OK, now pictures or English please. Seriously, I'm really interested to know but that just flew right over my head.
Maitland gave a good general explanation of Osgood-Schlatter disease, and she pointed the way to a nice multi-part discussion at the
Drills and Skills site:
An in-depth look at the pathophysiology and treatment of Osgood-Schlatter Disease - Part A
This illustration shows the location of the painful lesion, generally two or three finger-widths below the bottom of the knee cap:
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These two links include discussion of Osgood-Schlatter disease, along with some illustrations:
Google Image Result for http://www.hopkinsortho.org/orthopedicsurgery/images/Image14.gif
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Since Osgood-Schlatter disease resolves as kids mature, you might wonder if there's any point to therapy other than pain relief.
One reason is simply cosmetic:
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However, it turns out that adults who had Osgood-Schlatter disease as adolescents score lower than unaffected people on measures related to knee pain-related aspects of daily living and continued sports activity; a long-term follow-up study suggests that 1/4 of adults who [soapbox mode on]
did not reduce their activity level and seek appropriate therapy [soapbox mode off] for Osgood-Schlatter disease as adolescents had some limitation in activities.
About 1/20 of children and 1/4 of athletic kids get Osgood-Schlatter disease, although it seems that it could commonly be confused with other knee problems. I don't know how most gymnasts who suffer from this problem are treated; I have the impression that it's common to use NSAIDs and knee straps to allow the children to continue to train.
Since she was diagnosed with Osgood Schlatter disease, my twelve year-old has taken five weeks off from anything that hurts her in gymnastics: that includes most of everything except some beam work and uneven bars (excluding her dismount). She's working with a therapist who expects her to return to full activity within a few weeks. In the mean time, she's been adding skills on bars, including a new release and an eagle-grip Endo that seems to the untrained eye to be certain to lead to long-term shoulder issues

. I hope that we're not trading one problem for another. . . .