TRAINING EXERCISE - MOVING CABLE ON BOOM. EE CLIMBED 6' LADDER TO CHANGE CABLE ON BOOM SHEAVE. FELL OFF LADDER WHILE DESCENDING; HIT FRONT OF HEAD ON LADDER (OR ?), LANDED ON BACK ON GROUND. UNRESPONSIVE, BUT AWAKE WHEN AMBULANCE ARRIVED. REFUSED TREATMENT. WENT TO ER NEXT DAY; ADMITTED (3 DAYS). PRIOR BACK PROBLEMS AGGRAVATED BY FALL.