Planks provide excellent activation to the core muscles, including the abdominals.  A 2017 study published in the American Journal of Physical Medicine & Rehabilitation6 observed activation in the “upper rectus abdominis, lower rectus abdominis, external oblique, [and] lumbar erector spinae” in eight different isometric plank variations. That means planks make a great tool for increasing abdominal muscle size, strength, and endurance, but they shouldn’t be the only exercise you do. To put things into perspective, a 2018 study published in the Journal of Human Kinetics7 compared core muscle activation during the plank versus a 6-RM set of heavy back squats. The back squat set produced similar EMG activation in the rectus abdominis and external obliques, but “greater erector spinae activation,” leading researchers to conclude that “high-intensity squats rather than isometric low intensity core exercises for athletes would be recommended.” Put simply, doing planks is effective in building abs, but it should be part of a comprehensive program that supports building core strength and stability in other ways.  Your core training should include much more than just direct ab training exercises. Period.