External Oblique abdominal muscles are a pair of muscles that run along each side of the rectus abdominis. The muscle fibers run diagonally downward and inward from the lower ribs to the pelvis, forming the letter V and allow compression of the abdomen.
Internal Oblique abdominal 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 lower muscle fibers of the internal obliques run nearly horizontally and allow compression of the abdomen.
Depending on your level of fitness, you can modify the plank position to make it easier or more challenging. Mark Sisson describes four levels of mastery in the plank progression. I am at level 2, the forearm/knee plank. If you don't know how to do a plank then read how to do a proper plank.
To do the forearm/knee plank, assume the plank position. Tighten your abs using the isometric contraction technique and tighten your glutes. You should really feel your muscles contract. Then roll over onto your knees and forearm. Keep your elbow directly beneath your shoulder. Arch your body slightly in an arc toward the sky. Hold this position for 30 seconds to 2-1/2-minutes. This will work the obliques on both sides; one is pulling and the other is pushing. Then switch sides and repeat.
I follow Mark's set of plank - side plank - side plank. I do five of these sets at my current level of fitness, which is beginner. I follow up my planks with the Foot 2 Foot Crunch.
Foot 2 Foot Crunch
Passion4Profession states that completely isolating the lateral section of our abdomen is no easy task. However, the Foot 2 Foot Crunch exercise has been expressly formulated to stimulate the lateral muscles of the abdominal wall as much as possible. They claim that the Foot 2 Foot Crunch is one of the best exercises to train the oblique abdominal muscles.
The 3-D animation demonstrates how to do the Foot 2 Foot Crunch. Stretch out on the ground with your shoulders and head raised in a way that keeps your abdomen constantly tensed up. Again, this is where the isometric contraction technique will help you. Raise your trunk a little further and at the same time fold it over on one side until your hand can reach over to touch your foot. Return to the starting position and execute the same movement on the other side.
I do one set of 12 repetitions. Your level of fitness may allow you to do more or less. Always consult your health care provider when beginning a new exercise program.