When people traumatise a knee through having it bent the wrong way or smacking it up against something hard, it is fairly obvious why you will feel sore afterwards. When your knee starts hurting for no apparent reason and a trip to the doctor and an x-ray still does not explain the cause of the problem it might be a muscular one even if it feels deep inside the joint. This happens because uneven muscular tensions in your upper or lower leg can interfere with how well your leg can bend and weight bear.
If you have a problem knee (whether it is recent or long term) have a good feel of your leg muscles and compare the same locations with even pressure on both legs. Press into the muscles right and left and above and below your sore knee.
Feel along the full length of the muscle. Do you get identical sensations under the same pressure in the same locations? If not you might explain your knee pain, you may need a massage or some acupuncture.
If you have had a knee x-ray and there is evidence that you have some osteo-arthitis (cartilege thinning) but it has not yet worn to the point where you need surgery, there are things that can be done that can slow further deterioration and alleviate your symptoms.
Sometimes stiff leg muscles can make your knee sore, check your knees now, do your thigh and calf muscles feel exactly the same on both of your legs? Some Self Massage over your stiff leg muscles may improve how your knee feels even though your leg muscles dont feel particularly sore.
Sometimes knee pain is brought about by neglect, like through lack of exercise. Sometimes doing things when you are young that do not bother you at the time (like kneeling on cold hard surfaces or playing contact sport) can affect you later on in life. Sometimes our efforts to achieve stronger and better bodies can be a little misguided through poor exercise and sports practises.
Many times I have seen at gyms a bunch of guys standing around a leg-press machine all egging each other on to out-do one another, grunting and straining against heavy weights in the heels-to-butt position doing god knows what damage to their knees. Ego can really mess you up.
The practise of strapping dodgy ankles to stop them from twisting can damage your knees. Your ankles are naturally designed to move side to side to help us walk on uneven ground, our knees have no natural sideways movement. When ankles are strapped to prevent side ways movement we force our knees side ways which is a really bad idea.
Stiff knees can stop us doing many common place things like tying shoe laces, pulling weeds from your garden and safely squatting to lift things. Your knees are likely to give you many subtle hints that they are not happy before they really let you down. Fully squatting especially under load is not good for your knee joint.
Self Massage and correct exercise and posture are useful for knee comfort and function.