Q400 Captain was unable to control the rudder through the rudder pedals during descent. Passing 7;000 FT the pedals became usable and the rudder operated normally during the remainder of the approach and landing.

Narrative

I reduced power to Flight Idle and attempted to re-trim the aircraft. While pushing right rudder to correct for the left yaw; I noticed that the rudder pedal deflects only a small amount and then stops. It felt like a limit stop; not an increase in effort required to move it. After trimming the aircraft with the manual trim knob; I then tried rudder input to the left side only to feel and see the same result. I asked the First Officer to also carefully apply pressure to the rudder and reported the same limit to travel. The PFCS (Powered Flight Control Systems) [display] deflected only a half a needle width with the rudder pushed to the limit restriction. We called Maintenance Control and discussed the situation. Dispatch was informed and a decision to continue was agreed upon. We continued the flight and continued to check the rudder for freedom of movement. About 7;000 FT and turning left downwind to base; the rudder had a noticeable and physical release while being pressed and then functioned normally during the remainder of the approach; landing; and taxi to gate.