I've been spending quite a bit of time while I'm in the US doing on-line educational activities, playing catch-up on my required continuing medical education while I have some spare time and good internet access. I have to say that a lot of the activities available are of minimal real-life value to me, but this one is a major exception--in fact, I thought it would be of interest to many members of the forum.
"Chronic tendon injuries" are those lasting more than a few weeks. The four most common of these injuries are:
Lateral epicondylitis ("tennis elbow")
Rotator cuff tendinopathy
(By the way, pain in the main part of the Achilles tendon is different from pain where it attaches to the heel, but details.)
The biggest single piece of take-away knowledge from the article I read is that CHRONIC tendon injuries are NOT mediated by INFLAMMATION! (That is why these are now referred to as "tendinopathy" rather than "tendinitis". This single bit of knowledge changes radically the approach to treatment. The biggest change in the approach for most doctors is that anti-inflammatory medications are NOT effective in these conditions.
Now, the thing that is most relevant to this forum, is that the best-proven therapy for Achilles and patellar tendinopathies is eccentric strengthening exercise. Note that this is not merely stretching the affected tendon (which has been recommended in the past), but eccentric strengthening. Intuitively, I'd tend to assume that that would make either condition worse, but I'd be wrong! For Achilles tendinopathy, the recommended exercise is a sort of calf rise done with both feet on the edge of a step, then eccentrically lowering using only the injured side, then again using both feet on the concentric. The authors recommend doing 3 sets of 15 reps twice a day for 12 weeks, adding weight after 2 weeks. Seems like a lot of volume to me! There is also mention of repeating the exercise with the knees slightly bent, but they don't make clear if they want you to do that for the same volume as the straight-leg version every day.
For patellar tendinopathy, they recommend a sort of 1/4 squat done with both legs with heels elevated. The illustration provided shows a subject doing this while standing on a foam wedge. I'd think doing it on a solid surface with a small heel lift would be more stable. They recommend the same rep-set scheme as for Achilles tendinopathy.
A whole host of other possible therapies are discussed, but they are all either significantly less effective, or significantly more invasive and risky than eccentric strengthening.
They also discuss tennis elbow and rotator cuff injury, but there is no discussion of eccentric strengthening for those injuries. By the way, rotator cuff injury is the injury for which steroid injections has the best track record. This is risky for Achilles and patellar, and not very effective for tennis elbow.
I'm not sure if people who are not members of the American Academy of Family Physicians, or a paid subscriber can access the full article or just the abstract, but here's a link you can try:http://www.aafp.org/afp/2013/0401/p486.html