The Ultimate Core Workout - Planks
The almighty Plank! One of the best core workouts when done properly. Proper plank form means keeping your core and quads tight, and your hips lifted. For stability's sake, your feet (or knees) and arms (or elbows) should be shoulder-width apart. To keep your spine in a neutral position and avoid straining your neck, aim your chin about six inches in front of your body. As you hold the plank, imagine that you’re extending from the crown of your head through your heels. Lastly, don't hold your breath! It's key to remain engaged throughout the entire plank, so don't let yourself zone out.
Here is a video of a proper plank form and a few variations to the plank to challenge the body.