I just wanna reply to +1 this comment as it's an overlooked bit of all of kneesovertoesguy's content. He preaches the importance of your starting point being a spot where you don't experience pain and building from there. When I first saw his stuff I just thought he was another social media grifter, but I no longer think of it that way. The content might not be ground breaking, maybe it is. But I've found it helpful and personally rewarding. I also just want to reply to the thread OP in general with my own life experience and things I've learned that I wish I could go back and tell my 19 year old self about - that guy was OBSESSED with table tennis.
Obviously this is all just anecdotal and I'm not a doctor. As someone who has taken my knees from an absolute garbage fire (thinking I just had 'bad' knees) to generally fine and still improving, I can't recommend those exercises enough. I would take it farther and say that once you can improve your mobility and reduce the pain level to do some of these things, that general strength training supplemented with some kind of mobility training (don't have to go crazy) will improve your life tremendously as well even if you only have time for a couple hours a week. I've made these things a non-negotiable in SOME way for the past 8 or 9 months and without trying to sound corny, it's literally saved my life.
I've strength trained on and off a lot of my adult life. Used to care more about my power lifting numbers than I do now, but after I got to the point in my life of having a desk job it was hard to keep pain away completely in some way even while I was still strong. Lower back, knees, whatever. It would then get even worse when I stopped strength training. Then I'd get depressed, gain weight, get weaker, and pain would just get worse. Felt helpless. Try to start doing something, hurt too much. Gave up. Just thought the ONLY cure was strength training the way I knew how paired w/ dealing with certain pain levels. And eventually didn't want to do this anymore. While experiencing this when I hadn't left the sport yet, I really started listening to the attitude of people around me telling me I can only be so good, to be realistic, to set realistic expectations, some people just 'are' a certain level, and certain people aren't, etc. Eventually just stopped playing table tennis altogether. Would show up once every few months at first, then just stopped that too. I don't think like that anymore.
I'm 33 years old. I started table tennis when I was 18. recently off of a 5 year break from competitive play, 2-3 year break from just general club goer levels of play. Started playing table tennis again back in December as a way to start losing weight (I got big AF boiii), and ended up finding a practice partner. In part, I've learned some lessons dealing with ailments/injuries (shoulder dislocation, severe piriformis syndrome, etc) but while a year ago I would have thought my best table tennis level was way behind me, I now think it's still in front of me. Improving my knees allowed me to handle my body when playing table tennis and training at home. Playing more table tennis gave me more confidence and conditioning, and those things allowed me to be successful in other parts of my life. It helped me lose more weight. I'm now down 70lbs and climbing, and I can attribute the start of all this to simply learning I can actually improve things I'd written off as things I'm stuck with, like my 'bad' knees or my 'bad' shoulder.
Now I treat table tennis as if I'm a professional athlete (lol I'm obviously not): I take a dedicated time to warm my body/muscles up properly before I get on the table - I'm usually sweating before I pick up my racket. I put my work in on the table, and the first thing I do after just taking some general FH/FH or BH/BH to warm up my eyes and feeling is immediately into footwork to get my legs and core pumping before moving onto more table time. The session is a success as long as I pushed myself in some way. I (not as consistently) cool down/get a recovery session after I'm done. I static stretch before I go to bed. I do mobility work, a bit of yoga, and while I haven't been able to do general strength training the past couple months for home reasons, I plan to start back at that very very soon as I can tell the difference in performance not doing that compared to doing it. Overall it sounds like way too much for an adult with a full time job and family - It makes me sick typing it, but my thinking has adjusted to taking these things incredibly seriously because it really does enable me to be a completely different person and enables me to actually enjoy table tennis and handle practicing more than I ever have in my life. Even if I can only dedicate a little bit of time during the week, I make sure to get some of this done, for no other reason that to do something for ME.