Knee pain is often not due to knee osteoarthritis. The pain may be due to bursitis, inflamed fat pads & plica, ligamentous & iliotibial strain, and this: patellar tendinopathy. I've got this, and I no longer run marathons.
So what are eccentric exercises? That's the phase of muscular contraction while the muscle is lengthening (while concentric contraction occurs when the muscle contracts and shortens). Where patellar tendinosis is concerned, an example of eccentric exercise is to do sitting leg lifts, focusing on slow controlled release as the quadriceps are lengthened.
Here's an ultrasound shot of my right patella tendon in long axis. Note the hypoechoeic area. I'm seriously considering injecting PRP into this lesion.
Platelet-Rich Plasma in Arthroscopic Rotator Cuff Repair: A Meta-analysis of Randomised Controlled Trials
Moving on to the shoulder, rotator cuff tears, tendinosis, impingement & bursitis are the main bane. Referred pain from the neck is also not uncommon.
In this study comparing PRP with physical therapy in partial rotator cuff tears, the various outcomes (pain, range of motion, function) at 12 months were similar.
Elbow pain is overwhelmingly Tennis Elbow, and there is no "best" treatment. The popular toss-up is between needle fenestration (dry, saline/destrose, steroid, PRP) and ESWT. Entrapment of the posterior interosseous nerve (PIN) at the arcade of Frohse can mimic the pain, albeit more vague and diffuse. Hydrodissection may help in this case.