What's the problem?
What happens is simply that the amount of pressure applied to the bones, cartilage, muscles, tendons and ligaments around the knee is simply more than they can stand. That results in microdamage to these structures and results in pain and inflammation. Those symptoms send a message to your brain telling you to stop. Therefore our job now is to teach you how to control the symptoms and then work to restore function by improving the strength and flexibility of the joint. By restoring and improving your strength, flexibility and coordination we will then often be able to see steady increase in your activity level up to (and perhaps even beyond) what you could do before. And then, since you are well-conditioned and able to do what you want and/or need to do, your pain should not return. Without such a rehabilitation program, your pain will likely return when you try to become active again.
How did I get this way?
Injury? It is possible for this problem to start after an injury. If you simply rest an injury until it feels better and then restart activity, chances are that you are now weaker and less flexible (due to the injury, healing and inactivity) than you were initially.
Overuse? There may not be an injury that you can remember. Overuse might be simply a relative problem. In other words, you don't have to run a marathon to overuse your knee. If you are overweight and weak, you knee will be overused with a much lower amount of activity than if you are slim and fit. Sometimes an overall illness like the flu or other severe illness might produce lasting weakness which makes previously "normal" activity into "overuse." In a healthy person, overuse may simply be activity that is too frequent, too intense or out of the ordinary for you. An example might be a home project (painting a ceiling or excessive yardwork) or an unusually long period of any physical exertion.
Growing children and teenagers are susceptible: Relative overuse can often occur in growing children as well. When growing, one generally gets heavier and taller each day. If you don't get stronger too, you fall behind just a bit everyday. If you have an injury or overuse problem while growing, it is even easier to get behind in your strength and flexibility. This then makes it even harder to return just to the level of activity that got you into problems initially, much less to progress beyond that level of activity.
So what did I do wrong and how do I get better?
If you rest an overuse injury and then simply restart activity when it "feels better," again chances are that you are now weaker and less flexible (due to the injury, healing and inactivity) than you were initially. You are therefore even more susceptible now to recurrence of the problem. Do you see how this can be a vicious circle of pain, rest, weakness, recurrent activity, more pain, more rest, less strength etc? The key to treating this type of problem is to control the symptom by activity modification (rest, decreasing frequency and/or intensity of the activity), ice and, if needed, medications.
Then, a rehabilitation process must begin to gradually increase strength and flexibility of the involved extremity. It is often a mistake to focus just on the exact area of pain. Often, trunk and abdominal strengthening and flexibility exercises are needed as well. One needs to remember that the body works as a whole and rehabilitation must consider not just the "painful part."
Understanding the process of recovery is important to preventing recurrence and maintaining a healthy lifestyle of activity.