Just in case it helps anyone. On a recent visit to my rheumatologist I asked exactly how PMR creates pain and stiffness and does it affect the joints, muscles or vascular system as I have heard all three.
He said that it is actually an inflammation of the peritendon or peritenon which I gather is the sheath that covers the tendon enabling it (normally) to glide and protect the tendon which attaches to the muscles (ligaments control and support the joints). The tendons also attach the muscles to bones and joints and enable the lengthening and shortening of muscles in use
Anyway, I found this interesting as I always find that a visual picture of what is happening helps