You know plank works your core [1] like no other bodyweight move, so try this variation to make 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, bringing your palm back to the floor; repeat on the other side. This counts as one rep.
• Complete five to eight reps, keeping the core stable and the belly button pulled up toward the spine.
• The wider your feet are, the more stable you will be. Do not let your pelvis drift toward the ceiling.