Yes I'm abnormally stiff as well, the causes can be a combination of a few things, but it all boils down to the same issue, which is your nervous system, is restricting your movements to those it feels are safe for you,
Tight muscles or rather the muscles don't have enough volume to stretch and your ns, restricts movement to prevent damge, do exercises that increase volume and fill your muscles with bloobrand reprogram you ns
You don't have enough strength in seldom used muscles ( usually around the groin area)and your n s restricts movement to prevent damage, strength them and reprogram you ns.
Your ns,is very cautiousI and just restricts your movement to save damage, Either to the muscle or it thinks you will over balance,,usually by locking the opposing muscle, so if your stretch ing your hamstring it locks the quad, this stretch reflex kicks in after 2,3,4seconds so stretch only for 1,2,3 seconds as once it kicks in your stretch is a waste of time, as your now doing an isometric exercise of two muscles fighting each other, and one will win damaging the otherRight
As good a way as any, is hang a bag, just at the height you can kick and move it up an inch or so a week