Flat Abs ... Strong Core
When it comes to exercises, sometimes the simplest are the most effective, and this is certainly the case with the plank and its variations. I personally think the plank and its many variations are one of the most effective exercises for core conditioning. I am not sure there is any part of my body that doesn’t feel its working when I am holding a plank!
If you’ve never tried one, a plank may look easy, and it might be hard to believe it's very effective in strengthening and toning your whole body. Holding the correct position for a plank takes strength and endurance in your abs, back, core and upper body.
You can add intensity with more advanced planks as you master the basic ones and each one works different areas of your body. Here is a side plank with hip drop. Again it might look easy but give this a go for 30-60 seconds (beginner to advanced) and see how challenging it can be.
Obliques burn out!