Contemporary books on tai-chi, as well as writers of blogs and websites on the subject, used to say, first you have to learn the soft part of Tai-chi, then you can learn the hard part of tai-chi in some future day. Unfortunately, the future day never comes, but the students are not disappointed. The reason is that tai-chi is one of the best mind-body conditioning system nowadays, and a common folk doesn't need much power to satisfy his daily chores. Having said that, I don't think it "healthy" for people (old people and folks recovering from major illness excluded) to have too soft a structure after learning years of tai-chi. For those who are interested to build a strong body structure and some martial power (like lifting or punching power), I can suggest two reference books:
1. A Chinese book written by DaCheng Quan master Yu Yongnian (大成拳站樁與求物 - 于永年). One can find excellent and detailed power training techniques in his advanced zhan zhuang practice.
2. An English book "Tendon Nei Kung" by Mantak Chia (that I wrote a book-review in Amazon.com).
The first one refers to the method of building the necessary power, while the second one refers to the practice of "externalizing" that power. Both are needed. Yu is a lineage student of grandmaster Wang Xianzhai, while Chia said he learned many stuffs in Hong kong, and rumoured had it he also learned tai-chi nei kung from famous Wu-Cheng style grandmaster Cheng Tian Hung. Both have important things to say. I shall come back with more details of their teachings in later posts, stay tuned....