Mom won't straighten her legs, what is going on?

This is advice that I know often works pretty well in LBD, but I have no idea if it would apply to your mom. Instead of giving directions that the brain has to interpret ("unbend your knees") try a cue to an action they can do from muscle memory.

For us, for example, when Coy was stuck sitting on the edge of the bed the best thing to do was stand immediately in front of him and say "give me a hug." He knew instinctively how to do that and as he complied with that I helped steady him and he was standing! He might not know how to lean forward and put some of his weight on his feet, etc. but he did know how to hug! (My son started doing this first, and later I saw Teepa Snow recommend this.)

MAYBE (just maye) you could stand beside her with your arm around her shoulder and say something like "OK Mom, now we are going to the wheel chair" and if she starts to stand use the arm around her shoulder to help her. Maybe. Don't mention standing, or walking, or unbending her knee. Too complicated. Just "here we go" (for something her muscles remember how to do but her brain is confused about.)

This was helpful for LBD. If you try it, let us know if it has value beyond LBD.

I find that this is common with my mom from time to time. (The " i can't " thing i mean). I think it's symptomatic of the malfunction of the brain with dementia.

It's like the signals that are generated in the brain to straighten aren't making it to the muscles. I don't think it's volitional.

JG, I tried the "hug" this morning, I'm not sure it helped with the transfer but it seems nicer to give a hug than to always be so clinical... put your hand on the chair mom, NO, reach across... grab the waistband and... stand, no stand, NO STAND... swing into chair, hopefully not bruising anything along the way. Maybe with practice it will work, mom wants something to grab on to and I don't have handles :)
I may soon need to get OT to come in and give us some tips. Or maybe she will soon become altogether bed bound, another one of those lines I said I wouldn't cross.

Pam, I'd never heard of epsom salts in a lotion, I'll have to look for that.

There are good suggestions above.

I have noticed that with my cousin, who has dementia, she has progressed to the point were her brain signals don't get to her limbs. She can move her hands and fingers, but she is no longer able to reach into a gift bag to get her gift out. She looks at it. She wants it, but her signals don't work. She can't do it. On some days she may be able to do it, but most of the time she can't. It's funny that way.

I've noticed the same thing about her legs. She is able to move her legs and feet as she propels herself in her wheelchair, however, if you stand her up to transfer or get weighed, her legs are frozen and she is not able to move them. It's like they are jelly and she has no control of them. It's the brain signals that get mixed up. It's not that they don't want to move the muscle and it's not that the muscle is paralyzed. It's that the signal in the brain is not firing right. At least, that's how it was explained to me. Normally, when she is not able to move the way we are trying to get her to move, she gets scared and is unhappy that she has disappointed us. Of course, we reassure her that all is fine and that we got through it together. I can see that eventually, she will not be able to transfer from the chair.

I think you are right about the mixed signals Sunny and that soon she will not be able to stand at all. She has had episodes of this before, but if I was able to catch her at a good time I could get her to stand, walk of transfer, I guess the muscle memory was still there. I am afraid that the longer she goes without being able to do it the more her muscles will deteriorate, and she will never have the strength to recover. One more step down the road.

Mom's 96, she's never gotten a formal diagnosis but I have no doubt that she has advanced VaD. Add to that multiple compressed vertebrae, osteoarthritis, and a history of hardening of the arteries resulting in a coronary by pass decades ago and more recently multiple TIAs and perhaps one small stroke. Oh yeah, and an atrial flutter. We are currently treating her palliatively, so there will be no investigations into the root causes. I think that this is just another big decline that I will ultimately have to learn to adapt too.

Epsom salts for external. Magnesium pills or powder in a drink for oral ingestion, only if doctor recommends this for palliative care.
You will read also that Epsom salts can be taken internally for constipation, but don't do that for your Mom. It would be too harsh. Ask the doctor, the doctors do know how to keep her more comfortable, or the hospice nurse.
Keep her moving gently, as long as possible.
Keep your courage up. We care.

When one has dementia, the messages are not getting from the brain to the muscles. Don't blame her, she really cannot. Time to accept her contractures will get worse and help her as she is.

Ferris, I agree just trying to show in a child what he was told with how the brain works and Dementia makes it worse.

cwillie, get some Epsom salt lotion, rub it into the feet. It will help the leg spasms and associated pain. Keep her electrolytes balanced. Every little bit helps.