In a previous post on utthita parsvakonasana, extended side angle, I mentioned that most students tended to do the opposite of what I want them to do with their extended arm. There is only one muscle that connects the arm to the pelvis and spine. The latissimus dorsi connects directly to the spine and connects to the pelvis through the thoracolumbar fascia.
Whether or not you are using the latissimus dorsi in utthita parsvakonasana depends on the rotation of the arm in extension. I find that many people in the pose bend their elbow to the side and open the armpit in an attempt to open the chest. The chest should open independently of the arm and the arm should ideally move in the opposite direction of the trunk.
The video above references wheel pose in the beginning but it is essentially the same information for any use of the arm in extension. If you bring one of your arms directly above your head and turn the palm closer to your face you will be doing what I describe as deepening the armpit. This also engages the latissimus dorsi so that when you extend your arm up further the latissimus dorsi should tone all the way down to the pelvis. With your arm still overhead, if you turn your palm away from the body you will be opening the armpit and unfortunately losing any connection to the pelvis through the latissimus dorsi.
Using the latissimus dorsi in utthita parsvakonasana ensures that you can really access full extension of the side body. If the arm rotates in such a way that the latissimus dorsi isn’t engaged the extension of your arm begins at the arm instead of the pelvis if that makes any sense. We want to bring as much of the body into each and every pose as we can.
Since the latissimus dorsi is the only muscle connecting the arm to the spine and the psoas major and piriformis are the only muscles connecting the legs to the spine, these successful engagement of these muscles is necessary to make your yoga a full body practice.