Mountain climbers are a great total body exercise… and you are gonna alternate legs. The faster you go the more that heart rate is gonna come up. Again keeping the arms over or the shoulder over your wrist is were key to being able to maintain the plank. So you are looking not for your hips to be in the air, but for your hips to be low and your quad contracted. And then just moving in and out; nice and quickly. Getting that action in the legs, getting the action in the arms. Just a great total body exercise.