To do the Mountain Climber, set up in a high plank position with your hands under your shoulders and your feet together and legs out straight behind you. Your body should be in a nice straight line from your head to your heels. Brace your abs before you start so that you don’t feel this in your low back or let your hips sag as you run your knees in.
Then, maintaining the nice plank position, begin to run your knees in toward your chest. Bring your right knee in toward your chest with your left leg still out straight. Keep your butt down as you run the knee in.
Then straighten your right leg back out as you bring your left knee in toward your chest. “Run” your knees in as quickly as possible until all reps are complete. The faster you go, the harder the move will be.
Do not let your butt go up in the air or your hips sag as you run your knees in.