Athlexic
An athlete who continually thinks that he/she is too thin to play his/her sport. Note that the subject is usually convinced by his/her peers that the they are lacking in this area...and thus become athlexic. Similar to exercise bulimia, but instead of losing weight or maintaining weight...they want to increase weight.
I used to play rugby with Scott...we made fun of his skinny legs too much and now he is totally athlexic.