You can do this exercise either at the gym or at home. If you prefer to do them at home, you may be more comfortable performing this exercise on a yoga mat, towel or any other soft surface you can put between you and the floor.

This exercise works your rectus abdominis muscles, which are the major muscle group in your core and the muscles responsible for the six-pack look. Mountain climbers also work your deltoids, which are your shoulder muscle; your gluteus maximus, which are your butt muscles, and your quadriceps, which are located on the front side of your thighs.c 

1. Start off in plank pose: Lie on your stomach on your mat. Raise yourself onto your elbows and feet so that your entire body resembles a inclined plank of wood. Contract your abs.
2. Bend your left knee and step up towards your chest.
3. As you return your left leg to the starting position, you should also bring your right leg up to your chest at the same time.
4. Keep switching legs for a total of 20 reps.

*It's important to make your movements as smooth as possible, meaning eliminate as much bounciness from the exercise as possible so your ab muscles get the full benefit.