The author treated a case of patellar tendon rupture due to sports activity and examined the specific cause of its injury. Sasaki has reported that this injury occurs when the moment acting on the patellar lower pole reaches a maximum. However our case ruptured before the moment reached a maximum. The patient had spontaneous pain for 4 years. We consider that the cause of tendon rupture is degenerative change like jumper's knee rather than action of the moment.
