The Ab Plank, in which you perform a static hold on your forearms from a push-up position, engages your anterior core muscles, specifically the abdominals and hip flexors, in order to maintain your spinal alignment and prevent your lower back from over-arching and your hips from sagging toward the floor.
In the Performance U approach to training, if you can do a basic Ab Plank for 30 seconds, it's time to stop boring yourself and progress to advanced planks.
Every time you do Push-Ups, you're already doing basic Ab Planks. A Push-Up is an abdominal plank in terms of body position and core-muscle activation, because your torso is in the exact same position in both exercises. The only difference is that in the Push-Up, you're also involving your chest, shoulders and triceps, giving yourself more muscle activation, which makes them more beneficial and productive.
If you're already doing Push-Ups in your workout, it's unnecessary to perform basic Planks.
Try these more advanced plank variations.
Long-Lever Posterior Tilt Plank (LLPTP)
The LLPTP provides more intense training for your core musculature than the basic Plank.
To perform it, assume the plank position with your elbows spaced approximately 6 inches apart. Contract your glutes and rotate your pelvis by bringing your front hip bones toward your head and your tailbone toward your feet.
Imagine your pelvis as a bucket of water. A posterior pelvic tilt would tip the bucket to spill water out of your back, whereas an anterior pelvic tilt would spill the water from the front of your body.
The One-Arm Plank adds an element of rotation, because gravity pulls your unsupported side toward the ground. This creates a different loading stimulus than the two-armed Plank.
Begin in a push-up position with your hands and feet shoulder-width apart. Lift one arm off the ground without allowing your shoulders or hips to rotate, or your head or belly to sag toward the floor. Pause for five to 10 seconds before switching hands.
The 321 Plank combines the LLPTP with the One-Arm Plank in a series. Here's how it's done:
Note: In the video, my hips were a bit higher during the LLPTP than I coach my athletes. Ideally, your hips are in a straight line with your shoulders and ankles.
Schoenfeld and Contreras. "Exercise Technique: The Long-Lever Posterior-Tilt Plank." Published Ahead-of-Print. http://www.lookgreatnaked.com/articles/long_lever_posterior_tilt_plank.pdf