OBJECTIVES:The aim of this study was to investigate the effects of a 4-wk in-season exercise program of isometric or isotonic exercises on tendon structure and dimensions as quantified by ultrasound tissue characterization (UTC). DESIGN:This was a randomized clinical trial. Volleyball and basketball players (16-31 yrs, n = 29) with clinically diagnosed patellar tendinopathy were randomized to a 4-wk isometric or isotonic exercise program. The programs were designed to decrease patellar tendon pain. A baseline and 4-wk UTC scan was used to evaluate change in tendon structure. RESULTS:No significant change in tendon structure or dimensions on UTC was detected after the exercise program despite patellar tendinopathy symptoms improving. The percentage and mean cross-sectional area of aligned fibrillar structure (echo types I + II) (Z = -0.414, P = 0.679) as well as disorganized structure (echo types III + IV) (Z = -0.370, P = 0.711) did not change over the 4-wk exercise program. Change in tendon structure and dimensions on UTC did not differ significantly between the groups. CONCLUSION:Structural properties and dimensions of the patellar tendon on UTC did not change after a 4-wk isometric or isotonic exercise program for athletes with patellar tendinopathy in-season, despite an improvement in symptoms. It seems that structural improvements are not required for a positive clinical outcome.