What is polymyalgia rheumatica? Polymyalgia rheumatica (pronounced pah-lee-my-al-jah room-at-i-ca) is a type of arthritis that affects the muscles. It does not affect the bones, or the joints, which are the spots where two bones come together. It causes muscles to become stiff, tender and very sore. This is called inflammation. Even though the muscles are sore they do not become weak.