This is a very common disease affecting athletes who use explosive strength (in volleyball, basketball, and athletics for example). This can be the result of an acute event due to a functional overload, or the result of repetitive microtraumas. Clinically, pain is noted at the inferior pole of the patella accompanied by painful swelling. The pain builds gradually, decreases after warming up but then gradually intensifies, limiting performance. 

There are 4 clear clinical stages to this condition:

Stage I: pain appears after training, activity is not limited
Stage II: pain is initially present, disappears after warming, then reappears again after that
Stage III: the pain persists for the whole duration of the play/ movement
Stage IV: tendon rupture

The diagnosis is usually only clinical, supported by ultrasound scans of the muscle-tendon to get an idea of the degree of tendinosis, or an MRI for the most severe cases. For all but the most severe cases (complete tendon rupture), conservative treatments are more commonly used. The course of treatment here is quite a delicate process, where success is based upon the severity of injury and the time taken to start after the onset of symptoms.