An Achilles tendon tear or rupture is often the result of Achilles tendonitis that has gone without treatment. Over time, the inflammation of this condition gets worse causing changes within the tendon that weaken it and later result in partial tears.
Achilles tendonitis, inflammation of the strongest tendon of the leg, ankle and foot, can sneak up on you very slowly over time. Often patients fail to identify any injury or accident that may have caused it to develop. This condition results in pain and a loss of motion at the ankle. If you have pain behind your ankle and cannot bend the ankle beyond 90 degrees to the leg it is likely that you have a tight Achilles tendon. To check for this, a patient lies flat on his back with the knee bent.
The foot is positioned with the second toe aligning with the center of the knee. Pushing up on the foot the ankle will flex causing tension along the Achilles tendon. If the Achilles tendon is too tight, the ankle will not flex past a 90 degree angle with the leg. Often this tightness is painful and simple squeezing of the calf muscle can cause a lot of pain.
Over time, if untreated, Achilles inflammation, or swelling, actually causes a wearing of the tendon covering and later begins to weaken the tendon fibers much like a shoe string wears thin and before it breaks.
Read some articles that Dr. Molly S. Judge wrote about the subject