or fitness, with no considerable physical work in her life and with a job, sitting the whole day in the office. She genetically has very mobile upper back muscles (it is also a gender specific) without the effect of crossfit or weightlifting or calisthenics or physical work, which could have been made her muscles stiff. But because of sitting for years in elementary and highschool and then the university and for more years in an office, without streaching the hamstrings and the muscles of the hip (like you usually do in the crossfit or fitness world or any sport), she will feel some tightness in the hip muscles and knee flexors. So, when she begin to practice, she will feel herself naturally talented in backbends, because of her flexible upper back, and she will be able to do faster the feet-to head asanas or grab her feet in the backbend poses, such as Kapotasana, Rajakapotasana or Eka Pada Rajakapotasana (see the asanas here here and here). Her inner focus point (dristhi) in the movement will be somewhere in the upper back. That means, her backbend motions will be controlled by her upper back. She will naturally bend from the upper back to reach the asana. She won't need so much effort making her upper back more flexible to deepen the backbending asanas in the process of practice, but more on her hip and lower back.
These upper back and shoulder mobilization posts are basically for people with stronger, stiffer and more immobile muscles in the upper parts of the back, because of various physical activities like street workout, martial arts, crossfit, weightlifting or their job etc., for mobilization and gaining flexibility to reach the more complex backbends. Or it can help for advanced backbenders for the easy and effective activation before going into deeper asanas. Because their is another type of practicioner too, who has flexible lower back and hip muscles, but need more time for the upper back activation to be able to go into the more complex backbend asanas. I'm one of this type. However - thanks to the years of practice -, my upper back and shoulders became flexible enough for almost every kind of backbend, but because of martial arts, calisthenics and other kind of strenght training, I have really strong muscles there, which need much more activation work, before I can reach the more advanced asanas, like the backbends of Ashtanga 2nd, 3rd and 4th series. Also, my inner dristhi is always focused in my lower back. I move from my lower back all the time, maximize the bend in the lower back, and only after, moving with the upper back, shoulders and arms, grabbing my ankle, feet or touching my forhead with my heels.
You can see the difference between the two types of practicioner here and here. Also here.
Althought you can't see how they reach the position, pay attention to the difference of the bend and the activation of the upper and the lower back.
The following activation techniques in the Assisted soulder and upper back activation 2. post will give you useful tools for effectivelly activate and improve the shoulder and upper back mobility.