i have just two for now. do you know any good core exercises besides sit-ups and push-ups. i already do these ones and im looking for some variations to push myself harder. i would also like some information on some good circuit training that builds endurance and strength. keep in mind that the only equipment that i own is an elliptical machine and a smith machine with benchpress/squat, curls, tricep workouts, and an 8 pound medicine ball.