The plank performed on the knees is a static (isometric) exercise for strengthening the abdominals, back and shoulders. Isometric strength is important for stabilizing the trunk in various sports and exercises. The abdominal plank is a great exercise for strengthening the transverse abdominus. The transverse abdominus is the deepest layer of abdominal muscle and wraps around the whole midsection. When you contract the abdominals, your waist becomes thinner. This is the action of the transverse.