Plank is one of the most effective total-body moves,so here's a unusual challenge for you to conquer - a five-minute plank. Since holding a straight-up plank wouldn't be any fun (and could end up causing an injury), this challenge mixes up five basic variations to not only tone your abs and arms, or but to target the obliques,legs, and upper back as well. Here's an explanation of how to execute each one, or followed by the two-week plan.
Source: popsugar.com