Understanding why stabilization exercises (squats, dead lifts, pull ups ) are so important means understanding a little bit about the abdominal muscles. Think about the abdominal structure as a girdle or wall that supports your internal organs and back. There are four main muscles: the rectus abdominis, the external obliques, the internal obliques and the transversus abdominis. When you perform crunches, you are primarily strengthening the rectus abdominis. This muscle is most notably a mover--flexor of the spine in a supine or face-up position--and is not designed for extensive stabilization. The external obliques, internal obliques, and transversus abdominis are three supporting layers of flat muscles that play a substantial role in torso stabilization. However, these muscles are typically neglected in abdominal training!! Research has proven that strengthening the deepest transversus abdominis muscle, along with the obliques and even the pelvic wall--should be our primary focus for developing core stability and ultimately a more developed mid section. Squats achieve this by forcing torso stabilization and a greater contraction in the abdominals than a floor crunch. As do dead lifts and other multi joint exercises. However you can't glamorize these exercises to the masses when trying to sell ab equipment or magazines.
If crunches help achieve a ripped-looking set of abs, why bother with the functional stabilization exercises at all? Posture improvement is one reason and injury prevention as we age is another. The abdominals' ability to stabilize the pelvis and back is crucial for minimizing injury. Many back injuries occur because we lift or move heavy objects without adequate stabilizing strength. With core stability, you are working from a strong base. Consequently, you have improved function, decreased risk of injury, strong defined ab's and potentially more effective rectus training. So the Squat does directly work your abdominals and like yourself many don't even know it. Don't take my word for it simply try this....assuming you train regularly do a set of squats....now do your normal set of crunches or whatever you do for ab's.You will quickly see you cant do your normal reps, you exhausted not pre exhausted your ab's. So not a big deal...until the next morning. Hey bottom line, It's hard work but that is why we do it.
Richard Seymour