The most common reason causing knee pain with running is training errors and biomechanics errors. Let me explain
This is inclusive of volume and programming errors. Let’s start with volume of running. When training for a race or even just building up running as you get back into it, you want to pay attention to the volume you are running. Meaning the mileage you are running each run, the cumulative volume per week and per month. If you increase your volume too quickly, again looking at each run as well as your weekly volume, you are more likely to have pain.
Another training error is surface. I commonly see knee pain with running when people jump back into hill or stair running too quickly, or are doing it for too much volume that their body is not prepared for.
If you are experiencing knee pain with running, I suggest you audit both your runnign volume, and the terrain you are running on to see if you progressed too quickly.
When I am working with runners, the number one cause I see of knee pain is training errors. However, there can also be errors with a runners biomechanics. The most common biomechanic error I see with runners is overstriding. This means they are taking too large of a step, with their leg further from their center of mass when striking the ground. You will want to shorten your stride so when you strike the ground, your contact is closer to under your body. This does not mean you need to have a quicker pace. By increasing your cadence (number of steps per minute), you can continue to run at the same pace with shorter strides.
If you are having knee pain with running, look at your running volume, running surfaces, and your stride length.