If she has anything out of the ordinary it is probably hyperlaxity. Javasource has posted some info on this at times in the past, including some tests for it. I'll let you look into it rather than steer you wrong with my understanding. But at her age I wonder if the criteria even apply yet. Assuming this applies to her, it can lead to some problems for a pitcher, particularly with the shoulder. That certainly doesn't mean she shouldn't pursue it though.
If it’s Erlos-downlos ( not sure the spelling) (hyperlaxity) which is a ligament laxity disease then that would cause big problems. However, hyper-joint mobility won’t necessarily be an issue at all. There are major differences. One a disease the other more of a normal abnormality.
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