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Can anyone shed some light on this, or whether you have experience this before.

I'm RRMS - about 7 years since dx, but what I want to explain refers to when I'm pretty much symptom-free

When I am sitting or lying down, I feel ok , as in there's no spasticity. But when I stand up, my legs (around the hamstrings) tighten up and I can't move. I have to forcefully tighten my legs (it almost feels like I'm stretching my legs while standing up). After maybe 15-30 seconds, the spasticity goes away and I can resume walking as

Can anyone relate to this?

Yes.
My onset attack some 35ish years ago involved my legs. Since then I have suffered a certain amount of residual damage with muscle spasms and cramps in my legs. When I get out of bed every morning I do so carefully just in case the leg doesn't want to work quite right.