Today at the gym my friend(17) went on to the bikes, one of the more horizontal ones. About 20 secs into it she started telling me her knees kept cracking. Thought it might be her seating position but that never helped. Lowered the resistance (still no luck). She described what she felt as "It was almost like i could feel my knee cap coming off". Didnt hurt while walking, just on the bike, not sure about the other one. Basicly is there anyway to help her knees? Is there anything wrong with her knees? Shoudl she just avoid that exercise bike form now on?