Too many people hurt their knees in tai chi. I never could understand this, as I took up tai chi to CURE a knee injury! One day I finally understood why people were getting hurt. The culprit was improper execution of “twist steps.” When you twist-step, you need to make sure about a few things. First, the bent knee must never cave in. Straddle stretches, quad stretches and learning how to position the tailbone will fix that. But the BIGGEST problem is the motion and timing of the twist step. You can even see tournaments in China on YouTube wherein tai chi masters are doing this wrong! It’s not a matter of opinion in this case – it’s a matter of physics and evidence that any halfway decent sports medicine physician or orthopedist will immediately understand as well. Please do not repeatedly do a motion that will hurt yourself in the end. Fix this now, and… martially you will develop more power in your strikes, and for your health, you will strengthen, rather than weaken, your knee. Now, it DOES take some time to get used to it – especially on your non-dominant side. Just work on it, mindfully, and then it will become very easy.
Also make sure that when you pivot the twist step on the heel, you don’t sink back onto the moving leg. The bent knee should stay BENT and still. NO shifting back! Watch yourself in the mirror. The key is to relax, loosen the hips and stay sunken down like you are sitting. At the end, relax the straighter leg.