Even without any weights on the leg curl,it will help strengthen the back of your knee,just do lots of reps slow and smooth,and the same with the leg extension.If its too easy,as in you can do thirty and its not even starting to feel hard,then put a five kg disk on each side and go for around the twenty mark,but remember that rest is just as important as training,and only do it every other day,do three sets of fifteen to twenty each time,with about two minutes rest in between sets.You will soon find that the strenght in your knee will pick up.Once its easy for you put a little more weight on,another two and a half or even five kg if you feel like it,and do the same reps,just add another set when its comfortable to.You wont get much muscle growth,just stronger toned legs,you need to pack the protien into you to body build,as well all the other ingredients like carbs and fats,and more protien,so dont worry about getting big legs.Your reps would be low,like six to eight and you would need to fail on the seventh or eight rep with heavy weight on.