Anyone dealing with constant leg movements?

Is anyone out there dealing with anything like this?: My mom, with severe dementia, moves her legs constantly. When lying on her back, awake in bed, or when sitting or reclining in her reclining chair, she moves her legs like she's riding a bicycle, then she rolls up her pant legs, then unrolls them, then strokes her legs, then arches her back, then rides the bike again, and on and on in any number of sequences and combinations. Sometimes she will call my name and then point to a leg, or she has even asked me to "Please take this off". When it first started, it was not constant, and sometimes it meant she needed to go to the bathroom, or she ws hungry, or something hurt, etc. Now it is constant and she continues doing it immediately after she eats, goes to the bathroom, etc., even while she is eating, so it does not seem to indicate any particular need. It used to drive me crazy, but now I realize it is helpful in that it is giving her exercise and keeping her from getting bed sores. On the negative side, it makes it hard to feed her because the arching the back makes her slide down to where she is lying flat on her back with her head pushed forward, which makes eating and drinking an aspiration hazard. I have a blanket or sheet under her that I use to pull her up to a more upright position, but as soon as I'm done pulling her up, she arches her back again and slides forward. Also, after a number of pulls, my hands are aching, and the blanket has slid till it is no longer under her sufficiently and I can't use it to reposition her any more. It also results in the bedpad being no longer under her bottom, and the blanket and chair get wet. The latest addition to this behavior is if she is sitting in a regular chair, she immediatley starts arching her back and sliding forward to where if I don't push her back in the chair she will eventually just slide off onto the floor. She even did it in her wheelchair yesterday while I was getting things ready to take her to the doctor (she has always been safe in her wheelchair with a seatbelt up til now) and after a few minutes when I came back, she had her feet up on the bed like a footrest, her bottom hanging off the chair, and the only thing keeping her from falling on the floor was the seatbelt was under her breasts and armpits. It took me almost half hour to manage to wedge a chair under her bottom so I could keep her from falling on the floor and then get her back in the wheelchair, then later in the waiting room it was a constant effort to keep her from sliding off the wheelchair even with the seatbelt on. She is totally unaware she is doing this, so asking her to stop doesn't help. Now, even when she says she is cold, she moves her legs so much that she ends up kicking the blanket off, which then somehow ends up under her bottom (where the bedpad should be), and it ends up wet (the bedpads are no longer under bottom, so they are still dry!). Surprisingly enough, with all this exercise, she is stil so weak that when I get her up to walk her (using a rollator walker) to the bathroom or to bed and back, she is constantly on the verge of falling and it is extremely stressful and exhausting for me and for her. I could just give up and have her sit in her walker, but then she wouldn;t be doing any walking and would just get weaker. It's probably a vicious cycle, because I probably don;t get her up and walking as often as I should, so she has too much nervous energy and does all this leg movements as a result. My questions: Is anyone else dealing with these or similar behaviors? Any insights on the causes? Any solutions to the safety problems? Are there any products to keep someone from sliding forward in their chair? Any way to feel safer using a walker? I welcome any suggestions or comments!