Muscle Jerking???

[SteviNicks]

Hey guys, I’ve been trying to lucid dream for a while now and what I notice is that every time I keep my body still for sleep paralysis, after a while there’s this tension that builds on my leg, like a growing urge to move it, although I do all I can to resist, it always eventually spazzes out by itself and it is very annoying as it is very uncomfortable and makes it extremely difficult to stay still. So if any of you guys knows why it happens or any solutions I will be very grateful. Thanks oh i always lay on my back if that helps?