ALL 4 MOBILE BRIDGES WERE BACKING OUT OF A CUT, WHEN THE FRONT 2 BRIDGES (#4 & #3) STOPPED AND STARTED MOVING FORWARD, WHILE #2 & #1 BRIDGES WERE STILL BACKING. SUDDEN STOP CAUSED THE #2 & #3 BRIDGE TO SLIDE TOWARD THE OPERATOR CATCHING HIS KNEE PAD UNDER THE MACHINE PINNING HIM AGAINST THE RIB. OPERATOR HIT PANIC BAR, BUT IT FAILED TO DISENGAGE. IMMEDIATE REPAIR TO PANIC AFTER AID