Why can't Anne participate in gym?

From the text:

I didn't arrive home until five, since I went to gym with the rest of the class. (I'm not allowed to take part because my shoulders and hips tend to get dislocated.)

Anne was not allowed to take part in the gym class because her shoulders Tend to Discolate .The class played vollyball at the gym because it was anne's Birthday So She Got to decide what to play