What can I do about my kneecap hurting when I run?

Kneecap pain while running is often caused by tightness in the rectus femoris, which is the main thigh muscle, and your biceps femoris, which is the outside back of your thigh muscle (the outer hamstring). Tightness of these two muscles, increases the pressure on the kneecap (patella). There is usually also weakness in the inside thigh muscle (called your vastus medialis obliquus), and the upper outer hip muscle (gluteus medius). Weakness in these two muscles allow the thigh to move inward when the foot hits the ground. The combination of the thigh moving in (caused by muscle weakness), and the kneecap being compressed (muscle tightness), causes kneecap pain. To deal with this, here are the 4 main things that you can do:
1. Foam roll the outside of your thigh.
2. Stretch the thigh muscle (rectus femoris).  Try the kneeling hip flexor stretch and hold the stretch 30 seconds.
3. Stretch the outside thigh muscle (biceps femoris). Perform the 90-90 outer hamstring stretch and hold the stretched position for 30 seconds.
4. Activate the inside thigh muscle and the outside hip muscle (gluteus medius). Perform side lunges to balance, side to side tube walking and stability ball squats.
In addition, make sure you ice the knee when you are done running. If the pain persists, please contact your physician.