It seems everyone who exercises is looking for the best ab-exercise routine for developing flat, tight abdominal muscles. Every year there are dozens of a new exercises, fitness classes, products, gadgets or routines claiming to sculpt and strengthen the abdominal muscles like none other. And while some of these may offer a new approach to working the abs, many are ineffective and may increase your risk of injury.

To avoid falling victim to unproven and misleading abdominal exercise claims, it's important to have an understanding about the function of your abs, including where they are and what they do and how they can be exercised with the least risk of injury.

First, let's look at each of the abdominal muscles groups. The most well-known and prominent abdominal muscle is the rectus abdominis. It is the long, flat muscle that extends vertically between the pubis and the fifth, sixth, and seventh ribs.

A strong tendinous sheath called the "linea alba," or white line, divides the rectus abdominis down the middle, and three more horizontal tendinous sheaths give the muscle its familiar "washboard" or "six pack" look in very fit athletes.

The rectus abdominis helps to flex the spinal column, narrowing the space between the pelvis and the ribs. It is also active during side bending motions and helps stabilize the trunk during movements involving the extremities and the head.

The next group of muscles that make up the abdominals are the external oblique muscles. This pair of muscle is located on each side of the rectus abdominis. The muscle fibers of the external obliques run diagonally downward and inward from the lower ribs to the pelvis, forming the letter V. You can locate them by putting your hands in your coat pocket.

The external obliques originate at the fifth to twelfth ribs and insert into the iliac crest, the inguinal ligament, and the linea alba of the rectus abdominis. The external oblique muscles allow flexation of the spine, rotation of the torso, sideways bending and compression of the abdomen.

The internal oblique muscles are a pair of deep muscles that are just below the external oblique muscles. The internal and external obliques are at right angles to each other. The internal obliques attach from the lower three ribs to the linea Alba and from the inguinal ligament to the iliac crest and then to the lower back (erector spinae). The lower muscle fibers of the internal obliques run nearly horizontally. Along with the external obliques, the internal obliques are involved in flexing the spinal column, sideways bending, trunk rotation and compressing the abdomen.

Because of their unique alignment, at right angles to each other, the internal and external obliques are referred to as opposite-side rotators. When the trunk rotates left, the external obliques (on the right) contract. When the trunk rotates to the right, the external oblique fibers (on the left) activate the movement.

The deepest layer of abdominal muscles is called the "transversus abdominis." The transverse abdominal muscle wraps around the torso from front to back and from the ribs to the pelvis. The muscle fibers of the transversus abdominis run horizontally, similar to a corset or a weight belt. This muscle doesn't help move the spine or the pelvis, but it does help with respiration and breathing. This muscle helps facilitate forceful expiration of air from the lungs, stabilizes the spine and helps compress the internal organs.

The hip flexors are a group of muscles that bring the legs and trunk together in a flexion movement. The hip flexors are not technically abdominal muscles, but they do facilitate movements during several ab exercises.

The muscles that make up the hip flexors include: psoas major, illiacus, rectus femoris, pectineus, sartorius.

Many of the exercises being promoted as "ab exercises" actually work the hip flexors more than the abs. The hip flexors are strong powerful muscles that often overtake the abdominal muscles when performing some variations of abdominal exercises. In order to isolate the abdominals you need to minimize the involvement of the hip flexors and maximize the contraction of the abdominals.

One example of an ab exercise that actually focuses on the hip flexors includes the full sit-up exercise, especially when the feet are held down. This movement primarily involves the hip flexors and may cause the lower back to arch. This could increase the risk of back pain, particularly if you have weak abdominal muscles. Therefore, the full sit up is not recommended for beginners. Another example of an ab exercise that works the hip flexors is any leg-raising exercise done in a supine (laying face up) position. Again, this movement works the hip flexors far more than the abs and shouldn't be done until you have good abdominal strength.

Keep in mind that the best way to isolate the abs is by minimizing the involvement of the hip flexors while doing your ab workout. Now that you have a basic understanding of what the abdominal muscles are and how they work, you can design workouts that actually target these muscles.