the perhaps mistaken idea that arthritis in general is an autoimmune condition
Arthritis is an inflammation. In 'osteo', the inflammation is considered to be caused by "wear and/or tear": i.e., aging or injury. In "rheumatoid", the inflammation is not caused by wear and tear. It's considered auto-immune because there's no apparent reason for the body's inflammatory response. The specific symptoms can differ between rheumatoid and osteo arthritis, and they can differ from person to person. That's often one way that a doctor can tell. There are over a hundred types of arthritises. Some people believe that most forms of arthritis are the result of wear and tear (or aging). However, some argue that autoimmune issues (caused by genetics or diet) are always a factor --since not everyone develops arthritis.
One counter to the wear and tear argument is that exercise (i.e., more use) is almost always recommended for any arthritic condition. The amount and type of exercise will differ, but usage seems to be a cure, not a cause of arthritis. Okay, "proper exercise" versus exercising improperly seems to be a key. Anyway, all I can say in terms of treatment is 1) get a test to see if it's truly arthritis, not some type of neuropathy or condition. Rheumatoid arthritis can be painless, though physically debilitating. Pain can arise from non-arthritic conditions. If it's really osteo, then correct exercise is imperative to avoid further injury. If there's always pain, then there's either anti-inflammatories (nsaids or cortico-steroids) or surgery (depending on the body part affected), or replacement of the joint.
It's easy to advise someone with arthritis to exercise. Unfortunately, sometimes it's not possible because of the pain or lack of function. People with rheumatoid arthritis can be so disfigured that it's impossible to exercise, or even work. My father has it, and his hands are contorted to the point where they're almost useless. Rheumatoid can also have a congenital aspect and run in families. So, if it's in yours, it'd make sense to get tested for it, imo.
I would say that taijiquan would be good therapy for the knee ... but, in truth, if it hurts, it's probably not right. Frankly, imo, if you can walk, that's good exercise, too; and very practical. You can laugh ... until you can't do it
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"A man is rich when he has time and freewill. How he chooses to invest both will determine the return on his investment."