question about deformed joints
 

i created IK handle for my leg ( for bending of knee ) and for my foot ( for tapping of foot ) , then i allocate locators for the IK handles

then i parent the foot under the leg , then i smooth bind my skeleton to my model

i face this problem , that is deform skin when i move my leg back...
i try paint skin weight but it doesnt seem to help at all... it seems as if the joint comes out of the skin....

is my positioning of my joints wrong when i smooth bind?

pls refer to pic...

thks in advance