Take your plank work to the next level with this difficult variation that targets your upper body and your core at the same time.
Here's how to do it: Start in a plank position, resting on your forearms. Press yourself up away from the ground one arm at a time into a pushup, while maintaining perfect plank position. Press first with your left forearm, keeping your right palm on the ground. Then return to prone position. Press with your right forearm while your left palm remains on the ground.
Ready to make it even more difficult? Lift a leg while you do it, extending your hip muscles and forcing your upper body to account for an even greater imbalance.