EE STEPPED FROM TOP RUNG OF LADDER (5'4""HIGH) TO20""X 24""PLATFORM TO GET SAMPLE FROM A FINES FEEDER. EE TRIPPED ON EMERGENCY STOP PULL CORD THAT IS LOCATED 4""ABOVE PLATFORM. EE FELL, CATCHING HIS LT. HAND ON A HANDRAIL IN THE IMMEDIATE AREAEE TREATED BY MD FOR SPRAIN/STRAIN OF LT. THUMB.EE RELEASED TO MODIFIED DUTY. NO PUSHING,PULLINGLIFTING WITH HIS LT. HAND.