Osgood-Schlatter disease may be a common explanation for knee pain in growing children and young teenagers. It's characterized by inflammation within the area slightly below the knee. This area is where the tendon from the kneecap attaches to the shinbone (tibia). The condition most frequently develops during growth spurts.During the expansion spurts of adolescence, certain muscles and tendons grow quickly and not always at an equivalent rate. With physical activity, differences within the size and strength of the quadriceps muscle can put more stress on the expansion plate near the highest of the shinbone. The expansion plate is weaker and more susceptible to injury than other parts of the bone. As a result, it can become irritated during physical stress and overuse. The irritation may result during a painful lump below the kneecap. This is often the most sign of Osgood-Schlatter disease.