I had chronic tendonitis in my elbows, both of them. Medial epichondilitis (golfer's elbow) and lateral epichondilitis (tennis elbow) in my right. Over the years I've had cortisone shots in both elbows, enough that the doc won't give anymore, I'm at my lifetime max (cortisone, it turns out, is a cataboloc steroid and breaks down tissue, which is why it's used for inflammation). So I went to a sports medicine orthopeadist who got me doing wrist curls with extremely light weight to start with, then progressing a pound at a time, 20 reps in every direction, one set (of each) every day. I didn't believe it would work, but did it anyway, starting with 5 lb dumbbells. When I got up to 27-1/2 lbs (I bought magnetic "plate mates, 1-1/4 lbs each to stick on the ends of my hex dumbbells so I could go up in weight 1-1/4 lbs at a time) I finally started to feel better. I'm now up to 35 lb dumbbells and I no longer do the curls every day but have zero pain most of the time. That's right, ZERO pain. I also used MC IGF-1 at 100 mcg directly into the tendon (medial epichondile) a few times. This really seemed to jump start the healing process.
I know it sounds counter intuitive to exercise the very thing that hurts, but I'm telling you, this has been a chronic problem for me for 10 or more years and I've tried literally everything to fix it including ice, sleeves, blah, blah, blah. You name it, I've tried it. The ONLY thing that worked is the exercises. I started with weight so light it just barely hurt after 20 reps. I would have been embarrassed to do it in a gym, they were so light (really, like 5 lbs, no shit). Slowly worked my way up, going from 5's to 8's, to 10's, etc, every stepbof the way using pain as the gauge: can Ibjust barely feel it after 20 reps at this weight? If it hurt more than that, I went back down in weight. It took a long time, weeks and weeks, but it finally did work. Try it, and stick withbit, I'm POSITIVE it will help your elbows. I wish I could offer a suggestion for knees, but I haven't had tendonitis there, so I got nothing for that...