The plank is one of the best ab exercises you can do at home. It works ALL of the core muscles that run from the pelvis along the spine and up to the shoulder girdle, including: upper rectus abdominis (the upper abs), lower rectus abdominis (the lower abs), internal obliques and external obliques (side abs, along the sides of your body), transverse abdominis (your deepest core muscles). Planks are essential to building a strong core!