Unlike crunches, sit-ups, cable woodchoppers, ab v holds, hanging leg raises and the like, which primarily work the rectus abdominis (the vertical “six-pack” muscles) and obliques, planks blast the transverse abdominis.
The transverse abdominis (TA) is a deep, sheath of muscle fibers that forms the innermost wall of the abs; in isolation, it’s physically invisible to the eye.
It’s responsible for one thing — abdominal compression. In a physiological sense, that means coughing, peeing, vomiting, childbirth and other savory bodily functions.
For aesthetic purposes, a strong TA won’t produce the abdominal crevices, caverns, and undulations characteristic of a six-pack, per se, but rather it sucks the midsection into a tight, compact, slender package. It’s essentially the body’s corset — a strong TA naturally compresses the girth of the stomach.
A strong, iron-clad TA not only produces powerful suction and a pancake-flat core, but it also helps decrease lower back pain, preserves posture, and amplifies power and stabilization on standing-based lifts (think military press, squats, deadlifts, etc.). Ab planks are the most effective, most direct way to tear up the TA.
If you’re someone who DOESN’T like the “six-pack” look and the block-esque abs that go with it, and simply want a flat, defined stomach, the TA absolutely should be your primary point of emphasis.
Here’s a great video demonstrating how to master the plank: