This is very common, particularly in Frontal Lobe Dementia. I know there is a discussion topic on it, but I don't know what the title of the discussion is. I'll try a search for you.
My hb is still in the early stages but I do notice stiffness. I use to be able to squeeze him from behind to 'pop' his back but forget it now. He is so stiff, not relaxed at all so it is like hugging a tree! I know he says he can't straighten his shoulders saying it hurts his lower back. and he is always complaining his legs are stiffening up so has to get up and walk around.
At this point, my husband is so stiff that is really is like a wooden board. The aids have a very difficult time getting him dressed, undressed and in and out of bed because he is unable to move much by himself and is so stiff it is difficult to move him. It is also painful for him to be moved a lot.
He is on Bacofen (sp?) and that helps. His hands are affected and he is getting Botox shots in his arms every three months. This has at least helped to keep his hands more relaxed.
My husband has FTD and his whole body is very stiff and like therrja the aides have trouble getting out of bed and into his chair as he does not bend in the middle very well.