This is not a valid reason to test muscle tissue. The healthy girl should decide for herself whether to go out for sports, and the coach at the school will test her ability at that sport compared with that of other girls to determine who makes the team.
I don't see anything wrong wtih testing out muscle tissue if there are no health concerns associated with it. But if the results show that the girl has the potential and the girl has no interest in sports what so ever, then it should ultimately still be the girls choice whether she wants to try out or not. and if she already told the parents beforehand that she did not like sports, then they shouldn't have her preform the tests. In another scenario, if the girl does want to try out for sports and preforms the tests but the results show that her potential in the sports arena is extremely poor, she should still have the option to pursue whatever she wants regardless of what her parents thinks. Just my two cents. I'm not sure if I addressed your question but I hope I did :)