Binding and weight painting

Worked with binding skin to skeleton and control, found it very simple along with the weight painting it went well and was easily understood step by step thanks to the tutorials which allowed be to focus on each individual step at a steady pace allowing me to connect everything up. The only problem I have with my character is the movements of the characters knees because his legs do not has a real pivot point so the characters legs from the ankle of the skin  along with the skeleton and i have no idea why, am still am trying to come up with solution and still can't find one, I am guessing I should have designed the character with better knee's for the skin but not entirely sure what has gone wrong could be just the movement settings, will need to check.

I the future makes sure that every design is developed according to the function of its area to avoid problems and any mistakes.