Had Osgood-Schlatter after popping 7"in 6 months at 13 years old. Extremely tender and painful. That growth took all of the tendons and ligaments my knees and stretched them to the breaking point like rubber bands.
Dr. advised against competitive athletic activity for 6 months because any solid contact including falls or banging knees had the potential to cause multiple ruptures. Did a lot of swimming and no running during that time. Tendonitis on the other hand is generally soreness from overuse or stress. Not a lot of fun but in my view not as serious a condition a Osgood-Schlatter since truly serious injury is probably not in the equation unless the continued overuse or stress causes a breakdown.
I am not a Dr. and these are 40+year ago memories. No doubt medicine has changed and my experience might not match today's treatment.
I haven't talked about Osgood-Schlatter since the '80s - thanks for the trip down memory lane.