For the sake of simplified explanation we can divide the muscles into two kinds: 1) the mobilizing muscles which are those that are involved in movement and 2) stabilizing muscles which are those that stabilize the body. The first can be called mobilizers and the second stabilizes or postural muscles.
The mobilizers are those that enable us to move and can the fast-twitch. They can contract and relax in a short interval but they get exhausted quickly. The stabilizers enables us to maintain the posture and can slow-twitch. They do not get exhausted easily but are quite slow. They are located deeper in the body than the mobilizers.
We have high conscious control of mobilizers which we can put easily in use with our intent. The stabilizers, on the other hand, are used unconsciously. But stabilizers have two properties that are very useful. First, they can make the body structure really strong. Second, most of them are designed to stabilize and balance our body against outside force (especially the force of gravity).
The purpose of Zhan Zhuang is to become aware of the stabilizers and then gain some conscious control over them. As we said they are controlled unconsciously. The beginners of zhan zhuang in the beginning of practicing use the mobilizers to keep the upright posture and that is why they get exhausted very easily. The secret to long standing is to relax all mobilizers and use only stabilizers to keep the upright posture and feel as if a magic force is keeping us in the upright posture. Since we are unaware of stabilizers we feel as if our body is standing itself. In the beginning of practice it is a good point to distract our conscious mind to something outside like watching television in order to use at minimum mobilizers because as these are all the muscles we can be aware of.
For long standing we have to train ourselves to hold the posture without the mobilizers but only with stabilizers. That's why we suffer in agony in long standing practices where finally we made the breakthrough into a relaxed stance in which the mobilizers are relaxed and the weight is putted on stabilizers. Also that's why it is said in zhan zhuang practice to not use any muscles but relax all of them.
Why zhan zhuang is good for health? Because from an early age will start misusing mobilizers to take on the task of stabilizing the body. The mobilizers, finally, at some certain age, will deform the natural skeletal structure resulting in bad posture. Through zhan zhuang practice we re-educate the body to use stabilizers which in the long run will correct the posture and muscolo-skeletal problems.
Anxiety Disorder is caused by musculo-skeletal problems which seem to be caused by the misuse of mobilizers that are taking the role of stabilizers. The musculo-skeletal problems makes the nerves to be pinched (vagus nerve and others).