Photo credits: Top, James Michelfelder.
1. Get into a pushup position, bending your elbows 90 degrees and resting your weight on your forearms or planting your hands on the ground (as shown). Your elbows should be directly beneath your shoulders, and your body should form a straight line from your head to your feet. Rest your feet on Valslides or a towel (optional).
2. Squeeze your glutes and tighten your core. Then, push your body backward with your forearms as far as you can. Pull yourself back to the starting position and repeat. The farther back you push, the harder you hit your core.