EE WAS CLIMBING DOWN FROM A CATERPILLAR 980 HI-LIFT. WHILE CLIMBING DOWN ONE OF THE CHAIN LINK STEPS BROKE. WHEN THE STEP BROKE IT CAUSED ALL OF THE MAN'S BODY WEIGHT TO PUT PRESSURE ON HIS LE FT LEG, WHICH WAS ON THE STEP BELOW. ALL OF HIS WEIGHT ON ONE LEG CAUSED A PAIN IN HIS LEFT KNEE.