You know plank works your core like no other bodyweight move,so try this variation to get it more challenging. This exercise will strengthen your core while also toning your arms and upper back. In a plank position, reach your left arm back and then circle it overhead, or bringing your palm back to the floor; repeat on the other side. This counts as one rep.
total five to eight reps,keeping the core steady and the belly button pulled up toward the spine.
The wider your feet are, the more steady you will be. Do not let your pelvis drift toward the ceiling. Related:
Your Arms and Abs Will Transform After This 2-Week Challenge
Source: popsugar.com