I'm 5'11'' 170 lbs., I have a beer belly, I'm trying to get rid of that and gain some upper body strength. My personal trainer has me doing a full body workout and I have to do some core body workouts. Do I have to do this? Or why can't I just lift weights and run cardio? Some of the core workouts I don't like are her leg exercies since she says she doesn't like those leg machines since it's bad for your knees which Im' sure it is, I've experienced that, but I don't like having to workout my legs with the exercies she showed me. I feel weird and uncomfortable doing those.