d5aa8617_University_of_Chicago_Medicine__Possible_cause
[Type of overuse injury] Sinding-Larsen-Johansson disease [Symptoms] Knee pain, especially after jumping activities [Possible cause] 
This disease is caused by a fracture of the kneecap due to repetitive extension on the patellar tendon in the knee (the tendon pulls away from the bone).
Overuse Injuries - The University of Chicago Medicine
Possible cause