My 12 yr old son plays football. He recently began complaining about pain in his Achilles. He was diagnosis with osgood-schlater and we thought perhaps he was developing tendinitis but there is no swelling and the pain is only sharp right after he runs. He describes it as sharp and only for a short time maybe a minute. He is able to move his ankle in all directions without any pain except when he point his foot to the left and downward that he feels some pain. What could be causing this and what stretching or movement should he do as well as avoid.