Chinese are much onto longevity. There are numerous programs in (Mainland) China's TV channels focusing on different aspects of the subject, by different types of experts, modern or classic, East or West, this lineage or that lineage, self-made or inner circle. Recently I watched a program run by western medical experts (Chinese doctors) on the subject of harming your knees and saving your knees. The video program starts with a quiz: "Which exercise harms your knees most?" Choices are tai chi, running, walking steps, lifting heavy weights and zhan zhuang.
The medical answer is zhan zhuang!
It is nice to note that zhan zhuang is now considered a stand-alone complete exercise of its own by the general public (at least in China). The bad thing is it is considered the worst exercise for your knees!
Comments include: "Zhan zhuang is worse than tai chi (which, according to what was said previously, is pretty bad) because the weight on your knees in tai chi will shift constantly while that of zhan zhuang remains at a single point."
The classic way of teaching zhan zhuang (as least for the younger) is to stand for at least half an hour per session, and for serious practitioners an hour. The medical doctor's advice is not without empirical grounds. Some people did harm their knees through (inappropriate ways of) doing zhan zhuang and tai chi (with the misnomer: tai chi knees).
With proper training method, the practice of zhan zhuang (and tai chi) can actually heal or at least alleviate weakness or pain in knees. The rationale behind is that in proper training of chi related disciplines, a student has to open a joint (relax the joint), find a point of maximum resistance (realigning the joint), and (use gravity) to rest one's weight on the stronger point of maximum resistance (conditioning the new support point). At all times, a student is to engage the power of his breathing muscles.
In training procedure, the first hurdles to overcome is opening your hip joints (kua). With prior opening and conditioning of your hip joints, you can proceed to tackle your knees joints, cautiously at first if you have knee weakness or pain (consult a good teacher when in doubt).
Needless to say, the western medical practice for knee-saving is surgery, and the medical doctors further reassured the audience that surgery can be done relatively safe even for most people in advanced age. My view is that the medical doctor does offer some good advice. My further advice is "Why not try it first with doing (proper) chi kung?" If it works for you, you can skip a major surgery. If it doesn't work for you, at least you have learned a new skill that can promote your longevity. Surgery is always there as an option of last resort.
As a final advice, modern physiotherapists do have little tricks/exercise to alleviate (rather than cure) knee pains. Chi kung teachers should therefore be open-minded to incorporate such exercises if they find them appropriate to their students' conditions.