Really want to get into the deepest parts of your core? How about a plank? There are difficult varieties like the hardstyle plank and eight-point plank, a favorite of pro surfer Lakey Peterson. There ...

Plank is the most straightforward exercise out there: you go down on all fours and keep your body straight as long as possible. Or, wait, are you supposed to do planks on your elbows? What about side ...