As far as I know, ‘spasticity’ is typically severe tensing (…or flexing) of the muscle over long enough periods of time to cause ‘locking’ of the limb, so that you’re unable to fully relax (…or un-clench) the muscle.
I get this (…pretty much permanently now) in my right calf which makes it impossible to walk or even balance properly. Just can’t bend or relax the leg anymore.
I don’t think it causes pain normally, but I could easily imagine it causing cramps when it tends to cause muscle to contract when it’s not supposed to ?!!
Never heard of it causing burning pain, though !
I’m sure others will be along shortly to explain this symptom far better than I have.