Conventional ab exercises incorporate mostly shortening (concentric) and lengthening (eccentric) of the abdominals, which is great for the aesthetic of the mid section but does not serve as well for the stability of the mid section. Exercises like sit ups, leg raises, crunches and side crunches incorporate the types of muscle contractions mentioned above. These exercises place emphasis on the outer abdominal wall like the abdominals and the internal and external obliques, in turn, not optimally training the deeper core muscles.
The deep core muscles include the transverse abdominus, spinal erectors (multifidus), pelvic floor and diaphragm. These muscles work together to create stability in the midsection/trunk. Trunk stability means that the body doesn’t need to compensate with other muscles to stabilize when performing certain movements, moving limbs or playing sports. These compensations can lead to tightness, niggles or injuries in the muscles that are doing the compensating and not the jobs they were intended to do. Deep core stabilization exercises train just that while also incorporating isometric contraction of the outer muscles like the abdominals, internal and external obliques, which is something that people tend to miss when training.
Deep stabilization exercises include; table top holds, dead bugs, planks, side planks, bird dogs, paloff holds/presses. A pre-requisite to these exercises is learning to use all the deep core muscles efficiently and in unison. Learning to breathe and use the diaphragm to create an outward pressure while maintaining the ribs and pelvis in line/stable is essential to grasp before starting these exercises. This is called intra abdominal pressure (IAP). If this is not learnt prior, it is more likely to use the abdominals and lower back muscles to compensate and stabilize for you rather than the intended muscles. An example of this would be a person performing a plank but with a lower back arch and an anterior pelvic tilt. This person would also eventually complain of a sore lower back. The correct way to perform a plank is to breathe into the midsection creating that outward pressure while maintaining the ribs and pelvis in line, then hop into the plank position while maintaining this stability, holding that position for as long as possible until form breaks.