Chaturanga Dandasana: Strengthening the Arms, Shoulders and Core
Because you are holding your body basically parallel to the ground in this posture, to have good form and hold your spine in postural alignment and your shoulders in a neutral position, you need to be working the connections between the strength of your core and the deep muscles of your spine. This means that if you have the strength to do the pose well, it can be a great pose for maintaining or increasing your core strength.
There are a few things that are going on in this photo that I like. The first thing is that my shoulders are centered midway between the front and back of my ribcage. That means they are not being pulled back or dipping forward. As a result I have maintained the natural curves of my spine so that my posture in this pose maintains the actual, natural curves of my spine. I have not flattened out my thoracic curve (the curve of my upper back) and I have not distorted my lumbar curve (the curve of my lower back). If I was standing upright with my upper arms parallel to my body and my forearms basically parallel to the floor, my spine would have pretty much the same alignment it has in this picture. This means I am using my core effectively to keep the curves of my spine from distorting. It also means I am using my legs to maintain alignment effectively. I also like my head position and where I am gazing because it maintains the natural curve of my cervical spine (the curve of my neck) without distortion or tension. The result is that my shoulders are relaxed and they are not dipping forward. I also like the distance away from the ground that my body is in this picture. If I go any lower my upper arms and elbows would be higher than the back of my body and this would cause a flattening of my upper back, a dipping of my shoulders and tension in my neck.
The last thing that I think is worth mentioning about this photo is how relaxed the work looks. It is work to hold that position but you would not know from looking that I am working hard. This, again, is that concept of effortless effort that the Yoga Sutras describes in defining asana as stirasukham asanam.
Look closely at my toes. In this second version of Chaturanga my toes being pointed makes the work of the pose a little more challenging.