209 Danish professional Super League soccer players (12 clubs) were randomized to either undergo prophylactic eccentric training and stretching or to the control group, who did not perform any eccentric training, to compare efficacy in reducing the asymptomatic ultrasonographic changes in the patellar and Achilles tendons. The results indicated that eccentric training resulted in a reduced frequency of ultrasonographic abnormalities in patellar tendons, but not Achilles tendons. The asymptomatic players undergoing eccentric training with abnormal patellar tendons, identified by ultrasound, were at an increased risk of becoming symptomatic.