The big question here: Can tendons be strengthened, similarily to how muscles are strengthened? Can you train them, put the under resistance of some sort, and they will become stronger? For example, an exercise I do to recover from my ankle sprain is stand on the affected foot while tossing a ball in a circular motion. As my ankle fights to keep me balanced, is it becoming stronger over time from this exercise, or does the tendon only become more flexible? Sub question: I have tight achilles tendons from my sever's disease, as well as a side-effect from my right ankle sprain. What exercises can I do to 'strengthen' my achilles?
For your purposes it would be better to think of your body as a unit and talk about your joint rather than a part of your joint. Training is focused on body movements. Medical treatment and therapy focus on treatment of injured parts. I would answer training questions, but I am not going to answer therapy questions. Information about what therapy (for injuries and medical conditions) you should do should come from medical professionals.
Primarily eccentric and isotonic training are great for strengthening tendons. While it is easy to prescribe exercises for various injuries, I think the question that we need to ask ourselves is that are we doing enough pre-habilitation, as in preventative exercises to reduce the incidence of various injuries we suffer as a result of playing soccer.
In Denmark, two physios have developed a sock specifically targeting achilles pain. Its a sock that have compression around the achilles, providing protetion and increased blood flow. The sock is being used by alot of football players, handball players in Denmark. Its called C-Sock. You should look into it, if you have persistent achillus pains.
