EMPLOYEE WAS IN PROCESS OD STARTING EQUIPMENT AND SINCE THE ROCK TRUCK HE WAS WORKING ON WOULD NOT START HE CLIMBED THE BOARDING LADDER AND REMOVED THE OUTER AIR FILTER IN ORDER TO SPRAY STARTING FLUID. WHILE DOING THIS, HE LOST HIS BALANCE AND FELL APPROX 7 FT AND LANDED ON HIS FEET. HE SUBSEQUENTLY SPRAINED HIS LOWER LEFT LEG.