Are there any ab exercises or oblique exercises that can be done on a pull up bar mounted in a doorway? Thanks.
Yes,
Hanging leg raises (front, to the side, in a circular manner, weighted, etc…) learn how to do them with a full ROM if possible (keep legs completely straight and touch insteps to bar). This requires a much higher degree of mobility/flexibility and strength than just bringing the legs/or knees up to parallel with the floor.
Front lever (probably the most intense ab exercise I’ve yet to come across, though full ROM dragon flags and standing ab wheel come close), a gymnastics exercise where you basically hang from a chin-up bar and then (while keeping your arms completely straight) pull your body up until your body is parallel to the ground.
Once again, this is extremely difficult and will take quite a while to be able to achieve. If you’re just primary goal is to be a bodybuilder then it’s probably not worth the time and effort. But if you want to be able to do a skill that’s not only going to give you an extremely strong midsection but also cause people to accuse you of secretly being Spiderman then front levers are a great exercise.
Weighted chins (not an ab isolation exercise, but if you’re pulling heavy enough weight it will put stress on the abdominals).
What sentoguy has said are prob the best to do, i would recommend spliting the sets with another exercise, kind of like a superset. its up to you but i find i can get more out of my abs this way.
best of luck
L-Pullups: hold your legs parallel to the ground and do pullups.
http://media.crossfit.com/cf-video/L-pull-up.wmv
Here’s a video showing knees-to-elbows, windshield wipers, and some other stuff:
http://media.crossfit.com/cf-video/CrossFit_Knees2Elbows.wmv