A well trained pair of shoulder joints is essential to martial artists as well as for general body work-out. What do I mean by that? There are two criterion: Firstly the joints must be strong enough to withstand pressure (like able to throw strong punches, able to lift up heavy weights and less likely to get dislocated in accident), to achieve this the tendons and muscles surrounding the joints must be strong. The second criteria is the strong joint must be able to move without undue obstructions in all anatomically possible directions, i.e. the joints must not be tight.
In a previous post, I mentioned that if one's joint is weak and loose, doing exercise that requires excessive ranges of motion or positioning (e.g. using a pec deck or Nautilus pullover machine or doing yoga asana) may cause more harm than good. Here, zhan zhuang and tai-chi will be a very good exercise to strengthen one's weak and loose shoulder joints.
But how about a strong but tight joints? The convention wisdom is that yoga asanas can do a good job in loosening them. How about tai-chi, can it be used to loosen tight joints? Traditionally the task of loosening one's tight joints (in particular one's shoulder joints) is through pushing-hands exercise with one's teacher or sifu. Your sifu with controlled pressure at his hands/arms will stimulate equal force from you and he will gently open your tight shoulder joints through appropriate circular movements targeting at the tight areas. One draw back is: it takes too much time and can only be performed by an experienced practitioner, your sifu. Most people don't have this luxury nowadays.
So, how can a modern tai-chi practitioner opens his joints using techniques within the tai-chi system? The system is called "tai-chi system of joint-loosening" (太極鬆功). Unfortunately most tai-chi teachers are not familiar with it, or put too little emphasis in this area. One good reference on this system however can be found in a book by late Wu-style Master Wu TuNan: his famous book (in Chinese) "Wu TuNan Tai-chi-gong" (吳圖南太極功). A comprehensive system of "joint-loosening" technique is presented and explained clearly there. Those who are passionate about tai-chi as a work-out system can certainly find treasures there.