Hanging Knee Raises: Grab the chin-up bar using both hands with a grip a bit wider than your shoulders,
   cross your ankles and bring your knees up to your chest or as close as you can get.
   Pause at the top of the movement for a second and then slowly lower
   your knees by relaxing your abdominal.
   Don't lower your legs all the way. Repeat the movement using just your abdominal to raise your knees.
   Make sure that you don't start swinging. You want your abdominal to do the work, not momentum.
   Make sure your pelvis moves, your lower back stays neutral or slightly rounded, not arched,
   and that your abdominal are doing the work, not your hips.



   Hanging Leg Raises: Just like knee raises except you keep your legs straight.
   This requires good hamstring and lower back flexibility.
   Make sure your lower back stays neutral or rounded.
   There is an isometric variant done by gymnasts called the "L-Support", which basically consists
   of taking the leg raise position with the legs held straight at a level just above the hips.
   The position is held for 10 seconds. When you can complete this easily, try a higher position.

   Pulldown Crunches: Drape a towel or rope around the bar of a pulldown machine
   so that you pull the weight using it instead of the bar. Kneel facing the machine and grab hold
   of the towel and put your hands against your forehead.
   Kneel far enough away from the machine so that the cable comes down at a slight angle.
   The exercise is the same movement as an abdominal crunch, but using the weight instead of gravity.
   The emphasis is still on crunching the abdominal, pulling the breastbone towards the pelvis
   and making sure you exhale all your air at each contraction.