You know when you go to a specialist doctor for a very specific kind of pain/ailment that you can't fix with over the counter cold remedies or a old passed down home remedy from an old timer...the kind that is not seen by a naked eye? Only an x-ray of some type or a probe can see what the issue may be in a case like that. When all the testing and examinations of this testing are finished, you get the results of why the pain/ailment is persisting from the specialist. Our bodies are very finely detailed machines that function as a place for us to live and breathe life to the fullest and pain(s) cause them to not function properly. Like a knee for instance. Normal every day life over time can wear out parts within the area of the knee. Unless there is swelling going on, someone would not know that is an issue or pain just looking at it. But once you start moving and walking with a limp that is when others can see an issue had occurred or is ongoing. People may guess at what is wrong by without the proper testing/exam from the specialist doctor tgat deals with knees, you will not know the course of action to take for the care of your knee. It could be as simple as resting it a few days to knee replacement which requires rehabilitation after the replacement is done. Both of my parents have had knee replacements and they both had to get out of the bed within a few hours to start rehab process of learning to walk again on a rebuilt knee.