This indeed is an important issue, and some practitioners (including sifus) did hurt their knees. Not immediately but from long term incorrect practice. The situation that may cause hurting one's knees is doing the bow stance (弓步) with rear foot pointing 45 degree from front, and when one turns one's body from 45 degree to facing front (e.g. doing Brush Knee Push Step 摟膝拗步).
This is how it can be done:
One must focus on single weight before the turn. When doing right leg front/right turn, one's body weight should go down 100% on one's right sole. There is indeed a subjective feeling that the rear (straight) left leg takes on some weight, but that weight is actually a side support to avoid toppling (like when a building is standing along [like when the two supporting buildings on each side have been taken down], beams must be erected on each side, like pointing at some 45 degree, so that the building will not topple on either side). When doing the turning, the heel of the rear/left foot should be mindfully pressed to the ground, the ball and socket joint at the end of the femur should be mindfully opened. The foot may turn (slightly) towards the front with the pressing heel acting as a fulcrum. With one focuses as such, a right turn will act as opening (and strengthening) the left hip's ball and socket joint instead of acting upon on the knee causing possible damage there.
Like everything else in tai-chi and qigong, a practitioner has to mindfully practice what has been instructed and has to understand the logic behind. How long would it take him or her to do it correctly will depend on the situation of each individual. Needless to say, one also need some common sense.
|Wu Chien-ch'uan (吳鑑泉): Brush Knee Push Step 摟膝拗步|