
Alongside the abs on each side are the obliques, the muscles that allow you to twist your torso. (Actually two sets, the internal and the external obliques.) Certainly these need to worked out as part of your routine. However, if you have love-handles, realize love handles are fat deposits between the obliques and the skin. No amount of firming of the obliques will affect them, except as a general part of burning calories caused by any exercising. (And there are much more strenuous exercises than sit ups or crunches.)
The fourth set of muscles are the transverse abdominus. These act like a corset in that by connecting with the spine and pelvis on all sides, they are the biggest stabilizer of the spine, allowing it to remain vertical. ANY time you go to move a section of your body, the transverse abdominus contracts first in order to stiffen the core. Even for tiny muscle movements like working your fingers, first the trans abs fire.
In popular nomenclature, working the trans abs is referred to as "scooping the belly", "sucking in your gut" or imagine trying to pull the belly button in to touch the backbone. You should do this EVERY time you do any exercise which will noticeably increase overall body strength.
The innermost layer is the transverse abdominals which simply wrap around body like a cummerbund, corset, or weight belt.
The internal obliques ("oblique" means running at an angle) are the next layer, the rectus abdominals ("rectus" means straight) next, and the external obliques are the last.
The fibers of the transverse abdominals are attached in a way that only lets them pull on the entire band so the only thing this does is to compress the abdominal area. "Only" is a relative term here as this muscle fires first before you can move anywhere else, stiffening the torso so the other muscles can pull on something solid. On the other hand, I don't know of any way to isolate them to work them, except the idea of "sucking in your gut", "pulling your belly button in try and touch your spine" or "scooping your gut". This should be done with EVERY exercise you do, or even when you just sitting here reading this.
Both sets of obliques let the torso twist as well as also stiffening the torso.
All three sets of muscles actually stop short of the middle of the abdominal area with the connection continuing with layers of connective tissue (called the "aponeuroses") The rectus abdominals run vertically through this otherwise muscle-void area, attaching the middle of the rib cage with the front of the pelvis. Contracting of this muscle allows a pelvis thrust (with its sexual overtones) and also draws the rib cage closer to the pelvis. As would be expected, these muscles are worked by "crunches" (named from the idea of trying to have the ribs crunch the pelvis and/or crunch the torso).
In terms of appearance, the rectus abdominals have a narrow band of connective tissue running up the dead center, called the "linear alba" (literally meaning "white line" from its appearance in an autopsy compared to the red muscles. And there are three thin straps of connecting tissue running crosswise, separating the rectus into four separate sections. It is this crosswise and vertical strapping that gives the sought-after "six-pack" (actually an "eight-pack").
There is also the quadratus lumborum which while located in the small of the back, the lumbar region, is really part of the torso area.
Internally, you have the "hip flexors" or psoas and iliacus, known jointly as the illiopsoas. "Psoas" comes from the Greek meaning loin (and iliacus from the Latin word for loin). When you sit up, it is these muscles that pull the pelvis toward the stomach. These are NOT the muscles you want to work when doing crunches.
Few people want to bulk up their ab muscles, but do want to make them clearly defined, the so-called "6-pack". I used to think this meant more reps, 12-20, instead of the standard 8-12. However, doing hundreds of reps and/or working them every day is NOT suggested, as these muscles are just like every other muscle.
Now I think that because of the bands of connective tissue across the rectus abdominals, if you bulk these muscles up (8-12 reps with heavy weights), you WILL get a defined ab area (assuming you don't have it covered with a layer of fat). However, you still can have a bulging stomach, which you might see on some heavily bulked up body builders. I think it is the strengthening of the transverse abs that will tighten and slim the waist.
The crunch is considered the "gold-standard" of all abdominal exercises. A crunch is basically the first part of a sit-up, when you get your shoulders off the ground. Past this point, you are just working the hip flexors. The top of the pelvis needs to be tipped toward the back ("posterior pelvic tilt") in order the work the rectus abdominus the most, which is a reason to keep the knees raised or flexed. This also decreases any stress on the lumbar region of the spine (lower back).
There is also the reverse crunch, where the legs are raised instead of the upper body. This can be done on a slant board with the legs raised perpendicular to the torso, and then they are brought up as if you were curling into a ball with the knees up near your face.
The Roman chair is popular, but some consider it not as effective as a combination of crunches and reverse crunches and also can lead to lower back trauma if you bend too far back.
Side bends are considered the most effective for working the obliques. (Despite popular misconception, working the obliques too much does NOT thicken the waist but will actually tighten the area, giving a thinner waist.) Side bends also work the quadratus lumborum, which helps prevent injuries to the lumbar spine of the lower back.
Trunk twists involve sitting on a bench with a bar across the back of the shoulders, twisting the torso as far in one direction and then back in the other. The muscles are worked when they have to stop and reverse the direction, a ballistic movement. (The bar holds the arms out to increases the torque required. You could just hold your arms out but they would probably get tired before your really worked the obliques.
The rotary torso machine duplicates this motion, but puts lot more resistance on the obliques.
Lots of times, people come up in a crunch and twist one way on one rep, and twist the other, to work the obliques much more than a simple crunch.
A crunch can be done on the floor, a bench, on a swiss ball, etc. The pelvis must be tilted posterior to really work the rectus abdominals, and for this reason, most crunches are done with the knees raises. This also cuts down stress on the lumbar region of the spine (the lower back).
The hands can be placed on the chest, flat on the ground or the most common, behind the neck. For the last, you don't want to use your hands to pull on the neck as this cuts down the intensity of the resistance. In fact, the further the hands are from your waist, the more intense the exercise. You could even hold a weight behind your neck to increase the exercise, but the temptation is to use to much weight and start using poor form.
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