Low-Level Laser Therapy for Osgood-Schlatter or Sever Pain in Youth Athletes

The goal of this clinical trial is to learn if low-level laser therapy (also called photobiomodulation) works to treat knee or heel pain in physically active children and adolescents with Osgood-Schlatter disease or Sever disease. It will also learn about the safety of this treatment. The main questions it aims to answer are: 1. Does low-level laser therapy lower pain more than a sham (placebo) laser treatment? 2. Does low-level laser therapy improve daily and sport-related function more than a sham laser treatment? 3. What medical problems, if any, do participants have during the study? Researchers will compare active low-level laser therapy to a sham (placebo) laser treatment. The sham treatment looks and feels the same but does not deliver therapeutic light. This comparison will show whether the laser therapy works better than placebo. Participants will: * Complete screening and a baseline visit * Be randomly assigned to active laser therapy or sham laser therapy * Receive a series of treatment sessions over \[2 weeks\] * Answer short questionnaires about pain and function at baseline and follow-up visits * Have ultrasound imaging and/or provide blood samples for research measurements Participants will not know which treatment group they are in until the study ends.