The plank is the most basic of core exercise and one that almost everyone ignores. The plank trains the core by doing what the core is supposed to do: hold you upright. Everyone should be able to do this for a minute, but if you can do a straight plank for more than 2 minutes, you should start making them harder. Watch the other videos for some ideas on ways to step this exercise up a notch.
Coach Stevo's Top Tips
Plank
Long Plank
Weighted Plank
One-Leg Plank