The internal oblique is located directly underneath the external oblique. Its fibers run at nearly right angles to the external oblique fibers in the middle and upper portions of the abdominal wall and form an inverted V shape in the lower abdomen. However, the internal oblique fibers run almost horizontal. The internal oblique muscle runs from the lumbar fascia, anterior crest of the ilium, and the outer half of the inguinal ligament in the lower abdomen and attaches to the cartilages of the eighth, ninth and tenth ribs and linea alba.

In the hip joint, the major muscles involved are the psoas, iliacus, rectus femoris, and pectineus. They should remain under isometric contraction throughout the exercise.

Execution

Sit, straddling on exercise bench. Place the buttocks fairly close to the end of the seat with your hands 6 - 12 inches behind the buttocks while grasping the sides of the bench. Incline the trunk backward at a 45 - 60 degree angle and place most of your weight on the arms. Hold the legs out almost level with the floor.

When ready, inhale slightly more than usual and then hold your breath as you bend the legs and bring your knees toward the chest. When the knees are 6 - 12 inches in front of the chest, straighten the legs, return to the initial position and exhale. Once the legs are straight, repeat for the necessary repetitions. As you bring the knees up and down, be sure to keep the trunk stationary.

Muscle and Joint Actions

In the seated knee up, the lower abdominals contract concentrically to rotate the pelvis backward to allow the knees to approach the chest. The upper abdominals remain under isometric contraction to maintain trunk position and also come into play dynamically when the knees are brought very close to the chest.

To make knee ups more difficult and produce even greater development, bring the knees as close as possible to the chest. To do this you will have to maximally contract and shorten the abdominal muscles to rotate the pelvis back further.

Also, you can perform this exercise more rapidly, but make sure you go through a full range of motion.

Keep the trunk in place. If the trunk moves up as the knees come in, you will be using the hip flexors instead of isolating the abdominals.

Since hip joint flexion is also involved (to hold the legs in position), there is less danger of injury to the spine than there is in straight leg abdominal exercises. This is because your pelvic girdle is rotated posteriorally - giving you a slightly rounded back that automatically takes the pressure off the spine due to the hip flexor pulling. If you feel any discomfort in the spine, low back or the tail bone, you can relieve the pressure by using the Tush Cush: an angled cushion with an opening in the middle for the tail bone.

The Tush Cush also makes other exercises like sit ups safer and more effective.

Lean back only to a 45 - 60 degree angle. Any more than this, or lying down on the bench, can cause hyperextension or place excessive stress on the back. It's important that you always keep your arms straight and allow your upper body to be supported by the arms. With the upper body stabilized, bringing the legs in and out becomes easier and more effective for abdominal development. It also greatly diminishes the risk of injury.

Another variation of the knee ups is the leg ups. This is a more advanced exercise in which you straighten the legs and then raise and lower them. But to effectively do leg ups, you need great levels of abdominal strength and flexibility in the hips and lower back.

An even more difficult option is figure eights. To perform these, hold the legs above the level of the bench and arrange your feet into a figure eight, horizontally not vertically.

If you have difficulty doing knee ups, try doing them in your bare feet. With less resistance on your feet, this exercise becomes much easier. Then, when you can easily do 15 - 20 repetitions, put your shoes on and try again.