"OK, so we all dread these," Lagree said, "but I promise you, they work." But in order for mountain climbers to work properly, you need to have good form. "To begin, position your body in plank. Pull your right knee into your chest. As the knee draws to the chest, pull your abs in even tighter to be sure your body doesn't stray from the plank position. Then, quickly switch and pull your left knee in." Keep repeating. Lagree recommended working up to three one-minute intervals with quick rests in between.
Again, form is key for these to work. "A common misconception is that mountain climbers have to be done fast. Rather, whatever your speed, be aware of your body positioning and core stabilizers," Lagree said.