You have to ask yourself about 3 things, i believe anyone can help me if they know of others.
1 Are you eating healthy?
This means, are you getting enough protein, fat and carbs? Protein being of utmost importance.
2 Are you exercising properly?
This means compound lifts, and if you are only sticking to upper body lifts it may be part of the problem. What is the issue with your knees? I know there are some people that know a good amount about knee pain, hopefully they can chime in.
3 How is your testosterone level? Maybe you should take a look or try out Alpha Male.
These are in no particular order, just 3 things to look at, though I know the #1 priority is dealing with your knee pain. Let us know how it goes!