In my case, the muscles of the forearm that allow the wrist to be cocked up–the muscles of wrist extension–had begun to microscopically tear loose from the outside of my elbow where the muscle attaches to the bone. This place where muscle attaches to bone is not technically a tendon since it does not run through a sheath, but the pain is similar and referred to by the same term of “tendinitis.”
Another insidious form of tendinitis attacks the tendons that hold the upper-arm bone to the shoulder, usually when the shoulder has been overworked in some kind of overhead work, such as reaching up a steep slope to plant a pole. The tendons at the back of the shoulder rub the underside of the shoulder blade until they are raw. You can get the same result from a sudden, violent movement, such as falling on an outstretched arm.
A more severe type of tendinitis can occur if you ignore the developing problem until calcium salts grow in an inflamed area. The sharp pieces of calcium irritate the bursa sac, the tough bag that surrounds all joints to hold the lubricating synovial fluid. The irritated bursa starts to overproduce fluid. Eventually, the entire sac becomes inflamed and tense, and the whole joint aches. At this point the standard treatment of tendinitis may not work.