WHILE BOARDING THE UNTI TO REPLACE A VALVE, EE STEPPED ONTO THE SECOND STEP, AS HE LIFTED HIMSELF, HIS FOOT SLIPPED OFF THE STEP AND BUMPTED HIS LEFTKNEE AGAINST THE STEP RESULTING IN A SMALL BRUISE. THE SFAETY CLERK WAS NOTIFIED ON 2-26-01 THAT THE EE WORKED 2 DAYS OF LIGHT DUTY.