A recent research used ultrasound to measure changes in abdominal muscle tone when test subjects (who were mainly women) performed six abdominal exercises. The side plank (or side-bridge) and the abdominal crunch both resulted in the greatest increase in muscle tone in the transverse abdominis and internal oblique muscles of the women.
Based on the findings, the research concluded that side-bridges and crunches contract the muscles more than any other abs exercises. So leave the rest alone and follow these 9 variations of the side plank and crunch that will get your abs rocking hard. Be warned though, these exercises will make your abs sore for days (but the good kind of sore):