I look at the ab core from an everyday perspective, and their overall purpose within our body.
They serve an important support/stabilizing function in various movements (including some turning/twisting), and have a high capacity for endurance due to the nature of the specific job functions.
They serve as a protector (and holding our vital organs--something like a belt, and they are very close to these organs, and is one reason it takes low body fat to reveal them).
Rectus abdominis
The rectus abdominis runs from your sternum to your pelvis and essentially helps pull your rib cage and your pelvis closer together.
The rectus abdominis is the actual visible “2-pack, 4-pack, six-pack” (as many call them) that you see in someone with a well-developed core and a low body fat percentage.
The transversus abdominis
The transversus abdominis acts as a natural weight belt, holding your insides in (thus displaying one of the reasons body fat has to be low to see the core), and act in stabilizing the trunk.
Attempting to develop the transversus abdominis helps "pull in your stomach area" giving you the appearance of a smaller waist.
One popular misconception is that people think that the upper abs and lower abs can be worked separately.
The fact is that you cannot isolate the upper or lower core. The rectus abdominus is one muscle group (or
one sheet of muscle) and the entire length of the muscle group is activated whether you’re "pulling the
upper body up" or "pulling the
lower body up (such as the legs)".
With that said, IMO, it is beneficial to work the core from a "variety of different angles" with respect to the muscle (s) that encompass them, to attempt to recruit max muscle fiber development throughout the "entire abdominal region" (including obliques).
The internal and external obliques
The internal and external obliques work to rotate the torso and stabilize the abdomen (turning/twisting type of movements,etc).
There are some lateral abdominal muscles and their role is to support the spine and maintaining a healthy lower back--which do not got too much attention.
When working the core (especially or "mainly" if you will, the Rectus abdominis), we can not leave out the role of the Hip Flexors.
Small but very powerfully strong that tend to engage (in varying degrees dependent on range of motion/execution of movement) anytime you’re "pulling the
upper body up" or "pulling the
lower body up (
such as with the legs)".
Hip Flexors. While having strong hip flexors are important (and believe me when I tell you these muscles can get strong), attempting to minimize the engagement or involvement (as much as possible) when during core exercises is important
so they will not "tend" to take over most of the stress and remove training stress from the core.
Why are the hip flexors important?
Its because the hip flexor muscles such as the (psoas), along with the core, act to pull your trunk towards your legs, and at the same time. However, the Psoas has a greater range of motion.
For example:
The psoas "can" get involved to the highest degree when your feet are supported and/or your legs are extended straight........such as in the traditional situp.
The psoas can take over the majority of the work. This is not saying th core doesnt get involved (it does), but at a certain point the Psoas takes over, when your upper body comes off the floor
by more than approximately 30° in crunching or sit-up movements. This is why I support 30 degree situps if one elects to do them.
Try an
un-anchored 30-degree sit-up, and you have a whole different world (as an example) as compared to an anchored FULL sit-up.
IMO, you can not "completely" remove the involvement of the hip flexor, but one should attempt to eliminate it as much as possible.....
while attempting to put more stress on the muscle being worked.
Having strong hip flexors are important (say for sprinters, as an example) so, don't get me wrong, but....
having an understanding of their involvement and how to minimize their recruitment will benefit your core training;
you can get more isolation. But, understand that eliminating them "completely" due to their design function is virtually impossible.
One of the reasons that many people who spend an hour during each workout doing hundreds of crunches/sit-ups (etc) fail to ever develop six pack abs is because the burn very little calories, and in the overall scheme of things, mean so little
when a properly setup fitness routine is included in the discussion. Additionally, some fail to adjust their routine to a more fat burning machine or at least metabolically driven--when fat loss is the focus point.
Of course one's bodily position has to be taken into account,
but my opinion is that the majority of one's time in the gym should be spent on the bigger push/pull compound movements (and in some cases where they are metabolically-driven to burn "more" calories) where your body is forced to work progressively-harder and burn more calories during and after the workout.
For Example:
One of my favorite exercises has always been the Renegade Row. It works a variety of muscles at one time (including the core), it will make you work, work hard, breath hard, and expend a lot of calories. It can sometimes make you wish for death, that will not come. Which is what we want.

It is what I term as Metabolically-driven type of exercise, and I love it for it.
Moving on.
IMO, you train the core like any other muscle group. This means not everyday. Some may differ in this opinion, but this is mine. Muscles develop during rest, not being "bombed" repeatedly without rest and recuperation. If there were a muscle group, that could take a bombing/exercising everyday, it would be the core though, so I regress a tad, but still, I believe they shouldn't be exercised each day (
directly, anyway).
Some direct exercises to consider:
- Hanging leg raises (with hunched back)
- Hanging knee raises (with hunched back)
- Lying leg thrusts (hip thrusts)
- Decline bench leg thrusts (hip thrusts)
- Reverse crunches (crunching hips off floor)
- Ab bicycles (alternating knees to elbows)
- Ab scissors
- Stability ball crunches (weighted for progression)
- Bench crunches
- Alternating (oblique) crunches
- Weighted cable rope crunches (with hunched back)
- Ab wheel
- Stability ball hip flexion (knee tucks)
- Abdominal vacuums (transversus abdominis development)
- "No anchor" Weighted sit-ups (30 degree only) Advanced method: Adding in "peel offs"
And my absolute favorite and love: The Renegade Row.
Other "indirect favorites":
Front Squats: There are some minor variants on how to hold the BB, but when the bar is placed on the shoulders in the front of the body (as compared to behind the head as in the back squat),
this position tends to place much more stress on the core and demands that the core provide stability and support while its performed.
And, more so, than the back squat, IMO. It also tends to take stress off of the lower back. I prefer @SS-TO-THE-GROUND when performing them.
I want to mention that, virtually any squat variants, will get the core involved. So, if you do a program that does not include the Front Squat, do it with confidence that it will recruit the core satisfactorily. I just prefer the Front Squat, because it causes more recruitment in my opinion.
And, again I mention the fabulous: Renegade Row (upper body work with amazing oblique and core stability work). It kicks ass, and has been in my workout for 2 years now, and I am not about to separate myself from this beast.
As far as indirect core work, of course there are others, I am just mentioning two here.
Best wishes,
Chillen