Mountain Climber with Pushup

Start: Assume a pushup position with your hands slightly wider than your shoulders. Your body should form a straight line from your ankles to your head. Brace your abdominals—as if you were about to be punched in the gut—and maintain that contraction for the duration of this exercise.

Movement: Without allowing your lower-back posture to change, lift your right foot off the floor and slowly raise your right knee toward your chest. Return to the starting position, and repeat with your left leg. Then lower your body until your chest nearly touches the floor. Pause at the bottom, and then push yourself back to the starting position as quickly as possible. That’s 1 rep. Continue to alternate between mountain climbers and pushups until you’ve completed the prescribed number of reps.  

Want an even bigger challenge? Place your hands on a Swiss ball or a medicine ball when performing this move. The instability of the ball forces your core muscles to work harder than when you do this move on the floor, increasing the difficulty of the exercise.