3. Elbow and Knees Alternating
This exercise is usually done when you lay down while moving alternating knee and elbow. It can also be done easily while sitting on a chair. Sit straight with your hands around your head or on the sides of your head and then start twisting your body by lifting your leg and bringing left elbow to your right knee. Hold in the position for a second, then relax your body and then do the same with your other elbow and leg. Keep doing the same movement in a continuous motion.
4. Seated Bicycle Pedal
Although this one is easier said than done, I personally feel this among the most effective sitting ab exercises. Sit on the edge of your chair and give a support to your upper body on the armrests of the chair. Then put your legs in a position as if you are pedaling on a bicycle by bringing the knees near your chest and keeping your abs muscle contracted. Start moving your feet like a pedal and repeat it in a continuous motion. I think this is one of the most beneficial ab exercises to do at your desk working your entire core.