one cause of dark knees (as well as elbows, and inginual/singit) is
friction.
those are the parts of our skin which
folds & constantly rubs with whatever we wear & surfaces they contact with (like floor & table). they are also
usually dry which makes themeasily get dark.
moisturizing is the key to preventin dark knees/elbows. keep them hydrated throughout the day. if you are at work, you could apply body butter or lotion...
when you
exfoliate you should pay extra special attention to these areas. but
don't overdo it (excessive or rubbing to hard) cause
too much friction could also cause skin darkening
