Inclusion Criteria:  Tenderness on palpation of the tibial tuberosity or pain during resisted isometric knee extensions Insidious onset of pain or swelling of the tibial tuberosity for ≥6 weeks Provoked by at least 2 of the following positions or activities; prolonged sitting or kneeling, squatting, running, hopping/jumping, stair walking or during multidirectional sports Clinical diagnosis of Osgood Schlatter Markedly reduced sports participation OR severely affected by pain during sports participation  Exclusion Criteria:  Other primary pathology or complaints from other structures of the knee Other injuries, complaints or illnesses that may cause disability, or specifically restricts levels of physical activity or sports participation Previous surgery in the lower extremities or lumbar spine Congenital deformities, device implants og cysts og tumors of the knee Participants not willing to cease concomitant treatment Participants and their parents not able to understand and communicate in written and verbal Danish