What causes the pain?

When a muscle or tendon gets injured, either traumatically by a tear or gradually with overuse, scar tissue or adhesions will form between fibers. Scar tissue and adhesions act like glue to bind up the tissues, and inflammation develops, swelling up the tissues. The effect of this is to cut off the blood circulation to the area. No blood equals no oxygen. Without oxygen, nerves can't work properly, bringing pain, numbness or weakness.