Two years ago, I ruptured my Achilles tendon. This is definitely worse than a ruptured plantaris, which is a smaller tendon. I had to have surgery, then three months in a cast, and then six months of physical therapy. It was still sore and stiff so I decided to give up tennis. I was playing tennis when I was injured.
This is the information that I got from the doctor. She told me that this tendon is the largest one in the body and that there are several things that might have caused the rupture.
It might have happened because my calf muscles were weak and tired, or the muscles in the leg were not balanced in strength. Or, maybe, when I was going for a shot, my foot and toes went up from the ground and "pop," the ligament tore apart. I guess I am either too old, or I didn't warm up or whatever. It was a real ordeal.