In a study done by ASCM fellow and professor, Michele Olson, the effectiveness of muscular activation of popular abdominal exercises were tested compared to the standard abdominal “crunch.”

    The study utilized EMG (electromyography) to test the muscular activity in the abdominal muscles by measuring the electrical energy that triggers muscular contraction. The greater the intensity of the signal, the greater the degree of muscular activity. They used the EMG to compare the standard crunch to popular Yoga and Pilates exercises.
    Earlier research performed at the beginning of the ab “craze” demonstrated that the abdominal crunch was more spine-friendly compared to exercises, such as the sit-up, that require full spinal flexion. Also, findings showed that utilizing a stability ball, or Swiss ball, resulted in increased intensity compared to a regular crunch. In fact nearly a  40% increase in muscle output was found when utilizing a stability ball during normal crunches.
    Eventually as the fitness world gained interest in isolating muscles, the “bicycle” was found to be the best exercise for activation of the external obliques and “hip-ups” (or reverses as we call them) were best at activating distal (lower) portions of the abs. “Drawing in” the abs was also shown to stabilize the spine and decrease low back pain during ab exercise. (Drawing in refers to the same movement we promote when we ask you to “pull your belly button to your spine.”)
    More recent research aimed at finding the most effective and safe abdominal exercises, particularly those that engage the transverse abdominis and external obliques, found the following data regarding specific abdominal muscle activation compared to the standard abdominal crunch (standard value of 100%):

ExerciseRectus AbdominisExternal ObliquesDeep 
Yoga Boat111%154%
Yoga Dolphin Plank on Ball105%147%
Yoga Side Plank105%139%
*Standard Crunch*100%100%100%
Bird Dog (Quadrapeds)80%113%
Dynamic Side Bridge79%128%
Isometric Side Bridge59%122%
Pilates Double-Leg Stretch29%139%
     Overall, the data suggests that exercises requiring little to no spinal flexion (bending/crunching motion) and isometric contraction (no change in the length of the muscle) increased deep muscle response (transverse abdominis).
     Also, static exercises such as the planking poses that require the abdomen to behave as a “brace” are less likely to aggravate conditions such as low back pain because they require little to no flexion of the spine. (Bracing refers to the stiffening of the abdominal area, as when one braces to absorb a punch to the stomach).
    Therefore, if you are looking for a way to bump up the intensity of your abdominal exercises or are simply looking for something new, try adding a stability ball for your crunches, holding a plank or two, or try some new Pilates movements, like the Bird Dog or Double-Leg Stretch. These exercises are sure to make you “feel the burn” and will challenge your core musculature at a whole new level.
    As always correct execution of these exercises is necessary to prevent injury and maximize effectiveness and supervision of a certified and experienced trainer is recommended when beginning a new program. These exercises may not be suitable for all, and may need to be modified for individuals with health concerns or existing conditions. Always check with your physician before beginning a new exercise program. Attempt of these exercises is at your own risk. 
Olson, Michele. “The Anatomy of Investigating Abdominal Exercises.” ACSM’s Health and Ftiness Journal. Vol 17, No. 4. July/August 2013.