The condition is caused by inflammation and tension in the area below the knee, where the tendon from the kneecap attaches to the shinbone, or tibia. This inflammation causes painful swelling at the bump on the upper part of the tibia, called the tibial tubercle. The growth plate, soft cartilage near the ends of bones where new bone forms as children and teens grow, slowly widens because of the tension, which also contributes to the appearance of a tender bump.
Osgood-Schlatter disease often develops in children and teens who participate in sports such as gymnastics, basketball, and soccer that involve a lot of running, twisting, or jumping. These movements can increase stress on the tendon from the kneecap, called the patellar tendon. However, less active children may also experience this condition.