Mountain Climber

How to do it

1. 1

   Start in a push-up position — hands under your shoulders, body in a straight line, arms straight.

2. 2

   Drive your right knee toward your chest, then quickly switch — bring it back as you drive your left knee in.

3. 3

   Keep alternating legs at a steady pace — the faster you go, the harder it gets.

4. 4

   Keep your hips level and your core tight throughout — breathe steadily and don't hold your breath.

Where you should feel it: Your stomach, shoulders and the fronts of your hips.