Sounds like spasticity to me...my legs start to stiffen after a few minutes of standing or if I take the pup for a walk. It hurts!! Not like a knot-like spasm, just tightness and pain.

I go home and try to stretch the legs out with a yoga strap or by pushing against the wall doing an achilles' stretch. It seems to alleviate the tightness.

Hugs, dear