I’m not very good at sports in general, but I feel that if I don’t have one I will have very unimpressive EC’s (I only have a bunch of clubs, afterschool martial arts and a job). I was rejected from tryouts in both 9th and 10th grade and I doubt I’ll be good enough to join a team this year, but how much would having a sport impact me? Can I pad it with something else?
Thanks for the help 
While I’m sure it depends on your school, I would think that in most cases picking one or two EC that you are really involved in (not just doing it to pad a list of EC) is what is important. Not necessarily sports. If sports is your thing and you show commitment and dedication to practices and time and keeping your grades up and even do volunteer work with your team that is good. But if you aren’t into sports and you are involved in music, or theater, or science, or tutoring, or something outside of school then that also shows commitment and dedication. I wouldn’t force yourself into sports just for the EC. Granted - my kids aren’t trying to get into super elite schools either so someone may override me.
Unless one is a recruited athlete, the impact is zero, zilch, nada compared to any other EC.
No, you should pursue EC;s which interest you. AO’s will see through any attempt to “pad.”