Low plank position.

The low plank position, also known as the elbow plank position, is obtained by bending the elbows  and placing the arms perpendicular to the floor, while supporting the opposite side with the tip of the toes(or metatarsals), maintaining the back and legs straight. It is a challenge after ~2 minutes if one is fit.

Of course the biceps triceps and quadriceps are worked, but the core, especially the transversus abdominis is the most benefited from this exercise. My record is 4:30 minutes but it keeps increasing as I continue gathering sessions. One series every other day can help one improve the sitting posture, let alone the abs and arms workout.