My son is seven months old. He will all of a sudden tighten up both arms and clench his fists. He does this a few times a day. I think more if he is frustrated. He also moves his head back and forth quickly when he's waking up or going to sleep. He is often stiff in the legs when you pick him up. He prefers to stand then to sit or lie down. Does anyone know what jerky movements indicate as an infant, is this something he will grow out of as his neurological system develops? Does anyone have any input for me?