For the Mountain Climbers, start in a standard push up position with your hands and feet on the ground. From here, slightly elevate your hips and bring one knee as far forward to your chest as possible. Next, while returning the leg in front to its starting position, bring the opposite limb forward to your chest. Try to keep your hips as low as possible throughout the entire exercise. Repeat this motion for the allotted time and then rest.
This exercise focuses on the core. As your knees come in for each individual side, the abdominal muscles are being fired. The shoulder muscles are also being used to stabilize the body throughout the exercise.