30 DAY PLANK CHALLENGE - FROM 20 SEC TO 5 MINS! This challenge will help you to hold the plank pose from 20 seconds to 5 minutes in 30 days! Plank exercises are a popular element of isometric training, which involves contracting your muscles against stationary resistance. It will work you upper abs, your lower abs, your obliques and your lower back. You will feel your core getting stronger. It is hard, it works your entire body, but the results are noticeable!