Employee had to climb 2 fixed ladders in order to access head pulley of choked stacker belt to apply belt dressing. Performed action 4 times (up/down) throughhout 5-hr period. 2nd trip heard knees ""pop"" but did not report to super at the time. Self-medicated/rested over weekend, but pain persisted-requested treatment 2/10/09 - was not determined as work-related until 2/19/09.